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Natural resources research IV

In this series:

I. A review of the natural resources of the African Continent. I. Enquête sur les ressources naturelles du continent africain. n . Bibliography of African hydrology ¡Bibliographie hydrologique africaine,

by/par J. Rodier. m . Geological map of Africa (1/5 000 000). Explanatory note/Carte géologique de l'Afrique

(1/5 000 000). Notice explicative, by/par B . Furon & J. L o m b a r d . IV. Review of research on latérites, by R . Maignien.

Review of

research on latérites

R . Maignien, Office de la Recherche Scientifique

et Technique d'Outre-mer

Paris

unesco

Published in 1966 by the United Nations Educational, Scientific and Cultural Organization Place de Fontenoy, Paris-7e

Printed by Vaillant-Carmanne, Liège

© Unesco 1966 Printed in Belgium NS.65/XII.4/A

Foreword

Since the earliest days of the Organization, part of Unesco's science programme has been concerned with certain problems relating to the natural environment and natural resources. One of the first steps was to establish an advisory committee for Arid Zone Research and one for Humid Tropics Research under the guidance of which many symposia were organized and a number of reviews of research published by Unesco.

It became evident, however, that certain activities undertaken under these programmes in fact concerned an entire continent or the whole world and that other activities not covered by any of the existing programmes should be undertaken. For these reasons all the activities were regrouped within a single division known as the Division of Natural Resources Research. Moreover a 'Natural Resources Research' series was initiated and to date three volumes which concern the African Continent have been published. Forthcoming publications will be related to natural resources in Latin America and to various other fields of activities of the Division.

Following recommendations of the Advisory Committee for Humid Tropics Research, Unesco organized a symposium on latérites which was held at Tananarive, Madagascar, during the last week of September 1964. Prior to this meeting a review of research on latérites was prepared at Unesco's request by R . Maignien, of the French Office de la Recherche Scientifique et Technique d'Outre-Mer ( O R S T O M ) . This study served as a basis for discussion during the symposium and is now published as Volume IV of the 'Natural Resources Research' series. It is hardly necessary to give a reason for such a review, prolific literature having developed since F . Buchanan (1807) gave the name 'latérite' to the mantle of ferruginous deposits which covers large areas in southern India. This is partly due to the distribution range of the material which comprises India, Malaysia, the East Indies, Australia, Cuba, the Hawaiian Islands, the tropical regions of Africa and South America, not to mention geologically ancient deposits which are even more widely spread. The reason for this abundance of literature seems to be that latérite—and its far more economically important variant, bauxite—is not uniquely identified with any particular parent rock, geological age, single way of formation, climate per se, or geographical location. It is rather a response to a set of regionally varying physico-chemical conditions which are still far from being fully known in their interaction. Hence untimely generalizations on the basis of too limited experience occurred while important

publications were sometimes left out of account. Further research in this field, •which interests at once geologists, mineralogists, biogeographers and geomor-phologists, should be based on a wider use of existing scientific literature, and the present review is published with this end in view.

It goes without saying that such a review necessarily reflects the opinions of its author, and will thus give rise to further discussions. M a y these contribute to a deeper insight into a subject which is of great importance both from the scientific and the practical points of view.

Contents

Acknowledgements • • • • • • • • • • • • • » 9

Background, definition and scope of the problem . . • • • • • 11

Morphological and analytical characteristics of latérites . . . . . 17 Indurated occurrences 17

Morphological and physical characters 18 Chemical and mineralogical characters 20 Mineralogical characters 22 Location of indurated occurrences in soils 24

Lateritic soils 26 Characteristic profiles 26 Analytic characters 37

Relations between tropical soils and indurated latérites . . . . 45 Lateritic soils 45 Ferruginous tropical soils 46 Hydromorphic soils 47 Yertisols 47 Colluyial crusts 47

Relations between the morphology of incrusted horizons and the m e d i u m in which they form 48

The role of constituent sesquioxides 48 Morphological factors in relation to incrusted horizons . . . . 49

Global distribution of latérites. Relations with environmental factors . . . 52 The distribution of latérites 52

Africa 53 America 54 Asia 55 Australia 57

Relations with environmental factors 58 Climate 59 Vegetation and fauna 61 Rocks and original materials 64 Relief 66 Age 73

Origin of latérites 76 Origin of latérite components 77

Alteration product components 78 Pre-existing components in the rock 91

Accumulation of latérite constituents 92 Accumulation in the profile without outside contribution . . . 93 Accumulation by outside contributions to the profiles . . . . 101

Development of accumulation horizons 103 Development of microstructures 103 Latérite induration 104 Latérite degradation 108

Classification of latérites. Correlations Ill U . S . S . R . system 113 French system 115 Portuguese system 116 British system 116 Australian system 117 U . S . A . system 118 Belgian system 120 SPI system 121 F A O system 123

Utilisation of latérites 126 Fertility of latérites 126 Value of latérites as a source of ores 132 Utilization of latérites in civil engineering 132 Use of latérites for stratigraphical purposes 133 Hydrological properties of latérites 133

Appendix 134

Bibliography 136

Acknowledgements

I am very grateful for the assistance received from the following distinguished scientists who were good enough to provide m e with information on the main studies with which they were familiar: Dr. C H A R L E S E . K E L L O G G , Deputy Administrator for Soil Survey, Soil Conser

vation Service, United States Department of Agriculture, Washington 25, D . C . , U.S.A.

Dr. L Y L E T . A L E X A N D E R , Chief, Soil Survey Laboratory, Plant Industry Station, Beltsville, Maryland, U.S.A.

Dr. J. J. FRIPIAT and Dr. M . C. G A S T U C H E , Laboratoire des Colloïdes des Sols Tropicaux de l'INEAC, Institut Agronomique de l'Université Catholique, Louvain-Heverlée, Belgium.

Dr. C. S Y S , Centrum Voor Bodemkartering, Rozier 6, Ghent, Belgium. Dr. J. K . T A Y L O R , Chief of Division of Soils, Commonwealth Scientific and

Industrial Research Organization, Private Bag No. 1-6, P .O . Adelaide, South Australia.

Dr. M . J. M U L C A H Y , CSIRO, Western Australian Regional Laboratory, Private Bag, P .O. Nedlands, Western Australia.

Dr. R . D U D A L , F A O , Via délie Terme di Caracalla, Rome, Italy. Dr. E . V . L O B O V A , Dokuchaiev Institute, Pyjevsky per 7, Moscow, U.S.S.R. Academician I. P. GERASSIMOV, Institute of Geography, U.S.S.R. Academy

of Sciences, Moscow, U.S.S.R. Dr. F. F O U R N I E R , Bureau Interafricain des Sols, 57, rue Cuvier, Paris-5e, France. Mr. B . D A B I N , Services Scientifiques Centraux, O R S T O M , Bondy, Seine, France.

Background,

definition and scope of the problem

The term 'latérite' first appeared in scientific literature a little over a hundred and fifty years ago. Despite a chequered history it has remained in c o m m o n usage, so that w e should expect it to cover certain recognized, well-defined occurrences; yet even a brief study of attempts to narrow down its meaning shows that this laconic term is often applied to widely different phenomena. Before starting to explain it, therefore, it seems necessary to compile a list of all the observed occurrences which for one reason or another have been claimed as latérites during the past century and a half. Such a list would define the scope of the subject and so provide a solid basis for future research.

The word 'latérite' was suggested by Buchanan (1807) to denote a building material used in the mountain regions of Malabar (India). Its appearance is that of a ferruginous deposit of vesicular structure, apparently unstratified and occurring not far below the surface. W h e n fresh it can readily be cut into regular blocks with a cutting tool. O n exposure to the air it rapidly hardens and becomes highly resistant to weathering. Because of these properties it is frequently used as a building material comparable to bricks. The word for it in some local dialects means brick earth and the n a m e 'latérite' is merely a translation from the Latin later, meaning 'brick' and so relating solely to the use to which these blocks are put (Prescott and Pendleton, 1952).

There is some dispute in regard to the authorship of the term. Prescott, h o w ever, suggests that Babington (1821) was theÜTSt to use it scientifically. Buchanan soon realized the disadvantage of using too specific a word for a material of which the analytical characters were little known and during the period 1807-14 he used the words 'latérite' and 'brickstone' indiscriminately. The unusual feature of the occurrence first described by Buchanan under the n a m e latérite was that it had a soft consistency in situ but hardened rapidly on exposure. Buchanan himself noted that morphologically similar occurrences observed in Bihar were already indurated in the soil. It was only towards the end of his life that Buchanan restricted the use of latérite to a material of soft consistency which hardened on exposure to the air.

It can be seen that latérite was originally applied to a particular type of morphology and to a remarkable property of the rocks. Even though it was very soon acknowledged that a ferruginous argillaceous occurrence was involved, it was in this sense that the term rapidly found currency in India and then throughout the world well before the turn of the century.

11

Background, definition and scope of the problem

Excellent descriptions are to be found in the works of Newbold (1844, 1846), and by 1890 Lake was in a position to review a considerable number of studies of Indian latérites in a report on the geology of South Malabar. This period is associated with the names of Babington (1821), Benza (1836), Clark (1838), Wingate (1852), Kelaart (1853), Blanford (1859), Buist (1860), King and Foote (1864), Cole (1836), W y n n e (1872), Theobald (1873), Voysey (1883), and Mallet (1881) (cf. Sivarajasingham et al., 1962).

Important criteria had already been established. Latérite was the alteration product of various materials including crystalline igneous rocks, sediments, detrital deposits and volcanic ash; it could also be a lacustrine deposit.

Thus it very soon became axiomatic that latérites were of extremely diverse origins, and they were identified in terms of physical characters as surface or near-surface occurrences in various regions (Australia, Africa and South America).

The difficulties entailed in use of the term soon became apparent. Blanford (1859) mentions that in some cases the subjacent lithomarge m a y harden on exposure to the air. Talbott (Prescott, 1931) further complicated the problem by applying the term in Australia to indurated occurrences containing silica and limestone.

More precise definition was now an imperative necessity. Mallet (1883) had suggested that latérite was ferruginous and aluminous, but it was Bauer (1898) who established its main chemical characteristics. O n analysing latérite samples from the Seychelles and finding that they contained small amounts of combined silica and of aluminium in an hydroxide form, he compared the composition of latérites with that of bauxites. A little later Warth and Warth (1903) published a report on their findings in regard to samples from India. Some of these latérites contained small amounts of alumina but were rich in iron oxides; conversely, others contained large amounts of aluminium oxide but were relatively poor in iron; intermediate between these extremes was a complete series of different combinations. Similar results were obtained from analysis of various samples from other tropical regions of the world (Du Bois, 1903 ; Holland, 1903).

Geologists soon became interested in the possible value of latérites as an aluminium or, in some cases, manganese ore (Fermor, 1911) and numerous analyses were performed on samples offering or seeming to offer some minera-logical interest. This was the period associated with the names of Richtofen (1886), Oldham (1893), van Bemmelen (1904), Chautard and Lemoine (1908), Arsandeau (1909), Harrison (1910), Lacroix (1913) and others. The general tendency of all these authors was to define latérites in chemical, sometimes in mineralogical terms. Their efforts gave rise to much controversy, particularly in regard to the nature and relative proportions of the characteristic oxides.

Croor (1909) and later Evans (1910) wished to confine the term latérite to alteration products containing free alumina. Evans writes that 'though the chemical composition of latérites varies within wide limits... one feature remains constant—the small amount of combined silica in proportion to the alumina present, and it is in this respect that latérites differ from clays, which also occur as tropical decomposition products'. Other authors insisted on the importance of the iron oxides. Neustruev (quoted by Lacroix, 1913) maintained that a latérite must contain more than 86 per cent of Fe 20 3 . Fermor (1911) raised this limit to 90 per cent; latérites, in his view, were made up of the oxides of iron, aluminium, titanium and manganese; but the presence of iron oxide was essential, as Holland had pointed out in 1903.

12


Attempts at physical definition of latérites were thus practically abandoned in favour of chemical and mineralogical definitions.

Harrison (1910) widened the scope of the term still further by including unhardened iron- and alumina-rich earths.

Again, Lacroix (1913) took into account only the total content of hydrous oxides. H e distinguished: (a) true latérites, containing more than 90 per cent of hydroxides; (b) silicate latérites, containing from 50 to 90 per cent of hydroxides, and (c) lateritic clays, containing from 10 to 50 per cent of hydroxides. These latérites were described as quartziferous when they contained quartz neoformations.

Walther, however, writing towards the end of the same period (1889, 1915, 1916) returned to a morphological definition, recalling that the term latérite had originally referred to the red, brick-like colour and meant 'resembling bricks'. H e therefore proposed to extend the definition to all red-coloured alluvion and eluvion products and, specifically, to tropical red earths. This new definition was enthusiastically welcomed, especially by agronomists: hence a new source of confusion.

The pedological approach to latérite study dates only from the 1920s. B y that time, the words 'latérite' and 'lateritic' had come to be used in a variety of meanings, for the definitions were based sometimes on morphological, sometimes on physical and sometimes on chemical characters. The many attempts to standardize usage had met with varying degrees of success.

Harrassowitz (1930) associates latérites with a characteristic profile developing under tropical savannah and forming the following four levels, in ascending order: (a) a fresh zone; (b) a zone of primary alteration to kaolinite; (c) a lateritic bed proper; (d) a surface zone with ferruginous incrustations and concretions.

Erhart (1935) considered the presence of a transitional horizon between the parent rock and the latérite to be essential, but this contention was disputed.

Van Bemmelen (1904), followed by Harrossowitz (1926) and finally by Martin and Doyne (1927) brought the Si02/Al203 ratio into the definition. In latérites this ratio was narrower than 1.33. Iron was not an essential element. These criteria, together with the Si02 /R a03 ratio, were adopted by Joachim and K a n -diah (1941). The use of the different ratios w o n favour and was widely adopted (Botelho da Costa, 1954; Aubert, 1954; Aubert and Duchaufour, 1956; Segalen, 1957; Camargo and Bennema, 1962, and others).

The value of these ratios, however, has more recently been disputed by Pendleton and Sharasuvana (1946), G . W . Robinson (1949), Van der Woort (1950) and Waegemans (1951 a, b), on the grounds that they result from a combination of alteration processes, neosyntheses, differential migrations and mechanical reworking (Maignien, 1961) and are therefore indicative rather than absolute criteria.

Pendleton (1936) strongly opposed the chemical definitions and reverted to Buchanan's original concept: 'One in which a latérite horizon is found in the profile'; that is to say, one in which there is an incipient or immaturely developed latérite horizon. A lateritic soil is one in which there is an underdeveloped latérite horizon which will become true latérite if the appropriate conditions persist long enough (Pendleton and Sharasuvana, 1946). Latérite is an illuvial horizon (Mohr, 1932; Pendleton, 1936; Prescott and Pendleton, 1952).

While these controversies continued, studies based on the chemical and agro-

13


nomical criteria multiplied, sometimes producing contradictory results. This extremely fruitful period is associated with the names of Bennett (Bennett and Allison, 1928), Hardy (1931), Senstius (1931), Marbut (1932), Sir John Harrison (1933), Fox (1936), Lombard (1937), Baeyens (1938), Scaetta (1938, 1941), Greene (1945), Sherman (1949, 1950) and others.

With the development of pedological studies immediately after the Second World W a r more and more examples of latérites were found, both in Buchanan's definition and in the sense of tropical soils with a SiOg/AIjC^ ratio narrower than 2. D u Preez (1949) gives a definition very similar to Pendleton's but minimizes the importance of free alumina. This definition covers numerous morphological aspects but nevertheless does not mention varieties which, although soft when in situ, harden on exposure to the air—a characteristic of all the indurated, sesquioxide-rich occurrences. Mohr and Van Baren (1954) extend the definition to alteration products, whether indurated while still in the soil or only after exposure.

Kellogg (1949) restricts the word 'latérite' to ferruginous materials which harden after exposure and to fossil forms of these materials. The four principal forms recognized by the United States Soil Survey Staff are: (a) soft mottled clays that change irreversibly to hardpans or crusts when exposed; (b) cellular and mottled hardpans and crusts; (c) concretions or nodules in a matrix of unconsolidated material; and (d) consolidated masses of concretions or nodules.

In view of the complexity of the examples and occurrences in question the present trend is to distinguish between alteration processes and the induration processes characteristic of latérites.

'The two phenomena, the physical process of plating and the physicoche-mical process of ferrallitization (lateritization) are completely different: the former can occur only in materials rich in iron and manganese hydroxides alone, without aluminium hydroxides, whereas the lateritization or ferrallitization process proper to lateritic soils consists of a combination of phenomena resulting in extremely thorough alteration of the soil-forming rock and individualization of such components as silica, oxides and metal hydroxides and hydrates, especially those of iron, and alumina, manganese and titanium. The latter are preserved or accumulate in a surface horizon or at small depths, the silica falling partly to the base of the profile' (Aubert, 1954).

Certain chemical characters of lateritic soils thus occur in all the crusts, but they m a y equally occur in other soils of different structure: conversely, soils without the chemical characters of latérites m a y contain indurated horizons or ferruginous or manganiferous concretions (Maignien, 1954).

D'Hoore (1954) and Maignien (1958), studying the formation of indurated horizons in tropical soils, associate them with differential migrations of ses-quioxides connected with the movement of water in the soil. The induration processes are promoted by lateritic alteration but are not specific to lateritic media.

It appears increasingly difficult to give a purely morphological, purely physical or purely chemical definition. The word 'latérite' covers a wide variety of aspects of tropical soil formation and is too general. Cerasimov (1962) uses it to include all the soils of intertropical regions. Many investigators tend to give it a genetic definition and would prefer to abolish the term in order to avoid any possibility of confusion. Scrivenor (1930), Vine (1949) and G . W . Robinson

14

Background, definition and ecope of the problem

(1949) substitute the connotative -word 'ferrallitization'. Botelho da Costa and Azevedo (1949) support this and point out that 'this term is very useful, since it refers to physical and physicochemical characters of soils and can therefore be used on a high level of generalization'. The French school of pedology follows this practice.

Kellogg (1949) introduced the term 'latosols', which is still widely used to denote these soils.

In 1958 Sys invented the term 'kaolisol' to denote intertropical soils possessing the physicochemical characteristics of latérites. Latérite in Buchanan's sense is called 'plinthite', a word introduced by the United States Department of Agriculture ( U S D A ) (1960) and meaning a sesquioxide-rich horizon poor in h u m u s , mixed with heavily altered clays, quartz and other diluents occurring as red mottles, usually in platy, polygonal or reticulate patterns. Plinthite changes irreversibly to hardpans or irregular (hard) aggregates on repeated wetting and drying or it is the hardened relics of the soft red mottles.

In 1960, in the seventh U S D A approximation, the term 'oxisol' was introduced, comprising both those soils which had lately become k n o w n as latosols and also most of the soils covered by the term 'groundwater latérites', which had in the past been called latérites. These oxisols are characterized by an oxic horizon, by which they are diagnosed. The presence or absence of plinthite is not characteristic at high levels of classification. The oxic horizon has the theoretical properties of latérites and lateritic soils; these properties require more precise and complete definition, but Sys (1962), summarizing and expanding them, defines a ferrallitic diagnostic horizon of kaolisol type. This is a tropical soil horizon situated between the A horizon and the level of altered rocks and presenting the following characteristics: (a) a low or non-existent reserve of alterable minerals; (b) a granular clay fraction composed almost exclusively of kaolinite and/or oxides, with gibbsite frequently but not invariably present and sometimes with large amounts of aluminosilicate gels; (c) Si02/Al203 ratio sometimes close to 2 but often narrower; (d) m a x i m u m intensity of red or yellow coloration, due to a relatively large accumulation of iron oxides ; (e) m a x i m u m clay content, due mainly to a m a x i m u m of alteration rather than to migration. This horizon m a y reach a thickness of 5 - 10 m .

The presence or absence of an indurated horizon either in situ or after exposure is disregarded. Kaolisols are defined by the intense kaolinitic neosyntheses characteristic of humid tropical climates.

In conclusion, it seems that the difficulties involved in applying the term 'latérite' in too broad a definition to occurrences associated with specific environments or processes are leading to the abandonment of Buchanan's term. At present, the terms 'crust' and 'hardpans' (ferruginous and/or aluminous, sometimes manganiferous), plinthite, ferraUitic soils, kaolisols, latosols and oxisols are all used as synonyms for 'latérite' in the very broad sense. Yet the term latérite is itself very widely used, always in relation to red formations, of varied morphology but always indurated, which are rich in sesquioxides of alumina, iron or manganese. This sense is close to Buchanan's use of the term.

Alexander and Cady (1962) use it in that sense and give the following definition: 'latérite is a highly weathered material rich in secondary oxides of iron, aluminium, or both. It is nearly void of bases and primary silicates, but it m a y contain large amounts of quartz and kaolinite. It is either hard or capable of hardening on exposure to wetting and drying'.

15


Summarizing, w e m a y say that induration is the sole specific characteristic of latérite, since the other criteria can be found in other clay soils or materials.

Sivarajasingham's synthesis (1962), which otherwise includes the principal criteria of Kellogg and Alexander, is based on this conception. There is no restriction to the definition in regard to morphological aspects, soil-formation processes or specific conditions of time and place. Used in this sense, the term covers the various forms indicated by Kellogg. It does not, however, include any of the following : (a) any earthy material rich in sesquioxides and bearing the name latérite or lateritic soil but not hardening on exposure; (b) the iron-rich masses or nodules combined with significant amounts of humus characteristic of podzols ; (c) indurated masses cemented with silica, carbonates or substances other than sesquioxides, even though they m a y contain particles or nodules in an advanced state of decomposition; (d) certain hardened granules occurring in a slightly decomposed material.

Discussion of 'latérite' would entail deciding whether the term is to be restricted solely to indurated occurrences, in the U S D A sense, which does not otherwise correspond exactly to Buchanan's, or strictly to the latter; or, again, whether to include all indurated occurrences or those not possessing the particular chemical and mineralogical characters which seem to correspond specifically to tropical environments.

Personally, I think the last-mentioned solution the most promising. It is the only one which allows for precise definition of the different processes and conditions leading to the occurrence. If we start with the purely morphological aspect, even if w e specify the physicochemical characters associated with it, we cannot place the problem in its proper context, which, despite all other considerations, is geographical. The limitations contained in the definitions discussed above merely show where the scientific difficulties lie. Alteration, for example, is frequently mentioned: but what type of alteration and in what degree? It seems to m e dangerous to consider even a thoroughly defined occurrence in total isolation. In the natural sciences observed phenomena cannot be separated from their geographical environment.

For all these reasons the present work deals with 'latérites', rather than with 'latérite'. I shall discuss the problem in the widest sense, in full awareness of the difficulties raised by so poorly defined a phenomenon.

16

Morphological and analytical

characteristics of latérites

Indurated occurrences, or latérites sensu stricto, and soils described as 'lateritic' in the wider sense will be dealt with alternately for ease of exposition.

I N D U R A T E D OCCURRENCES

Extremely varied forms of these occurrences have been identified. Newbold (1844) gives the following description (cited from Prescott and

Pendleton, 1952) : 'The latérite of Beder, generally speaking, is a purplish or brick-red, porous rock, passing into liver brown perforated by numerous sinuous and tortuous tubular cavities either empty, filled, or partially filled with a greyish-white clay passing into an ochreous, reddish and yellow-brown dust; or with a lilac-tinted lithomargic earth. The sides of the cavities are usually ferruginous and often of a deep brown or chocolate colour; though generally not more than a line or two in thickness, their laminar structure m a y frequently be distinguished by the naked eye. . . . The interior of the cavities has usually a smooth polished superficie, but sometimes mammillary, and stalactiform on a minute scale. The hardest varieties of the rock are the darkest coloured, and most ferruginous. The surface masses of the softer kinds present a variegated appearance. The clay and lithomarge exhibit lively coloured patches of yellow, lilac, and white, intersected by a network of red, purple, or brown. The softness of this rock is such that it m a y be cut with a spade; hardening by exposure to the sun and air, like the latérite of Malabar'. The extreme heterogeneity of the material is indicated by the length of this description.

Pendleton and Sharasuvana (1946) distinguish two physical forms (vesicular and pisolitic) with a wide range of intermediate forms.

D u Preez (1949) defines latérite as 'a mass that m a y be vesicular, or concretionary, or vermicular, or pisolitic, or more or less massive, consisting essentially of iron oxide with or without clastic quartz, and containing small amounts of aluminium and manganese. Although its hardness varies it can usually be broken and shaped readily with a h a m m e r ' .

It would not be difficult to increase the number of descriptions. T o do so, however, would only be to confirm the extreme diversity of these occurrences, which m a y be basically ferruginous, or aluminous, or argillaceous, and m a y sometimes be rich in clastic quartz or neogene quartz. They m a y equally well

17

Morphological and analytical characteristics of latérites

be manganiferous and frequently will not contain crystalline silica. Nevertheless, there are some specific characters that emerge out of the extreme heterogeneity of these materials.

MORPHOLOGICAL A N D PHYSICAL CHARACTERS

Induration

All descriptions and definitions refer to induration in situ or on exposure to the air. Degrees of hardness ranging from products that are practically unconsolidated and scarcely coherent to the hardest blocks which can be broken with a hammer only with difficulty are found on examination of samples. Induration is an empirical criterion, since it is impossible to give quantitative expression to any character related to the mechanical properties of the material. The usual definition of induration is a state in which the hard brittle consistency of the medium is not affected by humidity. Several degrees of induration can be distinguished, depending whether the material can or cannot be broken easily in the hand, cut with a spade, or split with a hammer (Maignien, 1958).

Several factors affect the induration of samples:

1. Composition and the extent of crystallization of the components : the higher the sesquioxide content, the greater the induration: hardness increases as iron content increases; the hardest latérites are also the least hydrated.

2. The arrangement of the various constituents : compact crusts are more indurated than loose crusts; homogeneous materials are harder than those whose components are segregated; the presence of foreign bodies reduces over-all strength.

3. The extent of ageing: older occurrences are frequently harder than recent occurrences of the same type of latérite.

Structure

Latérites vary greatly in structure, but can be reduced to the following three structural patterns: (a) the indurated elements form a continuous, coherent skeleton ; (b) the indurated elements are free concretions or nodules in an earthy matrix; (c) the indurated elements cement pre-existing materials.

These structural types exhibit great variability in relation to the shape and size of the elements involved and the degree of induration.

The following forms can be distinguished: 1. Homogeneous or continuous structures (rocks whose original components

have been replaced by the sesquioxides of iron or aluminium, and even sometimes of manganese).

2. Heterogeneous or discontinuous structures, which are the most common, and which are divided into : (a) loose rocks, usually gravelly (the concretionary or fine gravel horizons frequently encountered); (b) coherent rocks (conglomeratic, conglomerate-type, vesicular, scoriaceous or spongy).

3. Regular structures (platy, pisolitic and oxolitic). Alexander and Cady (1962) give some highly instructive details concerning the arrangement of microstructural elements in latérites. Under the microscope certain microstructures are found to be present in many samples of latérites with the most varied macroscopic structure. Thin sections reveal spheroidal or slightly

18


elongated particles embedded in an even finer mass of particles which m a y be extremely dense or spongy. The rounded elements are completely fused and m a y be separate or highly clustered. Their outlines vary from specimen to specimen and are well defined when smooth, but far less so when irregular. The mass m a y be homogeneous in appearance without any specific distinguishing features, or m a y exhibit regular patterning (rectangular, grid-like or reticulate). The patterned materials cover the pores or form a fine surface layer on the nodule, and m a y consist of pseudomorphs of original minerals that are sometimes recognizable. Although the rock structure m a y be retained, it m a y well be entirely absent. Quartz particles are sometimes to be seen and some samples contain decomposable minerals protected by a film of decomposed material.

Colour^

Latérites vary in colour, but are usually bright. The shades most frequently encountered are pink, ochre, red and brown, but some occurrences are mottled and streaked with violet, and others exhibit green marbling. A single sample m a y exhibit a whole range of colours merging more or less perceptibly into one another in a variety of patterns and forms.

Latérites owe their colour to iron oxides in various states of hydration and sometimes also to manganese. Save for extremes of coloration corresponding to red hematite when 7.5 Y R in Munsell's colour code and to goethite or yellow lepidocrocite (2.5 Y R ) , there does not seem to be any relationship between coloration and the index of hydration (Waegemans and Henry, 1954). Iron compounds yield a grey-black colour and manganese compounds a velvety black in a reducing medium, while in an oxidizing medium iron yields ochre, red or black, and manganese violet. Alumina is white in the pure state, but is often found mixed with iron in hardpans, where it gives rise to the characteristic rose tints. Silica, which is ordinarily whitish and impregnated with the hydroxides of iron, m a y yield a red or rust colour. Kaolinites fix iron on their surface and become deep red (7.5 YR—10 R ) . It is the mixture of these differently coloured constituents which produces the extremely varied coloration of latérites.

Although estimation of colours can give only a rough idea of composition it does make it possible to estimate the level of evolution and the formation conditions. Ferruginous crusts which are red or ochreous in their young stages darken as they age, and are then brown and sometimes almost black. Aluminous crusts, on the other hand, become lighter in the course of time. From another point of view ferruginous crusts are darker (brown) under conditions of poor drainage than under conditions of oxidation (red).

Density

Density varies within fairly considerable limits (2.5—3.6) in relation to chemical composition, increasing with iron content and decreasing with alumina content. The specific gravity of oxidized forms is higher than that of hydrated forms.

Comparison of the apparent density of latérites is useful from several points of view, firstly for estimation of the intensity of leaching of certain materials and for determination of the order of magnitude of active surfaces. Apparent density is also always higher in the surface levels of latérites than at depth.

19


Old crusts are denser than recent crusts. The looser the structure, the lower will density be for latérite of the same composition: cemented formations are denser than those of scoriaceous structure, which in their turn are denser than those of alveolar structure (Maignien, 1958).

CHEMICAL AND MINEEALOGICAL CHARACTERS

High content of the sesquioxides of iron and/or of aluminium relative to the other components is a feature of latérites.

These essential components are mixed in variable proportions. Some latérites m a y contain more than 80 per cent of Fe 2 0 3 and little A1 2 0 3 (a few per cent), while others m a y contain up to 60 per cent of alumina and only a few per cent of Fe203.

Although alkaline and alkaline-earth bases are almost entirely absent in most cases, this is not an absolute criterion. In particular, some ferruginous crusts formed in alluvia and some concretionary horizons in ferruginous tropical soils m a y contain significant amounts.

Combined silica content is low in sesquioxide-rich latérites, but m a n y varieties, such as Buchanan's latérite, can contain significant amounts. In Guinea (Maignien, 1958) some crusts and indurated gravels contain more than 20 per cent of combined silica.

This combined silica is predominantly in the form of kaolinite, the characteristic clay of most tropical formations. It was on this basis that D'Hoore (1954) made a theoretical calculation of free A1 2 0 3 content from combined silica content employing the formula : free A1 2 0 3 = total A1 2 0 3 — (silicate Si02

X 0.849). Use of this formula led to the statement that alumina was present principally in combined form in latérites of Buchanan's type.

Although alumina is sometimes the main constituent, the sesquioxides of iron are the most common and the most frequent.

Some latérites contain manganese, sometimes in sufficient quantity to be exploited as a mineral (Ivory Coast, Gabon). The same applies to titanium' which is frequently identified, and to a lesser extent to vanadium and chromium.

Combined water determined by loss on ignition is always present in appreciable amounts (10-30 per cent), and is more abundant in aluminous than in ferruginous latérites.

Although quartz is sometimes absent or present only in small amounts it is more usually an important component. This quartz is mainly residual, especially on formations derived from acid igneous rocks, although quartz is also often found in latérites formed from quartz-free rocks, in which case it is of alluvial or aeolian origin, or sometimes neogenetic (de Chetelat, 1938 ; D e Craene, 1954) or phytoliths (D'Hoore, 1954; Delvigne, 1963).

"Warth and Warth (1903) state that Indian samples contain more than 20 per cent of quartz on average.

Certain relationships can be established between the components by comparison of all this information: (a) a direct relationship between the amount of combined water and alumina content, but no such relationship for iron; (b) quartz is always poorly represented in aluminous latérites; (c) crusts with a high quartz content are particularly ferruginous.

20


There does not seem to be any clearly defined connexion between the relative content of silica, iron and alumina and the level of induration. Varying levels of induration, from the softest to the hardest occurrences, are to be found with identical iron content. Moreover, in crusts of comparable induration Fe 2 0 3

content is more often than not inversely proportional to insoluble iron content.

Secondary products (variously altered rock or mineral debris present as enclosures in the indurated mass) are to be found alongside the main constituents of indurated horizons, where they contribute to the morphology and composition of the crusts. Such debris is often derived from partly dismantled former crusts, but m a y also be rocks impregnated with sesquioxides in situ. A wide range of combinations is observed, and although these components do not fundamentally contribute to the genesis of the formations in question, study of them does throw considerable light on the history of latérite, and especially on relationships with subjacent or superjacent formations.

There have been several attempts to classify latérites in terms of their chemical composition (Fermor, 1911; Lacroix, 1913; de Chetelat, 1938), but Fox (1936) has demonstrated that such classifications are inadequate, other than in relation to deposits that m a y be exploited for their mineral content. Classifications based on chemical composition cannot be used to distinguish between indurated and softer formations.

So far w e have only considered the chemical composition of bulk samples and have not drawn any distinction between the separate parts. Castagnol and Shan-Gia-Tu (1910) investigated variations in the chemical composition of latérite as a function of variation in texture and colour. H e found that the most highly indurated brown constituents were rich in iron, that grey earthy constituents were clay-enriched, and that whitish areas were alumina-enriched. In pisolitic or nodular crusts the concretions were either similar in composition to the enclosing matrix or differed in composition especially in containing less combined silica and more of the oxides of iron. Prescott and Pendleton (1952) were of the opinion that nodules contained less free alumina than massive forms, and that their manganese content was lower. Segalen (1957) observed, when comparing the composition of concretions with that of the surrounding soil in Madagascar, that three types of concretion could be distinguished : (a) hardening of a pre-existing material on the addition of a moderate amount of iron and appreciable deposition of silica; (b) hardening accompanied by significant addition of the oxides of iron and little or no deposition of silica; (c) hardening accompanied by addition of the oxides of iron and significant deposition of silica.

Alexander, Cady et al. (1956), who studied samples from Africa, state that the nodules examined had a high sesquioxide and low silica content. They also considered iron and alumina. The present author has had occasion to study the pisolitic crusts of Kindia (Guinea), and found that the pisoliths were composed of pure alumina or the oxides of iron. Nevertheless, many concretions can contain uncombined and combined silica in significant amounts (Joachim and Kandiah, 1941; Waegemans, 1954).

These data illustrate the extent of variations in the composition of concretions, nodules and crusts, as indicated by Bennett and Allison (1928).

21

Iflorphological and analytical characteristics of latentes

MINERALOGICAL CHARACTERS

Chemical analyses are often too crude to reveal the composition, nature and origin of latérites (Campbell, 1917). Latentes exhibiting the same physical properties may differ greatly in their chemical composition and, conversely, latérites exhibiting a similar chemical composition m a y possess different physical properties.

M a n y investigators have also attempted to supplement chemical determination by mineralogical investigation. Harrison (1910) and Lacroix (1913) studied thin sections in polarized light. Hardy (1931) dealt in his research with the absorption of colorants. Since 1945 considerable light has been thrown on the mineralogical composition of latérites by increasing application of differential thermal analysis, temperature balance, and the study of X-ray diffraction diagrams. This research is particularly associated with the names of Humbert (1948), Van der M e r w (1951), Fripiat et al. (1954), Alexander et al. (1956), Sega-len (1957), Bonifas (1959), Paquet, Millot and Maignien (1961), and Pecrot et al. (1962).

The information actually assembled enables the constituents of laterites to be divided into major elements which play an essential part in the formation of indurated horizons and minor elements which have no effect on the process. The former consist of the oxides and hydroxides of aluminium and iron, and sometimes of manganese and titanium, silica and often also clays. The latter are textura! elements of the soils in situ, residual or clastic products.

Free alumina is to be found in various forms, of which the most c o m m o n is gibbsite : Y A 1 ( O H ) 3 . Boehmite, a monohydrate with the formula yAlO(OH), is less often reported, probably owing to difficulty of identification. Boehmite crystals are very small and it is impossible to examine them by optical methods. Diaspore (yAlO(OH)) occurs in flakes or oval sections, but is fairly uncommon in laterites. It has been reported in Portuguese Guinea (Weisse, 1952). Corundum (a A1203) is found only in laterites affected by volcanic glass. Amorphous forms have been identified under various names, especially cliachite (Hanlon, 1945), and have been incorrectly described as alumogels (Lacroix, 1913), although they are in fact disordered clusters of cryptocrystalline aggregates giving the appearance of isotropy on optical examination. They aTe mixed to varying degrees with the hydroxides of iron and range in colour from whitish-grey to reddish brown. They can be given the formula Ala03, n H 2 0 (n ranging between 1.54 and 1.51). X-ray examination of these ground masses often reveals the presence of boehmite, and sometimes of gibbsite, and also possibly of allophane.

Iron is present in the crusts in equally varied forms, of which the most usual are goethite (ocFe20(OH)) and haematite (Fe203). Bonifas (1959) reports mag-hemite (yFe203) on dunite in Guinea. In addition to these minerals derived from the evolution of laterites, residual iron oxides such as magnetite (yFe304) and ilmenite (FeTi02) are also found. As in the case of alumina, there are also oxides of iron that are amorphous in appearance and difficult to determine. This applies in particular to cryptocrystalline goethite aggregates which retain variable amounts of water and give rise to red products with the formula Fe 20 3

X H 2 0 (limonite).

Titanium present as rutile, anatase or ilmenite can often be detected on examination of thin sections. These are residual forms (de Chetelat, 1931); the hydrate form has also been detected, but the mineralogical forms have not

22


been established. The same applies to manganese, which is readily concentrated in concretions, and which gives rise to thin purplish-black coatings of manganese dioxide.

Silica, which is often inherited from the original material, is usually present as quartz. The presence of secondary quartz has been reported (Harrison, 1933), but its significance is open to dispute. Various forms of chalcedony and opal have also been noted, in addition to colloidal silica. Free silica is also often present in phytolithic form (D'Hoore, 1954).

Combined silica is frequently present in latérites as clay, especially kaolinite (0H4) Al2Si203, or clays of the same group as the halloysites occurring in pulverulent form usually associated with kaolinite that is to some extent crystallized.

Although many authors deny the presence of significant quantities of illitea they are frequently identified in African latérites, where they occur as products inherited from the parent rock, as contamination products, or as a stage in alteration, which explains w h y these minerals are usually to be seen in young latérites or in those formed near horizons affected by alteration (Maignien, 1958; Gastuche and Fripiat, 1962). Traces of vermiculite are sometimes reported.

Detailed studies of latérites in relation to the distribution and form of their components have been carried out by Alexander et al. (1956), Alexander and Cady (1962), who have shown that the finely divided ground mass is not usually orientated and is therefore difficult to identify by pétrographie methods. H o w ever, it m a y happen that the identifiable parts embrace almost the whole of the material. Thus, it is possible to identify grains and aggregates ranging from kaolinite slightly stained by oxides of iron and verging on goethite or practically pure hematite to highly iron-impregnated kaolinite. Variation in optical density is related to the extent of iron impregnation. It also seems that the induration of the mass m a y be related to the extent of crystallization and the continuity of the crystalline phase of the iron with which it is impregnated, which most often occurs as goethite. Spherical particles resembling nodules in course of formation by centripetal enrichment (Bryan, 1939) are impregnated with iron to a higher extent than the surrounding matrix. Some have surface coatings of crystalline goethite or interior concentric films. Other rounded bodies embedded in the matrix are concretions or pisoliths of gibbsite, boehmite, goethite or haematite. Orientated materials appear as an irregular lattice consisting mainly of goethite or haematite. The frequent pseudomorphs of gibbsite after felspars and of goethite after amphiboles and pyroxenes retain the structure of the minerals and rocks on which they have developed. Such pseudomorphs are particularly prevalent in recent latérites, whereas concretionary and pisolitic forms are to be found mainly in older occurrences (Alexander et al., 1956).

Gibbsite and boehmite are frequently to be found filling fissures and pores both in the matrix and in the nodules. These fissures and pores can equally well be covered by an orientated kaolinitic layer, more or less iron-impregnated, and even sometimes by orientated goethite. In the most highly indurated faciès haematite covering pores takes on a banded appearance.

Latérites derived from quartzose materials exhibit quartz grains randomly distributed throughout the ground mass and the concretionary products. Quartz apparently of external origin is to be seen in latérites formed on non-quartzic rocks (Alexander and Cady, 1962). Millot and Bonifas (1955) have observed neogenetic quartz consequent upon the alteration of felspars.

23


LOCATION OP INDURATED OCCURRENCES IN SOILS

Indurated latérites are usually found at the surface, especially in tropical regions where they give rise to extensive grassy clearings in timberland. In Black África and especially in Guinea these clearings are k n o w n as 'bowal' (Aubreville, 1947). It is, however, clear from the study of profiles that indurated latérites are also to be found in layers of various thicknesses at various depths within the soil, and also in some sediments, sometimes at considerable depth, and often in successive strata.

Prescott and Pendleton (1952) state that the average depth of latérites is approximately 60 c m . in Ceylon, and that in Siam its depth ranges from a few centimetres to 180 c m . Latérites at depths of 180 c m . have been reported in Queensland by Humbert (1948). In Black Africa hardpans are found at depths of between a few centimetres and 6 m . or more (Maignien, 1958). Geologists have reported lateritic levels at even greater depths. In particular, hardpans are to be found at depths of multiples of 10 m . in Miocene-Pliocene formations. At Casamance (Senegal) lateritic concretions are to be found between 27 m . and 29 m . and between 11 m . and 15 m . (Dubois, 1949; Fauck, 1955).

There are m a n y such observations in the literature. In general it can be stated that hardpans occur on average at depths of between a few centimetres and less than 10 m . Between these two limits the depths most frequently encountered are between 1 m . and 2 m . for continuous crusts and between 50 c m . and 2 m . for concretions.

It is generally accepted that surface latérites have been uncovered by the erosion of soft superjacent horizons.

These encrusted or concretionary formations m a y vary in thickness between a few centimetres and multiples of 10 m . Oldham (1893) reports latérites 60 m . thick. The thickest crusts are usually to be found near tectonic faults reflected in the relief. In Black Africa crusts m a y be more than 10 m . thick in such situations. It has, however, been shown by more detailed study (Maignien, 1958) that such indurated occurrences are thinnest in the centre of a plateau and thickest along its edges, and are wedge-shaped in cross-section. In general, old crusts are always thicker than recent crusts.

The transition from the hardpans or concretions to softer subjacent or superjacent formations varies, and m a y be either abrupt or gradual. N y e (1954, 1955) states that the latérite to be found at depth is encountered firstly as discrete nodules in an earthy matrix. These nodules increase d o w n to a certain depth, below which they diminish without ever coalescing. Joachim and K a n -diah (1941) describe profiles in which the nodules give w a y to a cemented piso-litic or nodular mass. Even so the transition can occur within the space of a few centimetres, especially in the case of sheet (nappe) crusts.

In general, transition would appear always to be more abrupt to the superjacent horizons than to the subjacent horizons. The transition to the superjacent horizons approximately follows the form of the relief. The extremely undulating and not infrequently discontinuous pattern observed below corresponds to progressive and often nodular transitions.

The connexions between hardpans or concretions and the superjacent or subjacent levels are not always apparent. In some cases crusts correspond to horizons leached of sesquioxides strictly related to the surface horizons (Marbut, 1932; Pendleton, 1936; Mohr , 1944; M o h r and V a n Baren, 1954; Maignien,

24


1962) . B u t o n occasion there m a y b e n o relationship, or only a slightly apparent genetic relationship b e t w e e n these levels, either because the superjacent horizons are later or altered, or, e v e n m o r e frequently, because the sesquioxides w h i c h c e m e n t s o m e of the horizons of a u t o c h t h o n o u s soils originate outside the profile (d'Hoore, 1954; Maignien, 1958; Mulcahy, 1960; Alexander and Cady, 1962).

The relationship between indurated formations and subjacent levels are also extremely varied. The connexion m a y be unapparent or, conversely, extremely clear. In the latter case the chemical and mineralogical composition of the two formations is approximately the same and the only difference is to be found in the induration. Crusts can occur directly on rocks in situ (Holland, 1903) without any idea of continuity.

Summarizing, all w e m a y say is that no general and exact principles can be established concerning the depth and thickness characteristics of lateritic formations or concerning their relationships with hardpans. These varied aspects can be explained only in terms of differences in mode of formation.

Indurated lateritic formations are typified by the predominance of hydrous alumina (gibbsite and boehmite), and the sesquioxides of iron (goethite and haematite) mixed in variable proportions with more or less crystallized kaolinites, and contaminated to some extent by residual or clastic products, the most important of which is quartz.

The distribution of the principal constituents in fifteen samples of crusts (Schaufelberger, 1953) is the following: kaolinite, 15; gibbsite, 7; boehmite, 2; diaspore, 1; goethite, 12; non-identified hydroxides of iron, 3; phyllite minerals, 5.

T A B L E 1. Summary table of the characteristics of aluminous and ferruginous latérites (Maignien, 1958)

Characteristic

Site

Induration

Colour

Density

Structure

Chemical composition

Aluminous latérites

Old forms

Slight to moderate

Whitish-rose to red

Low

Basically scoriaceous

Strongly hydrated, > 20 % little insoluble material

Ferruginous latérites

Principally deep-seated forms

Moderate to h e a v y and even

very h e a v y

Rust to dark b r o w n

High

Extremely varied : pisolitic, alveo

lar, lamellar, etc.

Slightly hydrated (10 %), plenty

of insoluble material

Mineralogical consti- Principally gibbsite, boehmite, tution goethite, little kaolinite

Quartz absent, or if present clastic and not abundant

Kaolinite and goethite; principally variable a m o u n t of haematite; variable a m o u n t of gibbsite, often absent. Quartz often an important constituent, residual or clastic; Varying amounts of phyllite minerals.

25


' L A T E R I T I C SOILS

CHARACTERISTIC PROFILES (Figs. 1 and 2)

Harrassowitz (1930) believes that 'lateritic' soils have a characteristic profile. This opinion is shared by many authors and, in particular, by Erhart (1935), who insists on the invariable presence of a transitional horizon between the parent rock and the crusted level, the latter flowing from the former.

Mohr (1932) and later Pendleton (1936) reacted against this interpretation and asserted that concretionary horizons and crusts are illuvial formations developing as a result of leaching of the upper soil horizons. This information was taken up by D'Hoore (1954) and later by Maignien (1958) who stressed the importance of the migration of components within and between soils in explaining the formation of hardpans.

If 'lateritic' soils are defined as soils possessing concretionary formations or sesquioxide-rich crusts in at least one of their horizons, the definition is extremely wide. In addition to the red and yellow soils of the humid tropics, such a definition includes concretionary ferruginous tropical soils and ferruginous crusts with different morphological and analytical characters (Maignien, 1962) and sheet (nappe) crust soils. Walther (1889, 1915, 1916) was the first person to cast doubt on the need to include the presence of a concretionary or encrusted horizon in the definition of such soils, when he classified all red tropical soils as latérites. Some of the horizons of these soils do in fact possess chemical and mineralogical properties comparable, except for induration, to those of concretionary or encrusted horizons. In particular, the Si02/Al203 ratio is narrower than 2, the saturation capacity of the absorbing complex is low and mineral reserves are non-existent. This interpretation has been and remains widely accepted.

Once the tropical origin of latérites had been established, Russian investigators cited by Gerasimov (1962) extended the term to all tropical soils and even to the greatly altered red or yellow soils of the semi-humid subtropics (lateritic krasnozems and sierozems).

It therefore became increasingly difficult to restrict the definition, but in many pedological works of approximately the last twenty years less importance has tended to be attached to concretionary horizons or crusts in the definition of 'lateritic' soils. A new terminology has been developed to avoid any confusion: allitic and ferrallitic soils (Robinson, 1949), latosols (Kellogg, 1949), kao-lisols (Sys, 1960), and oxisols ( U S D A Seventh Approximation, 1960). Unfortunately these terms are not always entirely equivalent, and since the definitions are in some measure restrictive, the limiting cases sometimes extend to soils that are fairly dissimilar, although the general concept is fairly well recognized.

M a n y 'latérite' profiles have been described in different regions, but in many cases no mention is made of the upper part of the profile, and it is therefore difficult to study the relations between the various horizons, especially as very important relations are operative between adjacent profiles.

Since it is impossible to examine all the observed cases, the author has decided to restrict himself in the interests of clarity of exposition to selected profiles of African soils which would appear to exhibit specific characters (see Fig. 3).

26


-— —

_ _ —

—

: 1 J" T V V

: T : T T V

5 I :- -: 9 Y

l o l o

1 loi

Mottled horizon

r-^VM Fermentation layer (A )

Active granular humiferous horizon

Clay (with absorbed iron oxides)

Leached silty horizon

Accumulation of dehydrated (red) ferric iron compounds

O O O O O Free aluminium compounds

Pi 0

~j! U ~ | Slightly decomposed parent rock \~ -L -Lj (e.g., granitic debri:

4- + + +1 !• + + + + + + "H Unweathered silicates

N.B. The. abundance of the various elements is indicated by the wider or narrower spacing of the lines and the density of the symbols.

Red ferrallitic Soil

Ferrallitic cuirasse

Dr< zone

Hydromorphi' zone '(with formation of kaolinite)

-Decom- • position (alkali hydrolysis)

- Initial stage

leaching of F e 2 Q . 3

*—Leaching and rise of FeO-»

- Dense forest - rrThin forest-W U- Hardening -* W Herbaceous—»

Mottled horizon

savannah

1. Yellow ferrallitic soil 2 . -Ochre ferrallitic soil 3. Red ferrallitic soil

4 . Leached red ferrallitic soil 5. Ferrallitic cuirasse

\ (1) Leaching of S¡0 2

J (2) Leaching of F e 2 0 3 and A l 2 0 3

\ (3) Upward movement of FeO

Fig. 1. Ferrallitic soils. After Duchaufour (I960, p . 305).

Fig. 2. Development of ferrallitic soils. After Mohr and van Baren, complemented and modified by Duchaufour (1960, p . 308).

27


1. Lateritic soil on granite (after G . Aubert, 1954)

D a k p a d o u forest in the west of the L o w e r Ivory Coast, 50 k m . north of Sassandra; slightly degraded omhrophil forest.

Sub-equatorial climate without a distinct dry season, annual precipitation 1,700 m m . , m e a n temperature 27° C .

Gently rolling topography, section o n top of low plateau. Parent rock: syntectonic granite with a high content of ferromagnesian minerals.

0 cm. Forest litter-bed of decomposing leaves, twigs a n d branches apparently lying o n the soil.

0-110 cm. G r e y - b r o w n horizon, slightly h u m i c d o w n to 35 c m . , thereafter beige; fine sand and gravel; high content of very hard, round, dark ferruginous concretions, especially in the first 40-50 c m . ; below approximately 80 c m . the horizon becomes m o r e compact , the concretions less hard a n d the colour brick-red.

110-175 cm. Fairly abrupt transition to a hardened horizon which can be broken b y h a n d — fairly dull-brown to red bands joining u p a n d outlining cavities containing an ochreous to beige earth.

175-650 cm. A non-hardened horizon, m o r e compact at the base; brick-red with well-delineated beige, ochre or grey spots; still s o m e hard nodules, especially in the upper part of the horizon; riddled b y small channels. T h e quartz grains are less pulverized in the upper horizons.

650-840 cm. Transition to mottled clay, with poorly delineated mottles of a clearer beige or grey colour; quartz grains m o r e abundant ; s o m e whitened, friable elements at the base have retained the felspathic habit.

840-1100 cm. A n ochreous-brown horizon with a high content of quartz a n d white, p o w dery elements, which contains nodules of less decomposed rocks exhibiting altered pyroxines. Gneissose grass towards 9 m . ; white, friable, felspathic materials, quartz grains and greatly altered coloured elements.

Towards 1,200 cm. Gneissose parent rock with a high content of ferromagnesian minerals.

2 . Non-encrusted lateritic soil on amphibolite (after R . Maignien, 1958)

Bero massif, the road between Nzérékoré a n d Beyla (forest region of Guinea). Clearance-felled mesophyllous forest, dense herbaceous aftergrowth. H u m i d tropical climate of the Guinea forest region type; short but distinct dry season

(2 m o n t h s ) , annual precipitation approximately 1,850 m m . , m e a n temperature a p proximately 25° C .

Topography: hill relief, section o n top. Parent rock: amphibolite.

0 cm. Burnt herbaceous litter. 0-20 cm. Dull b r o w n horizon; argillaceous texture containing s o m e hardened ferruginous

concretions of the s a m e colour; fine debris of altered a n d ferruginous amphibolite. 20-60 cm. B r o w n horizon; containing fairly indurated ochreous-brown ferruginous con

cretions 1-2 c m . in diameter; fragments of altered a n d ferruginous amphibolite; argillaceous texture.

60-170 cm. B r o w n - r e d horizon; highly argillaceous texture abounding in surface-altered amphibolite debris with a nucleus of fresh rock.

170-290 cm. B r o w n - r e d horizon containing friable purplish-blue products; argillaceous texture, a little altered quartz (a quartz bar at a depth of 2 m . ) .

290-320 cm. 'Gingerbread' faciès with purplish-blue mottles. 320 cm. and below. Highly diaclastic fresh rock altered along the fissures and zonally.

3. Lateritic crusted soil on dolerite (after R . Maignien, 1958)

T h e village of Kolente, right b a n k (Central Guinea) .

28


Completely degraded natural vegetation, under cultivation (Digitaria exilia). Tropical climate of the S u d a n - G u i n e a type; pronounced dry season (5 m o n t h s ) , annual

precipitation approximately 2,000 m m . , m e a n temperature approximately 24° C . Highly dissected hill relief—cutting through a ridge along the road. Parent rock: dolente.

0-25 cm. G r e y - b r o w n horizon; gravelly with b r o w n hardened concretions a n d crust debris. 25-250 cm. Highly indurated crusted horizon, with a red-brown ground colour; well-

developed pisolitic structure, very compact ; s o m e oblique diaclases bounding large rocks with a b r o w n ferruginous surface film; induration decreases at the bot tom of the horizon mainly because of the cement surrounding the concretions; at the b o t t o m of the horizon the colour is a brighter ochreous red.

250-350 cm. Developing crust horizon; m a n y well-rounded brown-red concretions that are already quite hardened in a red clay matrix; in places a darker, fairly indurated ochreous-red skeleton is forming.

350-420 cm. Mottled horizon containing discrete concretions responsible for the induration, large diffuse white banks .

420-600 cm. R e d horizon; argillaceous; fairly well-developed prismatic structure. 600-650, 700 cm. Flaky alteration horizon of ochreous-yellow, porous, paind'épices (ginger

bread) dolerite faciès of low density. 700 cm. and below. Altered and fresh dolerite with manganiferous deposits between

slightly altered flakes.

4 . Crusted lateritic soil on granite (after R . Maignien, 1958)

Five kilometres along the road from M a m o u to Kindia, meridional fringe of the Fouta Djallon massif (Guinea).

Completely degraded m o u n t a i n forest—dense scrub aftergrowth. Tropical climate of the Sudan-Guinea type; pronounced dry season (5 m o n t h s ) , annual

precipitation 2,065 m m . , m e a n temperature 23.2° C . Dissected relief—section through a hill. Parent rock: syntectonic calc-alkali granite.

0 cm. Greatly reduced organic litter. 0-80 cm. Grey, very gravelly h u m i c horizon containing separate, hard gravel particles

a n d quartz debris in a n earthy matrix; sandy-argillaceous texture; slightly nodular structure; m a n y herbaceous roots.

80-170 cm. Highly indurated crust; matrix of brick-red ground colour with harder quartz-rich purplish-blue mottles; s o m e mottles whitish to yellow-ochre a n d m o r e diffuse; structure scoriaceous to pisolitic, dense at the surface a n d becoming looser with depth, where horizontally extending nodules form a poorly denned skeleton with a flattened alveolar structure.

170-220 cm. Brick-red horizon containing m a n y red-purplish blue nodules; m o r e n u m e rous friable, white to yellow diffuse mottles.

220-270 cm. Transitional horizon, containing fewer a n d fewer, but still quite highly indurated, nodules.

270-300 cm. Similar to the preceding horizon, except that the purplish-blue a n d white nodules are no longer hardened, although still quite separate.

300-340 cm. R e d horizon with large, fairly poorly delineated, grey, whitish a n d ochreous mottles; argillaceous texture; m a n y grains of altered quartz.

340-550 cm. and below. Argillaceous, highly mottled debris of gneiss alteration products in which traces of the original minerals can be seen.

5. Crusted lateritic soil on sericitic schist (after R . Maignien, 1958)

L a b e (Fouta Djallon, Guinea), quarry o n the outskirts of the t o w n o n the road to Pita.

29

Morphological and analytical characteristics of latentes

Completely degraded natural vegetation, wasteland pasture. Tropical climate of the Guinea (Fouta) type (mountain climate) ; great seasonal contrast

(5 m o n t h dry season), annual precipitation 1,715 m m . in 115 days, m e a n annual t e m perature approximately 21° C .

Topography—gently rolling plateau, profile at the edge of the plateau. Parent rock: sericitic schist.

0-15 cm. Grey horizon; slightly h u m i c ; high content of b r o w n , highly indurated gravel particles; s o m e blocks of rounded crust; sandy-argillaceous texture; structure tending to nodular; porous; m a n y herbaceous roots.

15-40 cm. Yellowish-grey horizon still slightly h u m i c ; apparently leached; high gravel content, with large ochreous-beige concretions whose cement is in course of dissolution; gravel particles red to violet in cross-section and very hard; the first two horizons have apparently been altered in situ.

40-130 cm. Ochreous to rust-ochreous, highly concreted in situ; the concretions are covered b y a yellow ochre ferruginous film indicative of hydrometamorphic processes of s o m e intensity; the concretions, which occur as separates, exhibit various colours in fracture (red, violet, rose); whitish-ochre to rust-ochre mottled bands .

130-200 cm. Crusted horizon; alveolar structure with isolated, m o r e ferruginous, redder and harder nodules; these nodules are again encountered in the subjacent red clay; the ground colour of the crust varies from reddish-ochre to rose-ochre ; the indurated skeleton is usually brighter along the tubular cavities.

200-500-550 cm. Developing crust horizon, far less indurated than the preceding horizon; fairly bright in colour in the upper part and becoming redder with depth; s o m e indurated concretionary nodules stand out in the ochreous-beige argillaceous matrix ; earthy material penetrates from the surface to this horizon along the tubular cavities and fissures.

500-550-650 cm. R e d horizon; argillaceous; yellowish bands and hard, small, yellowish-b r o w n discrete concretions, which are essentially ferruginous and manganiferous.

650-750-800 cm. Slightly redder mottled horizon with m o r e or less diffuse ochreous-yellow patches; argillaceous texture; pseudo-arenaceous structure with s o m e banded tubular cavities; a few slightly indurated violet mottles indicating traces of the parent rock.

750-800 to 900 cm. and below. R e d clay containing an increasingly large a m o u n t of highly altered sericitic schist debris.

6. Red earth on sandy-argillaceous sandstone (after R . Maignien, 1962)

Sefa, C a s a m a n c e (Senegal), D i a r o u m é road, 100 m . 'Unité de culture no . 1' road. Climate of the Sudan-Guinea (Casamance) type : annual precipitation 1,380 m m . in ap

proximately 75 days, pronounced dry season, m e a n annual temperature approximately 27.5" C .

Flat low plateau relief. S a v a n n a h forest (Pterocarpus, Kaya, Prosopis and Daniella).

Parent rock : mottled sandy-argillaceous sandstone.

0-12 cm. Brown-grey (5 Y R 4/2), slightly h u m i c , plentiful carboniferous surface debris and poorly decomposed leaf litter—abundant root clusters ; fine, fairly well-developed nodular structure; slight cohesion; porous.

12-28 cm. B r o w n (5 Y R 3.5/4); sandy; m e d i u m , fairly well-developed nuciform structure ; m e d i u m cohesion ; porous.

28-55 cm. R e d (2.5 Y R 4/6); hardened; sandy-argillaceous; coarse; slightly angular nuciform structure; pseudo-arenaceous; strong to m e d i u m cohesion, porous.

90-120 cm. R e d (10 Y R 4/6); sandy-argillaceous; indistinct segregation in the form of diffuse, unhardened concretions ; slightly developed nuciform to polyhedral structure.

> 120 cm. Progressive transition to the argillaceous sandstone of the Continental Terminal; fairly indistinct whitish-grey mottles and bands .

30


7. Ferruginous tropical leached soil (after Dommergues, Poulain and Moureaux, 1962)

Sehiou-Sefa region, 'Ferme Jachère', grass experimental station along the edge of a forest of Combretaceae and Cordyla.

Climate: precipitation 1,350 m m . Topography: plateau. Continental sands and sandstones.

0-6 cm. Brown-grey (Munsell 5 Y R 5/3); humic; sandy texture; finely nuciform structure; slight compaction; good internal drainage; tubular pore spaces; m a n y roots and insect passages.

6-13 cm. Bright grey-brown (5 Y R 6/2); sandy, slightly argillaceous texture; fine nuciform to polyhedral structure; invariably some organic content; medium compaction, greater than in the subjacent horizon; good porosity, plentiful roots.

13-31 cm. Yellowish-beige (5 Y R 6/4); still very slightly humic; sandy, slightly argillaceous texture ; nuciform structure ; macroporosity due to roots and insects ; moderate to strong compaction.

31-79 cm. Beige to reddish-yellow, slightly darker tint (5 Y R 8/4); sandy-argillaceous to argillaceous texture, accumulation of clay(B2); polyhedral structure; fairly high compaction; weak to medium porosity with fine tubular pore spaces.

79-117 cm. Beige yellow (5 Y R 7/6); sandy-argillaceous texture; nuciform to polyhedral structure; medium compaction; commencement of clearly delineated segregation of red ferruginous mottles.

117-150 cm. Ground colour beige (5 Y R 8/4) with red and ochre mottling; sandy-argillaceous texture; structure coarse polyhedral from nuciform; moderate compaction; aggregate porosity slight to moderate; m a n y mottles and concretions of moderate to bright hardness, red to dark red and less frequently dark violet, with the role of ochre mottles with slightly hardened concretions becoming progressively greater towards the base.

150 cm. and below. Mottling continued, texture with beige ground colour, appearance of bright grey spots without distinct limits, numerous predominantly red or rust concretions, some of which can be crumbled in the fingers leaving a harder central point; strength uneven owing to the presence of aggregates; aggregate porosity average; cohesion moderate to strong.

8. Leached tropical ferruginous soil (after R . Fauck, 1962)

Dongas region (Dahomey) fairly dense afforested savannah. Climate: precipitation 1,350 m m . ; long dry season (5-6 months). Topography: gentle slope below a hardpan outcrop. Overlaying gneiss containing altered biolite.

0-5 cm. Grey arenaceous; particulate, friable structure; slight compaction; numerous rootlets, fairly high organic content.

5-15 cm. Gradual transition horizon. 15-30 cm. Beige; sandy, slightly argillaceous; lamellar structure; a few bright red mottles,

small black concretions, and harder red concretions; moderate compaction. 30-50 cm. Beige; sandy-argillaceous to argillaceous-sandy; extremely high content of

concretions of average hardness, 0.5 c m . thick. Some larger concretions have a very hard centre. High M n content.

50-120 cm. Extensive mottling, mainly red with some ochreous-beige ; fairly large black concretion; some large quartz, some concretions with a very hard centre.

120-200 cm. Ochre and grey play an increasing part in the mottling ; clayey, with a high muscovite content; black mottles; the structure of the rock cannot be recognized. Pegmatite layer at 180 c m .

31


120-260 cm. Gneiss with a high content of decomposing green muscovite. Ochre mottles present.

260-320 cm. A s above, but less grey.

9. Sheet (nappe) crust soil on gneiss (after R . Maignien, 1958)

Dubreka (Lower Guinea), locality known as the 4 F A O Concession*. H u m i d climate of Guinea type; annual precipitation 4,000 m m . , pronounced dry season

lasting 5 months, m e a n temperature 25.5° C. Edge of low plateau, 4-5 m . above mangrove s w a m p , and beyond m a x i m u m high water

mark. Parent rock: gneiss.

0-45 cm. Grey-black; fairly high humus content; gravelly texture with numerous highly indurated brown ferruginous concretions; structure tending to the nodular; fairly porous; rootlet clusters plentiful.

45-135 cm. Well-indurated alveolar crust; the generally bright colour, which is almost white on the outside, is due to the presence of leached earth deposits ; in fracture the skeleton is rose-ochre and appears fairly highly ferruginous; undecomposed quartz minerals plentiful. The alveolar cavities, which are well developed, contain friable, practically white earth products.

135-235 cm. Alteration horizon; slightly argillaceous bright rock debris; alteration products practically white; numerous muscovite flakes.

235 cm. Leucocratic gneiss; high quartz content.

10. Sheet (nappe) crust soil on alluvium (after R . Maignien, 1954)

Ballay plain (Guinea), right b a n k of the Baling. Climate of Guinea type; precipitation approximately 2,000 m m . , dry season lasting

5 m o n t h s , m e a n temperature 23° C . T o p o g r a p h y : flood-plain. Herbaceous complex, consisting mainly of large grasses (Andropogon sp.). Parent rock: recent alluvium o n slightly fespathic sandstone.

0-20 cm. G r e y - b r o w n (10 Y R 5/2), high h u m u s content; l o a m y texture; moderately developed nodular structure; slight cohesion, plentiful grass roots; porous.

20-48 cm. B r o w n (10 Y R 5/4), still h u m i c ; siltic texture; medium-sized, fairly well-developed nuciform structure; slight cohesion.

48-85 cm. Brighter b r o w n (7.5 Y R 8/4); silty texture; slightly structured and compacted; w e a k cohesion; low porosity.

85-97 cm. B r o w n (7.5 Y R 8/4); m a n y extremely hard, non-agglomerated concretions, 1 c m . - 2 c m . in diameter, surrounding large quartz grains.

97-150 cm. Identical horizon, except that the concretions, which are still plentiful a n d well delineated, are less indurated, m a n y rust mottled a n d bands leading to a slightly hardened skeleton of carapace type.

150-200 cm. V e r y hard, essentially ferruginous, compact crust containing m a n y angular quartz grains; the hardest parts are brown-red (2.5 Y R 5/6) to brown-black (2.5 Y R 3/4) and form a n alveolar skeleton surrounding soft earthy materials, ranging in colour from yellowish (10 Y R 8/3) to yellow ochre (2.5 Y R 6/8).

200-310 cm. F a r less indurated crust, consisting in the m a i n of hardened rust ochre to yellow ochre b a n d s ; slightly laminated alveolar faciès; m a n y quartz grains in the skeleton.

310-450 cm. Mottled horizon of slightly indurated rust or red bands with diffuse limits in a whitish-grey clay matrix; low quartz content.

450-650 cm. and below. Slightly consolidated arenaceous debris; diffuse rust-coloured ferruginous impregnation. W h e n the plain is flooded the water table of the Bafing lies at 600 c m .

32


Non-leached Iropical ferruginous soil

30

>"Wh

Leached tropical ferruginous soil with cuirasse

80

70

Lçached tropical ferruginous soil with ferruginous concretions

\S¿ 3d

200

Slightly ferrallitic red soil

E3

Particulate humiferous horizon

Granular active

humiferous horizon

Clay accumulation

Leached horizon

Accumulation of iron sesqu ¡oxides

Ferruginous cuirasse

r7l]l'. j Localized precipitation iM—lUU of iron sesquioxides

N.B. The abundance of the various elements is indicated by the wider or narrower spacing of the lines and the density of the symbols.

FIG. 3. Typical sequence of the pedogenetic horizons of four tropical soils.

All these soils can be classified as latentes, either because at least one of their horizons is an encrusted horizon, or a concretionary horizon in course of induration, or because at least one of their horizons, although remaining soft, has a high content of the sesquioxides of iron and/or aluminium (Si02/Al203 < 2), or because they exhibit both of these characters simultaneously.

Only profiles 1-6 are accepted as lateritic (or ferrallitic) in the French classification (Aubert, 1956, 1962, 1963). Profiles 7 and 8 are ferruginous tropical soils with ferruginous concretions; profile 9 is a ferrallitic soil with hydrome-tamorphic processes at depth; profile 10 is a soil subject to temporary hydro-morphic processes with a sheet crust on alluvium.

In the Seventh Approximation ( U S D A , 1960), profiles 1-5 would probably be oxisols; the position of profile 6 would be open to discussion owing to its arenaceous texture; profile 7 would be an ultisol; 8 and 9 would be alfisols, and 10 an inceptisol.

Belgian pedologists would classify profiles 1-9 as kaolisols, distinguishing 1, 3, 4 , 5, 6 and 7 as ferrallsols, and 2 , 8 and 9 as ferrisols; it is difficult to accord a place in this classification to profile 10.

Pendleton (1943), Alexander et al. (1956) and Sivarajasingham (1962) would classify profiles 3, 4 , 5, 6, 7 , 8, 9 and 10 as latérites.

33


Russian pedologists would classify all the soils here described as lateritic

soils. Opinions are therefore fairly divided, even in relation to the few cases repre

sented by the profiles here described. It would be possible to describe many others, especially the very deep yellow soils found beneath equatorial forests, lit de pierres or 'stone line' soils, and soils with deep humic horizons.

They can, however, be grouped into several categories in terms of a limited

number of common characters.

1. Soils apparently derived directly from thorough and profound alteration of

original material, with or without an indurated horizon, in situ. This group corresponds approximately to ferrallitic soils, oxisols and kaolisols.

The general characters of these soils are: They are deep soils, some of which reach a thickness of 15 m . to 20 m . There is usually little differentiation of the horizons, and especially no deve

lopment of a crust or concretions; the transitions between horizons are gradual. The surface horizon is rarely entirely in situ (surface erosion, landslip). The

surface horizon is often gravelly or pebbly and the clay content is low. Vertical, lateral or oblique movements (settling, transport of elements in solu

tion or in pseudo-solution from or towards other profiles) occur in the subjacent horizons. There is no true textural B horizon.

Since the structure is usually poorly developed (pseudo-arenaceous or finely polyhedral, sometimes slightly powdery), permeability is good and there is some resistance to erosion.

At depth it is often difficult to detect relationships with the surface horizons. Such soils are often very old, and have developed in the course of several climatic cycles.

They are often highly coloured (red or yellow), with high value and chroma.

There is little difference in colour between the dry and wet states. They are generally profoundly and intensely altered. Few minerals, apart from

quartz and some rock debris, escape alteration. Argillization processes are intense.

These general morphological characters are subject to certain variations affecting the following four levels : the surface horizons, the horizons of sesqui-oxide accumulation, the argillaceous horizons, and the alteration horizons. Surface horizons. Transition from the organic litter to the mineral soil is always

abrup t. The surface horizon contains little organic matter (2 per cent on average) and what there is is usually well decomposed. Humic components, which are present in small quantities, are only slightly coloured, and do not penetrate deep into the soil (20 cm) . Organic content may , however, be considerably increased (to as much as 20 per cent) on base-enriched rocks (basalts, for example), and penetration into the profile will sometimes reach a depth of 1 m . Organic content also increases with altitude and with reduction in drainage. The raw humus which m a y form on greatly altered and acid materials is sometimes conducive to true podzolization.

The most frequently encountered colours in these horizons are red, brown-red, yellow and rust. The brightest soils are also those which have been most leached of bases and which are most acid. These horizons are not generally very argillaceous. Concretions, quartz debris and fragments of crusts are often encountered, mixed in various proportions with earthy materials. These large elements

34


are often found scattered over the surface or forming a stone line of some thickness at slight depth (1 m . ) .

Sesquioxide accumulation horizons. These horizons vary in thickness and in morphology, but always have a high content of the oxides and hydroxides of iron, aluminium, manganese or titanium. Clay content is sometimes quite significant. Traces of hydromorphism are frequently encountered. These horizons have often become wholly or partly indurated, encrusted or concretionary in

situ. They possess the characters described in the last chapter.

Argillaceous horizons. These horizons m a y sometimes be absent, especially on well-drained basic rocks. W h e n present they are often of some thickness. The structure is generally finely polyhedral. The aggregates m a y be covered by shining films (Sys, 1959), which will not be orientated (Gastuche and Fripiat, 1960).

W h e n the soil is permanently humid, signs of segregation appear. The horizon, which is mottled in appearance (mottled clay), can vary in thickness from a few centimetres to several metres, but is always thicker on quartz-rich rocks than on basic rocks. This segregation arises from inadequate drainage related to environmental factors (climate, forest cover, rock structure and topography) or to pedológica! evolution (argillaceous neosynthesis and settling).

The positions of the sesquioxide and argillaceous horizons in the profile are sometimes reversed.

Alteration horizons. These horizons, which are usually thick, are often altered by mass movements. They contain the minerals of the parent rock altered to varying degrees, and one sometimes encounters breccias, debris with a varying proportion of clay, and less frequently grits at the base (Bonifas, 1959). Pain

(Tépices faciès are often to be found in these horizons on basic rock (Lacroix, 1913).

In conclusion, the influence of the parent rock on the soil is often greatly reduced. However, in the case of soils that are little altered, and that are protected by an encrusted horizon, this influence m a y be significant. It is s u m m a rized in Table 2 (Maignien, 1958).

2. Soils with a concretionary horizon or ferruginous crust produced by less thorough

and less deep alteration. These soils correspond to ferruginous tropical soils, to fersiallitic soils, to ultisols and even to alfisols. Their moderate thickness (rarely more than 250 cm. ) , and especially their clearly delineated horizons and distinct interfaces, are in sharp contrast to the preceding group. They vary considerably in colour, but are in the main rather bright in the yellow to red range (7.5 to 10 Y R in Munsell's code). There is a considerable difference (2 to 3 units) between the dry and wet states in the value and chroma of these colours. Finally, these soils often have a textural B horizon which indicates the development of a concretionary horizon or of crusts which are essentially ferruginous, sometimes manganiferous, but never aluminous. These crusts are rarely more than 1 m . thick. The alteration horizons are never thick (50 to 100 cm.) on quartz-rich rocks (granites and gneiss). These horizons, which are slightly argillaceous debris, are whitish in colour, marbled with rust bands. Partially decomposed primary minerals m a y escape alteration (kaolinitization), and m a y be found throughout all profiles. Argillaceous neosyntheses are less intense than in the preceding soils. There is a pronounced tendency for the clay to be leached in the surface horizons, and for an illuvial horizon to form at depth. The structures

35

.Morphological and analytical characteristics of latérites

T A B L E 2 . Characters of encrusted profiles

Granites-gneiss

Surface horizon : greyish, slightly h u m i c , often leached of b a s e s — m a n y greatly indurated ferruginous particles of fine gravel, quartz debris.

Indurated horizon: hardened to s o m e degree, often with a crust faciès in B u c h a n a n ' s definition, rarely attaining extreme induration—structure m o s t frequently alveolar, often horizontally flattened or scoriaceous in the oldest crusts (loose crusts).

D o m i n a n t colours bright: red, rust, sometimes with violet mottles usually forming the m o s t indurated nodules—• red to rust-ochre moderately indurated bands forming a f ramework imparting rigidity to the whole. T h e cavities are filled with bright yellow to rust earthy material.

Rarely m o r e than 2 m . thick.

Extremely gradual transitions to adjacent horizons.

Subjacent horizons m o r e argillaceous, general morphology quite similar to the preceding, but not indurated— mottled colours forming a pattern of poorly separated mottles a n d bands .

Polyhedral to finely prismatic structure, cleavage planes subsequent to settlem e n t of the entire mass—considerable thickness (1-5 m . ) .

Alteration horizon, debris containing varying a m o u n t s of clay, mottled— traces of the original mineral altered to varying degrees apparent at the base—heavily impregnated with ferruginous solutions.

Somet imes of considerable thickness (up to 5-6 m . ) .

Gradual transition to the fresh rock.

36

o n different rocks

Dolerites-amphibolites

Surface horizon: brown-black, m o r e h u m i c , m o r e argillaceous—gravel particles often very abundant a n d mainly well rounded, little or no quartz.

Indurated horizon usually greatly hardened—pisolitic structure hardening in the upper part of the horizon a n d giving w a y d o w n w a r d s to scoriaceous structure, followed b y nodular structure near the b o t t o m of the horizon— or very c o m p a c t crusts.

Principal colours darker: red a n d b r o w n -red. T h e colours are considerably whitened to greyish or rose in old crusts with a higher alumina content. M o r e mottled at the base: dark red to rose or b r o w n to brown-black mottles a n d bands .

Considerable thickness, m a y exceed 10 m .

More abrupt transitions between horizons, especially in the upper part of the profile.

Subjacent horizons argillaceous, dark red without apparent segregation. Darker concretions m a y sometimes stand out, but are less indurated, and disappear gradually as depth increases.

Finely porous polyhedral structure—• material riddled with small tubular cavities and pores—polished, shining, ferruginous cleavage planes—thickness sometimes considerable, often more than 5 m .

Alteration horizon—pain d'épices faciès —rust ochre, apparent density of pore space very slight—gibbsite skeleton retaining the habit of disappeared felspars—fairly heavily impregnated with ferruginous solutions.

Concentric flakes of slight thickness around fresh rock.

Abrupt transition within the space of a few millimetres.


are fairly coarse, of nuciform to polyhedral type, and poorly defined. Internal' drainage is generally fairly poor, and these soils are highly susceptible to erosion by water. They develop mainly on acid rocks. O n basic rocks they are intermediate stages in development towards brown eutrophic soils, or vertisols, with which they sometimes merge (Maignien, 1963).

Only the presence of an essentially ferruginous concretionary or encrusted horizon justifies the classification of these soils as latérite, from which they differ in many pedogenetic characters.

3. Incrusted colluvial or alluvial soils. These soils differ from true latérites. They are little evolved soils, which exhibit varying degrees of hydromorphism, and which have some of their horizons cemented by the sesquioxides of iron. These indurated components are not derived from decomposition of the soil in situ, but have been brought in by the waters which impregnate these formations. Their classification as latérites is based purely on morphology. Genetically they are far removed from latérites, and it should be noted that ferruginous impregnation of this type can occur in any type of soil.

ANALYTIC CHARACTERS

Although many analytic results are cited in pedological literature, very few can be found which cover the whole profile from the surface to the parent m a terial. Most studies deal with the relations between fresh rocks and indurated horizons, or compare the various encrusted horizons, or give information concerning the surface horizons down to a depth not usually exceeding 2 m . The names of Harrassowitz (1930), Harrison (1933), Hardy (1931), Lacroix (1913), Bonifas (1959), Segalen (1957), Leneuf (1959), Alexander et al. (1956; Alexander and Cady, 1962), and Précot et al. (1962) are associated with this research.

In general, most of these studies do not take into consideration variations along the horizons of a single profile. However, recent research has tended to indicate the existence of extremely strict relations between soils extending along slopes. This introduces the concept of the 'catena', invoked by the geomor-phological evolution of the territory. Adjacent soils on the same relief forms are interdependent. This concept introduces a geographical factor which is essential to an understanding of the phenomena of 'latérites'. These soils must be studied in their setting.

Developmental history must also be taken into account, since most latérites are very old, and sometimes fossil soils, which reflect environmental conditions which have now disappeared. Thus, the crusts of the oldest surfaces in Africa are pTobably of Tertiary age, and, even if still evolving, have certainly passed through several climatic fluctuations.

These few remarks show the difficulties involved in collating the data in the literature, in which results refer to objects which often differ greatly in terms of geography and history.

However, certain characters remain constant: (a) alterations are extremely extensive, and involve loss of bases and silica; (b) there is extreme individualization followed by accumulation of the sesquioxides of iron and/or aluminium; (c) kaolinitic neosynthesis is a constant feature that varies in intensity; (d) the absorbing complexes are extremely unsaturated and there is a pronounced tendency to acidification of the medium.

37


These characters are found in ferrallitic soils, oxisols and kaolisols, bat not in ferruginous tropical soils, encrusted fersiallitic soils and soils with sheet (nappe) crusts exhibiting s o m e degree of hydromorphism (Maignien, 1954, 1962 ; Botelho da Costa, 1949).

T A B L E 3. Alteration of nepheline-syenites, Iles de Loos (after Millot and Boiiifas, 1955)

Syenite in Bauxitic Fresh syenite course of 'Pumice* faciès Rose bauxite ferruginous

alteration crust

Si02

A1203

Fe203

FeO CaO MgO Na 2 0 K 2 0 Ti02

M n 0 2

H 2 0

T O T A L

Density

% 57.3 17.5 4.9 1.5 2.6 1.1 6.0 7.0 0.7 0.4 1.3

100.3

2.58

% 58.02 21.7 2.9 0.6 0.4 0.4 4.0 8.1 0.6 0.1 3.1

99.9

2.19

% 3.3

56.0 7.2 0.8 0.2

traces — — 1.7 0.1

29.7

99.0

1.54

% 0.4

58.5 6.4 0.5 0.4

traces — —. 2.1 0.1

30.9

99.3

1.74

% 5.4

44.3 23.1 0.2 0.2

traces — — 1.4 0.1

24.5

99.2

T A B L E 4 . Alteration of dunites, Kaloum Peninsula (Guinea) (after Bonifas, 1959)

Fresh dunite Altered dunite Pain d'épiées Transition Hardpan Crust facies zone

Si02

A 1 2 0 3

F e 2 0 3

FeO CaO MgO Cr203

Ti02

H 2 0

T O T A L

Density

Fe 2 0 3 in hydroxide state

% 33.9

1.60 8.55 7.40 0.50

36.70 0.09 0.08

10.60

99.42

2.80

3.00

% 34.60

2.00 16.80 4.80

traces 33.10

traces 0.08 8.75

100.13

—

7.10

1.60 1.60

84.50 —

traces 0.36 0.15 0.16

11.20

99.57

1.58

82.50

% 1.30 8.20

76.80 —

traces traces

0.23 0.12

12.85

99.50

—

—.

% 0.65 8.00

80.00 —

traces traces

0.80 0.40

10.30

100.15

—

—

% 2.70 9.60

74.00 —

traces 0.72 0.45 0.68

12.40

100.55

—

74.00

38


T A B U : 5. Alteration of calc-alkali granite to muscovite near Divo (Ivory Coast) (after Leneuf, 1959)

Total elements extracted by triacid (elements dried at 105c

Quartz + insoluble S i0 2

silicates A 1 2 0 3

F e a 0 3

T^a P 2 0 6

CaO M g O K a O Na a O H a O

T O T A L

Unaffected rock

%

93.42

2.58 0.87 1.00 0.07 0.11 0.05 0.07 0.50 0.50 0.54

99.71

Compact Extremely whitened friable internal yellowish

zone cortex

% %

90.79 88.08

4.14 5.10 0.77 2.43 0.80 0.80 0.08 0.07 0.09 0.05 0.34 0.34 0.08 0.01 0.32 0.28 0.26 0.24 0.85 0.96

98.52 98.36

' C.) whole rock

Friable whitened

block

%

76.07

8.79 6.12 1.10 0.07 0.05 0.36 0.04 0.35 0.20 3.67

96.82

Near-surface

whitened block

%

58.88

17.06 15.63

2.10 0.35 — — — — — 5.07

99.09

Fraction < 2^

Brown-red debris

%

0.51

43.48 33.55

6.25 0.62 0.13 — — —. —

14.21

98.75

T h e e x a m p l e s presented in Tables 3 , 4 a n d 5 are fairly characteristic. T h e y cover the different forms w h i c h c a n b e a s s u m e d b y the alteration of rocks in a lateritic e n v i r o n m e n t . T h e r e is great segregation of a l u m i n a in the first a n d of iron in the second, a n d formation of kaolinitic clay in the third.

C o m p a r i s o n of all the results f o u n d in the literature demonstrates t h a t b e t w e e n the fresh rock a n d the surface there is a n hydrat ion horizon followed b y a n oxidation horizon, that silica disappears to s o m e degree, that there is invariably a very significant increase in sesquioxide content, a n d that bases are almost entirely eliminated.

Silica

Loss of silica, which is more or less complete, is dependent on the drainage of the locality. Combined forms are more strongly eliminated than free forms. Losses affect firstly the silica of silicates, and then, to a lesser extent, the silica of quartz. There are in fact two alteration facies: (a) on well-drained rocks with a high content of bases, losses between the rocks and the alteration products are rapid and distinct ; (b) on rocks with a higher silica content (especially granites) losses cannot be traced throughout the profile.

W h e n drainage is inadequate, combined silica content in the form of argillaceous neosynthesis is found to be increased at the level of the parent material on acid and on basic rocks.

Bonifas (1959) and Alexander and Cady (1962) state that, in essence, kao-linite originates directly from micaceous and chloritic minerals. However, the reduced quantities of these minerals are not sufficient explanation for the quantities of clay encountered.

39


Silica eliminated by alteration is sometimes again encountered at the base of the profile in alteration zones (Blondel, 1952 ; Delaire and Reynaud, 1955). 'These formations seem to originate from precipitation of silica as a gel which evolves and crystallizes as it ages. The crystals thus formed seem to be enriched at the expense of silica contained in the solution, as is indicated by the presence of quartz zoned and entrapped along the edges of "chalcedonite" ' (Bonifas, 1959).

Sesquioxides

W e are concerned mainly with the sesquioxides of iron and aluminium. There is always an increase in oxidized forms. Concentrations are reduced up the profile. The accumulation which occurs m a y be either relative as a result of percolation of the more soluble elements, or absolute by immobilization of sesquioxides leached from the upper part of the profile or draining from adjacent profiles. The first process principally affects alumina, although this product is sometimes partly leached; the second affects iron and manganese. Differential leaching of the different sesquioxides and other constituent elements of the soil gives rise to selection through the profile and between profiles.

Bonifas, who compared a fresh dunite and its pain d'épiées alteration faciès which retained the original volume of the rock, compared constant weight and constant volume balances (see Table 6).

T A B L E 6. Comparison of a fresh dunite and its pain d'épiées alteration faciès

Constituents

Si02

A120, Fe CaO MgO Cra03

TiOa

H20

Constant weight

— 95.5 Constant + 40 — 100 — 99 + 67 + 100 + 5.6

Constant vo

— 97.5 — 44.5 + 185 — 100 — 99.5 — 4 + 13.5 — 40

These figures establish that the results are identical for silica, lime and magnesia on a basic rock in a well-drained locality. There is true elimination. The variation for iron is of the same type, but the increase is greater in the constant volume balance. For alumina, chromium, titanium and water the variation is in the opposite sense. Thus, whereas alumina seems stable in the constant weight balance, it is relatively mobile in the constant volume balance. C o m parison of the behaviour of iron and alumina reveals differences which have a bearing solely on the solubility of these elements, the former being more mobile than the latter.

In general, alumina is derived from the alteration of felspars. This segregation occurs principally in the form of gibbsite. Most of the iron comes from the alteration of ferromagnesian minerals (amphiboles and pyroxenes), and is found most frequently as goethite. These transformations have been

40


studied in detail on dolerites in Guinea (Lacroix, 1913; Bonifas, 1959). Alteration gives rise to a light, porous, non-friable yellow rock, in which the structure of dolerite can be seen. Abrupt transition from the fresh rock to this pain (Tépices

faciès occurs within the space of approximately 1 m m . Only a thin brown margin separates these two formations. Transformation of the fundamental minerals is direct, and does not pass through the intermediate stage or argillaceous constituents.

Bases

With the occasional exception of K 2 0 and N a 2 0 , bases are rapidly and completely eliminated (Leneuf, 1959).

This washing-through is more intense than for any other type of soil, but does not differ from ordinary leaching or from leaching in podzol formation. The result is extremely low contents of exchangeable cations and total cations— the exchange capacity of the absorbing complex is very low (10-15 meq. per cent). Saturation is generally below 40 per cent, and m a y fall to values of the order of 5-10 per cent. These data apply to the mineral horizons, since surface enrichment m a y be found under the influence of vegetation. The pH m a y rise to 7.0 in this case, but in the deep-seated horizons it is most frequently between 4.5 and 5.5.

Besides these fundamental and constant data, there are varying components— clays, minerals and more or less decomposed rock debris.

Clays

Analytical data reveal the presence of clays in all types of latérite, especially between the alteration zone and the sesquioxide accumulation horizon. It is possible for this order to be inverted. In particular, the sesquioxide horizon comes immediately after the alteration horizon on well-drained basic rocks, and the transition is abrupt. Above it there usually lies a clay horizon, followed by a new sesquioxide horizon. Several of these horizons are sometimes superposed throughout the profile. O n quartz-rich crystalline rocks and on shale rocks the clay horizon always follows the alteration horizon. These successions reflect drainage conditions at different levels of evolution.

The clays identified belong principally to the kaolinite group (kaolinite and halloysite). Vermiculite is sometimes reported. Bonifas (1959) and Leneuf (1959) report the fleeting presence of montmorillonite in alteration horizons. This phase is difficult to grasp. It occurs under conditions of deficient drainage when the mother liquors have a high content of alkaline earths.

Illites are also frequently identified. In Black Africa they are identified in 50 per cent of cases on average. These minerals, which do not seem to be characteristic of pedogenesis, are either transitional stages in the more or less advanced alteration of primary phyllitic minerals, or products inherited from the original material, especially on sedimentary rocks, or, finally, contamination products derived from external sources.

Microscopic examination demonstrates that kaolin m a y be derived directly from the alteration of micaceous and chloritic minerals (Alexander et al., 1956). However, the proportion of these minerals in the rocks is too low to explain the sometimes quite considerable amounts of clays in the soils to which they give rise. There is argillaceous neosynthesis from separates by hydrolysis.

41


The principal factors determining the nature of alteration minerals for a given type of rock are essentially drainage conditions and the ionic content of the percolating water. The role of the material is to set limits to the possible transformation. Précot et al. (1962) arrive at the same conclusions. O n acid rocks the influence of the rock is apparent in more extensive segregation of iron than of alumina. O n basic rocks the contrary is generally the case (Maignien, 1958).

Conditions of unsaturation and an acid j»H coincide with the appearance of kaolin (De Kimpe and Gastuche, 1962). Herbillon and Gastuche (1962) demonstrate that crystallization of aluminium hydroxide in gibbsite is greatly favoured by deionization conditions in the mother liquor. W h e n drainage is excessive a gibbsite layer forms round the altered rock. Sherman (1962), followed by Bates (1960), notes the high precipitation of the Hawaiian islands is favourable to the development of gibbsite. W h e n salt content remains high in the percolation water (in the case of poor drainage) gibbsite does not appear.

The good drainage conditions which favour the transport of silica and the elements known as mobile elements limit the neosynthesis of clay owing to the lack of Si02. These processes are defined with particular clarity on basic rocks, whereas on crystalline rocks with a high quartz content, silica deficit is little apparent, even when drainage is excessive, and neosynthesis of kaolinite predominates. It therefore follows that the relative proportions of free silica, ses-quioxides and kaolinite m a y vary greatly in relation to local environmental conditions. This limits use of the Si02/AI203 ratio.

More or less altered minerals and rock debris

Primary minerals are, in general, greatly and even sometimes completely altered. Quartz itself is partly decomposed. Particle-size analysis of the decomposition products reveals the almost total absence of sil fractions (2-20 ¡x). Alteration products are either of the size of clay particles (less than 2 ¡JL) or larger than those of fine sand (more than 50 ¡j.). It should also be noted that these coarse materials are composed in the main of quartz and several other resistant minerals, including zircon, rutile and ilmenite. This absence of the silt fraction is often taken as a characteristic of latérites. The silt/clay ratio is usually below 10/15 (van W a m b e k e , 1962).

The almost total absence of mineral reserves is another consequence of the extreme alteration of primary minerals. Because of this the content of exchangeable bases is often similar to that of total bases.

Précot et al. (1962) state that the sequence of alterability of the primary rock minerals is identical to Bowen's sequence (1956). (See p. 43.)

The complete sequence of alterability of the principal primary minerals can be presented as follows: olivine —¡- calcic plagioclases —> calc-sodic plagioclases —*• pyroxenes —> sodic-calcic plagioclases —¡* amphiboles —*• biotite —> sodic plagioclases —> orthoclase —> muscovite —>• quartz.

The field of alterability of biotite and hornblende is partly covered.

Intense and far-reaching alteration processes lead to the disruption of crystal lattices, in which the constituent elements appear as simple ionized forms : Si02, A1 2 0 3 , Fe203 , Ti02, C a O , M g O , K 2 0 , N a 2 0 , H + etc., which are immobilized or carried away, or which partly recombine among themselves in accordance with drainage conditions and the quality of the percolating waters.

42


Bowen's sequence

Discontinuous series Continuous series

A + Olivine Calcic plagioclases

Augite Calc-sodic plagioclases

I | Amphiboles Sodic plagioclases

I I Biotites Sodic plagioclases

\ / Orthoclase Muscovite I

Quartz

The extent of these alterations is related to the intensity and duration of

the processes. More or less decomposed minerals and/or rock debris sometimes

escape alteration and are found in the upper horizons of the soils. The fact that

they contribute to the chemical composition of the lateritic material is a further

limitation on use of the Si02/Al203 ratio. Such enrichment m a y equally be

stated to have arisen from contamination due to mass movements or mecha

nical alterations. However, there is often an extremely clear phase difference

between residual or clastic materials and lateritic materials.

In conclusion, the specific processes encountered in humid tropical environ

ments are conducive to the formation of a level, which m a y sometimes be very

thick, in which kaolinitic clays, the sesquioxides of iron and aluminium, quartz

and residual materials are encountered in variable mixtures. There is no chemical

or mineralogical difference between these levels and indurated formations.

The only differences relate to structure and induration.

A range of soils in which more or less thoroughgoing leaching processes affec

ting cations and sesquioxides, and the phenomena of mechanical alteration,

predominate develop from these horizons, which are sometimes referred to

as oxic horizons ( U S D A Seventh Approximation, 1960), or ferrallitic horizons

(Sys, 1962). Précot et al. (1962) do not record any significant modification

of the nature of the colloidal fraction under the action of OTganic matter. This

fraction is invariably determined by the nature of the percolating waters and

the intensity of drainage.

Lateritic evolution therefore contributes to the formation of a material of

some complexity characteristic of humid tropical environments. This material

can be likened to alteration crusts. This idea has been developed at length by

Gerasimov (1962) who states that 'The lateritization process gives rise to a

great many lateritic occurrences, which are a formation peculiar to continental

geologico-pedological phenomena'. H e also states: 'Contemporary lateritic

formation is to some extent similar to loess formation, inasmuch as the main

lithological properties of typical loesses and loess-like occurrences (like those

of latérites) are intimately related to erosion processes and to the characteristic

pedological formations of the arid regions of the temperate (boreal) zone'. This

idea was taken up in part by Sys (1962), who treats the ferrallitic horizon as

original material. M a n y other authors, who do not go thus far, stress the

43


complex nature of lateritic soils. Aubert (1954) notes, in particular, that th& interrelationships between the different levels of a single profile of such great thickness are far less certain than those between the horizons of soils in temperate regions.

There must, however, be relations between the various morphological and analytical characters of the soil profiles, and they must be related to the intensity of pedological processes. Many attempts have been made at elucidation relying on a variety of criteria, including the Si02/Al203, Si02/R203, and silt/ clay ratios, the saturation of the absorbing complex, and mineral reserves. None of this information provides any absolute indication. Structural studies m a y yield some positive elements. The work of Sys (1959) and of Van W a m b e k e (1962) is important in this respect. The present author would suggest that there are three types of structure specific to lateritic soils, and that they are related to the intensity of the evolution process: (a) pseudo-arenaceous structures corresponding to the least ferrallitic soils; (b) fine polyhedral structures corresponding to typical ferrallitic soils; (c) degraded structures, of powdery aspect, corresponding to soils in which laterization is deepest and most intense.

The geographical distribution of these three categories is evidence in support of this hypothesis. The first category is found where the climate is of the semi-humid tropical type, the second where it is humid tropical, and the third where it is humid equatorial.

Unfortunately these studies have been complicated by the present distribution of latérites, which does not necessarily correspond to the conditions of formation (Pendleton, 1936). Many sedimentary occurrences in the tropics owe their origin to lateritic alteration processes.

For example, some of the clay and sand formations of Black Africa arose from the sedimentation of lateritic material drawn from ancient massifs (Continental Terminal and Continental Intercalaire) (Michel, 1960), and possess the mineralogical characters of true latérites. Soils which develop on these formations possess inherited characters, even if pedogenetically different. These relations between pedology and geology are well accounted for in the theory of 'biorhexistasis ' (Erhart, 1956).

This is an important analytical element, since it affects morphology and is therefore readily observable, and permits of a closer approach to the problem. The question is one of the nature of the relations between sesquioxides and kao-linitic clays. Although as yet little studied, it would appear to be of great importance for its bearing on the behaviour of iron, which varies between soil groups. In lateritic soils, iron is highly segregated and intimately related to kaolinite (Fripiat et al., 1954; D'Hoore, 1954). In ferruginous tropical soils, on the other hand, free iron is present in labile forms and can readily be separated from clays. The same applies to hydromorphic soils. This information provides a partial explanation for the greatly sustained coloration of lateritic soils. It also underlies the processes of incrustation by leaching (D'Hoore, 1954; Maignien, 1958). The subject is one that is still in the realm of hypothesis, but that merits detailed investigation.

44


RELATIONS B E T W E E N TROPICAL SOILS A N D I N D U R A T E D LATERITES

Indurated occurrences are present in the genetic horizons of many tropical soils, where they m a y be crust debris or fine gravel derived from the erosion of hardpans (Greene, 1947; Ruhe, 1954), or ancient crusts in the course of alteration on which a new soil is developing (Mulcahy, 1960, 1961), although more frequently they will be formations specific to the genetic horizons which have developed in situ.

Several large categories of tropical soils possess concretionary or encrusted genetic horizons (lateritic soils as described in the last chapter, ferruginous or fersiallitic soils, certain hydromorphic soils and also, sometimes, vetisols). Some of these soils, however, are not concretionary or encrusted. Indurated occurrences cannot therefore be distinguished as a general characteristic of these soils, but must be studied separately within each category.

LATERITIC SOILS

Induration processes assume various forms, not always of the same origin, in lateritic soils. Concretionary and encrusted horizons m a y be found separately or together.

Lateritic soils with concretionary horizons

Concretionary horizons are usually near-surface and rarely more than 2 m . thick. Such horizons are a mixture of earthy materials and indurated materials, which vary in shape, but are usually rounded and have average dimensions of 1-2 c m .

These concretions are of two types : (a) true concretions formed by the successive deposition of sesquioxide films, usually ferruginous, around a nucleus, which is generally a quartz grain; (b) false concretions, consisting of altered rock debris impregnated by ferruginous solutions.

The former develop in the near-surface horizons, and the latter at greater depth, sometimes even below a true hardpan. The latter are therefore often nodular, in addition to being larger, less concentrated, and also less indurated.

Under the effect of mechanical alteration (erosion by water, disturbance by the roots of trees uprooted by tornadoes) these horizons mix with the surface humic horizons. The processes of dissolution and successive redeposition accentuate the pisolitic form, and the concretions assume a gravel faciès, i.e., a residual-product facies. The lustre becomes splendent, and they are darker in colour, brown to brown-black.

The relations between concretions and the surrounding soft materials are extremely clear in the horizons of formation. Segregation of sesquioxides leads to the formation of coloured nodules where drainage has slowed down. The percolating waters which circulate round the fringes deposit concentric ferruginous films which harden and isolate the material from the surrounding earthy mass. Analyses of the textural elements of the earthy mass and the concretions demonstrate the strict relationship between them (the same residual minerals, the same alteration facies). In altered surface horizons, on the other hand, this relationship is gradually obliterated, sometimes to the point of complete

45


disappearance, especially if there is contamination by materials from adjacent formations. These small gravel particles are sometimes deposited in more or less regular beds following the trend of the relief. The origin of the stone line thus formed is still widely disputed (mechanical grading, the action of the soil fauna, and especially of termites, scouring and spreading of desert type, followed by filling, etc.).

These concretions m a y also derive from the induration of a mottled clay horizon. Since this is a question of hydromorphic action, the position of such formations in the profile varies, and is related to a reduction in the rate of drainage which can affect any level (Waegemans, 1949, 1952) and is often connected with topography. Here there are once again strict relationships between the soil and indurated occurrences.

Lateritic soils with encrusted horizons

More or less continuous indurated horizons are also of two types, one of which is the pseudomorphic transformation of a rock in course of alteration. In this case the structure is provided b y fine gibbsite crystals which copy the form of certain original minerals (plagioclases). Ferruginous impregnations and kaoli-nitic neosyntheses in some degree fill out this horizon, which develops in contact with the parent rock, and therefore at great depth. The processes of partial dissociation and redeposition subsequently produce a scoriaceous appearance. A relative crust as defined by D ' H o o r e (1954), which m a y attain considerable thickness (as m u c h as 10 m . ) , is a typical example.

However , the most frequently encountered and classical faciès is a pisolitic or alveolar incrusted horizon deriving from the concentration of sesquioxides at a given level, generally between the surface horizons and the argillaceous horizons. The sesquioxides which cement the pre-existing textural elements are derived from the leaching of adjacent horizons and also, and more generally, from the leaching of adjacent profiles (Maignien, 1958). These indurated occurrences are often 2-3 m . thick. T h e textural elements are very varied, and m a y sometimes be concretions formed in the first stage of evolution.

There are extremely strict relations between the position of these crusts in the profiles and the circulation of water. In other words, although most crusts occur at slight depth, they can also be a feature of far lower levels in the soil. S o m e alteration debris are also cemented by sesquioxides and formed into crusts (Bachelier and Laplante, 1953). The rock itself is sometimes impregnated with ferruginous solutions and becomes encrusted (Maignien, 1958).

There are more or less strict relations between these types of crust and the levels of formation. The horizons function as a m e d i u m of reception and accumulation, and the pre-existent materials are cemented by sesquioxide solutions of diverse origin. Connexions are no longer effected solely through the set of profiles which are distinguishable along a slope, and incrustation is closely related to the geomorphological evolution of the terrain. These are absolute crusts in D 'Hoore ' s definition (1954).

FERRUGINOUS TROPICAL SOILS

The extreme ease with which iron circulates through the profile is a feature of ferruginous tropical soils, which form part of a group of soils that are eco-

46


logically less humid than those previously considered. This iron contributes to the formation of concretionary or incrusted horizons of illuvial origin. There is no individualization of aluminium but individualization of manganese is sometimes pronounced. Clay illuviation in the surface horizons, which is often intense, and which is followed by accumulation at depth, is a feature of ferruginous tropical soils. Leaching coefficients are of the order of from 2 to 6. Clay illuviation promotes the formation of a choked horizon which profoundly influences the distribution and immobilization of iron and manganese. Concretions or, in the case of lateral transport, an incrusted horizon subsequently form beneath this clay horizon (Maignien, 1961). These processes are accentuated by the subhorizontal relief on which they develop. External drainage is diminished and the soils evolve as true hydromorphic soils in the rainy season, with the formation of a suspended sheet which intensifies concretion and incrustation. These processes m a y , on occasion, invade all profiles, especially as the surface horizons, which are very susceptible to erosion (Fauck, 1955), are frequently affected by surface scouring.

Essentially ferruginous concretionary and incrusted horizons are of slight thickness (of the order of 1 m . ) . In general they develop at a depth of between 1 m . and 2 m . , but m a y also occur at around 30 cm.-50 c m . These soils are very similar to those described by Mohr (1932). Mohr and VanBaren (1954) recognize progressive formation of identical soils in three stages : (a) profiles without latérites, in which an impermeable substrate forms; (b) stages admitting of horizons with nodular latérite; (c) final stage, in which the mass is latérite.

There are strict relations between genetic horizons and indurated occurrences.

H Y D K O M O S P H I C SOILS

The incrusted horizons to be found at depth in some hydromorphic soils m a y at times be thick. These crusts, which are often alveolar, are essentially ferruginous, and at times slightly manganiferous. The formation of these crusts is strictly related to enrichment of the medium with sesquioxides by groundwater fluctuations. The depth and thickness of these formations is dependent on the ground-water table. These types of crust, which are frequently encountered in alluvial valleys (Maignien, 1954), are known as 'sheet (nappe) crusts', 'gallery crusts', and 'ground-water latérite'. The function of the original soil is that of reception. These crusts also often have a brecciated or conglomeratic structure.

VERTISOLS

Ferruginous or manganiferous pisolitic concretions are frequently encountered at the base of vertisol profiles in Black Africa. The author has even been able to observe the formation of true ferruginous crusts in the Upper Volta. The processes in fact diifer little from those noted in hydromorphic soils. The presence of a zone of saturation seems to be a decisive factor. A cation-enriched medium is conducive to the precipitation of the sesquioxides of iron in highly concentrated forms.

COLLUVIAL CRUSTS

Finally, the section would be incomplete without some mention of colluvial crusts. These crusts, which are found at the foot of slopes, are of two types :

47


either the secondarily cemented debris of former crusts, of colluvial or proluvial materials impregnated and indurated by sesquioxide-enricbed solutions. These crusts are known as 'slope bottom' (60s de pente) crusts, a type frequently encountered in incrusted terrain. They develop on gradients of less than 0.7-0.9 (D'Hoore, 1954; Maignien, 1958). They are very hard, of slight thickness (less than 1 m . ) , essentially ferruginous, and dark in colour, and have a characteristic foliated structure.

Finally, it should be remembered that crusts similar to those of hydromorphic soils are found on the bottom of lakes or mangrove swamps.

RELATIONS B E T W E E N T H E M O R P H O L O G Y OF INCRUSTED HORIZONS A N D T H E M E D I U M IN W H I C H T H E Y F O R M (Maignien, 1958)

T H E ROLE OF CONSTITUENT SESQUIOXIDES

Sesquioxides immobilized in original textural material fulfil three roles: (a) they cement the textural particles; (b) they impregnate the formations in situ; (c) they concentrate and form concretions.

Cementation

Cementation occurs most frequently in coarse elements, and is rarely found when the mean diameter of the soil particles is less than 20 ¡i,. It is therefore rare in clay soils. The textural granules are either original materials such as sands, gravels, and pebbles, or secondary materials such as pseudo-sands, concretions, fine gravel particles, and blocks of former crusts. It often happens that a soil originally consisting of fine particles is cemented as a result of concretion of one of its horizons.

Impregnation

This process occurs in material of fairly fine texture (coarse silt, fine sand, clayey sand, and sandy clay). Impregnation, which is more or less diffuse, follows the lines of least resistance where water can circulate. It is often an indication of slight textural variations. The indurated skeleton consists of particles that are usually coarser than the surrounding soft materials. Impregnation is sometimes complementary to the partial epigénesis of residual minerals. In this way the siliceous cement of some sandstones is replaced by the hydroxides of iron. Phenomena of the same order are to be found in schists. These processes find expression in dissolution of the silica to varying degrees and mechanical movement of the mass. Only the ferruginous skeleton is retained. The softened materials are carried away by water, leaving behind a more or less vesicular ferruginous rock, which retains traces of the original bedding.

Concretion

Concretion is produced by concentration, precipitation or deposition of sesqui-oxide films around various nuclei (quartz grains and mineral debris). The con-

48


cretions derive from impregnation and cementation, but since the phenomenon is highly concentrated it gives rise to fairly Tounded products which are segregated from each other.

Concretion m a y also occur by the induration of sesquioxides segregated from a more argillaceous material. It m a y also indicate the formation of pseudo-sands (the strict relations between clays and sesquioxides).

These three types of incrustation correspond to forms of absolute accumulation and are related to movements of iron in the soil. Alumina is a less frequent contributor to these processes, although alumina ooliths are sometimes encountered. O n the other hand, alumina plays an important part in the morphology of some types of relative crust. A n excellent example is provided by horizons of pain à"épices faciès impregnated to some extent with the sesquioxides of iron. Gibbsite crystals retain the original structure of the rock and form a more or less indurated skeleton.

MORPHOLOGICAL FACTORS IN RELATION TO INCRUSTED HORIZONS

Several factors affect the morphology of incrusted horizons.

Mode of formation

Relative accumulation. A scoriaceous structure is the most frequently encountered form of relative accumulation. The skeleton, which is bright in colour (white to rose), is bauxitic. Cavities (small bands) are partly filled with fine, red material, with secondary ferruginous impregnation. Pseudomorphs are encountered and the original structure is partly retained. Loss of the most soluble materials, which establishes the relative origin of these crusts, is confirmed by settling, shearing, sometimes by pseudo-colluvial appearance.

Absolute accumulation. The morphology of these crusts is basically affected by the physico-chemical properties of the medium in which they form. Such crusts are mainly ferruginous, and sometimes manganiferous. It should, however, be stated that iron and/or manganese sometimes impregnate or cement aluminous horizons. This makes the distinction between absolute and relative crusts a fairly fine one.

Physico-chemical properties

Base saturation. In acid soils indurated forms are diffuse and produced by impregnation. The following types are encountered: fairly complete cementation of a soft material, impregnation and the formation of crusts of alveolar structure (foliated crusts when there is significant arrival of sesquioxides by oblique leaching).

In soils with a higher content of the cations of alkaline earths, the oxides of iron and manganese tend to concentrate in rounded concretions (Castagnol and Shan-Gia-Tu, 1940). Sesquioxides are sometimes deposited around a nodule of rock in course of alteration which creates a sufficient concentration of alkaline earths in its immediate vicinity to promote precipitation. These phenomena are revealed with greater clarity on calcareous rocks.

49


Texture. R o u n d e d forms predominate in argillaceous and silted media. Diffuse impregnation forms are produced in a soft m e d i u m . T h e crusts formed in a m e d i u m of moderate texture (sandy-argillaceous to argillaceous-arenaceous) are to s o m e extent alveolar.

Level of sesquioxide accumulation

T h e sharpness of the forms is dependent on the level of sesquioxide accumulation and on the rate at which immobilization develops. A sesquioxide-rich m e d i u m is often related to a basic rock with a high content of alkaline earths. Such a m e d i u m tends to give rise to rounded forms. A sesquioxide-poor m e d i u m (granites or extremely quartzose gneiss) tends to yield impregnation forms. O n the other hand, the more abrupt the p h e n o m e n o n of immobilization, the more concentrated will the faciès be (crusts in a hydromorphic m e d i u m ) .

In conclusion, the information which has been given concerning the indurated levels of tropical soils can be tabulated as follows (Maignien, 1958).

LATERITIC SOILS

AN INCRUSTED HORIZON CONSISTING OF SESQUIOXIDES SEGREGATED 'IN SITU*

Relative accumulation

Well-drained medium. Alumina-rich crust; scoriaceous structure dominant; near-surface horizon; moderate induration; pain d'êpices faciès.

Medium with deficient drainage. Very slight incrustation; soils generally clayey; crust, when present, alumino-ferruginous ; pisolitic to finely alveolar structure, generally flattened, or concretions and nodules; near-surface horizon; slight to moderate induration; mottled clay.

Absolute accumulation

Well-drained medium. Absorption of iron on clays; pseudo-sands. Poorly drained medium. Aluminous crust more or less incrusted with iron, often highly

argillaceous; alveolar to nodular structure; position in profile related to level of permanent ground-water table; highly indurated.

INCRUSTED HORIZON CONSISTING OP SESQUIOXIDES ORIGINATING OUTSIDE THE PROFILE


Well-drained medium. Alumino-ferruginous crust; ferruginous films deposited on the original skeleton; clastic material plentiful; crust sometimes outcropping; porous; highly indurated.

Medium with deficient drainage. Whitening; pisolitic crusts, sometimes aluminous, fairly rare on plateaux.

FERRUGINOUS TROPICAL SOILS

INDURATED HORIZON OF SESQUIOXIDES LIBERATED 'iN SITU'


Well-drained medium. Highly leached soils with crust or concretions at the level of deep groundwater; alveolar, essentially ferruginous crust; highly indurated.

50


Medium with deficient drainage. Leached soils with ferruginous concretions or crusts; pisolitic structure and/or slightly foliated; depth 50-150 m .

E N C R U S T E D HORIZON CONSISTING OP SESQUIOXIDES DERIVED

F R O M A D J A C E N T PROFILES


Oblique leaching. Plateau-edge crust and slope-bottom crust; essentially ferruginous; clastic material plentiful; foliated structure; often outcropping; extremely highly indurated.

Effect of perched groundwater, temporary, pisolitic crust, sometimes slightly foliated; slight depth (50-150 c m . ) ; highly indurated.

O T H E R TYPES OP SOIL

Incrustation related to fluctuations of a temporary ground-water table; essentially ferruginous ; the structure is a function of texture and of the richness of the receiving m e d i u m .

51

Global distribution of latérites.

Relations with environmental factors

T H E DISTRIBUTION^ F|LATERITES

Latérites are widely distributed throughout the world, but especially in the intertropical regions of Africa, Australia, India, South-East Asia and South America. Distribution does not necessarily correspond to existing conditions of genesis. Many of these occurrences are subrecent or fossil, even in intertropical regions. Their extension indicates that conditions were favourable to their formation at sometime or other in the history of the world, but not necessarily simultaneously at all points. Quite apart from the very old (Permian and Carboniferous) red sedimentary formations which are suspected of being of tropical origin, iron ore levels and more recent materials exhibit the characters of late-rites. Muckenhausen (1962) reports latosols in southern Germany, and the 'red yellow podzolic soils ' of the United States can be largely classified as oxi-sols. Some of the buried soils under the Condroz sandy loams (Belgium) have all the characteristics of concretionary ferruginous tropical soils. There are many such examples.

It is now universally recognized that lateritic soils must have evolved over extremely long periods (Leneuf, 1959), of the order of tens and, sometimes, even hundreds of thousands of years. In particular, there is much evidence which tends to indicate that the Tertiary period presented conditions particularly favourable to lateritization, both in intertropical regions and in the world as a whole. W h y are these formations more abundant in the tropics? The prevailing formation conditions are certainly not the only factors. The differences are probably related to the action of Quaternary glaciers, which removed the alteration crusts and obliterated former influences. Since these glaciations did not affect intertropical regions, Tertiary lateritic formations have remained in addition to Quaternary formations.

Prescott and Pendleton (1952) attempted the first global synthesis of the distribution of latérites in the English usage of the term. Since this study, there has been much prospecting, especially in Africa and South America. In relation to Africa mention m a y be made of the soil m a p of the Inter-African Pedolo-gical Service of the Commission for Technical Co-operation in Africa South of the Sahara (CCTA), the last draft of which was put forward by D'Hoore in 1963. This is a synthesis on a continental scale, which takes into consideration the results achieved by all soil scientists who have worked in Africa. Carto-

52

Global distribution of latérites-

graphic Étudies are less far advanced in South America, but are sufficient to give a fairly good idea of the distribution of lateritic formations (Bramao and Lemos, 1960). The same applies to South-west Asia. Australian work is equally far advanced (Stephens, 1962).

Latérites are widely distributed in the semi-humid and humid intertropical regions of the globe, and in fossil state are found in drier climates, and sometimes even in a temperate climate (see the first chapter of this work).

As indurated occurrences, latérites extend beyond subhumid tropical climates to desert regions (the African and Australian deserts), where they are an indication of more humid influences in the past. W e are indebted to Prescott and Pendleton (1952) for the first global synthesis of the distribution of indurated latérites. There is little to add concerning their distribution in India and Australia, but many studies enable us to fill out our knowledge of their distribution in South-East Asia, Africa and America. In general, cartographic works combine indurated latérites and lateritic soils in the wider sense. A study of the distribution of intertropical soils would be needed to deal with this question. W e shall therefore restrict ourselves to general information concerning the distribution of 'latérites' in the major continents.

AFRICA

Available information concerning the distribution of African soils, and, inter alia, of lateritic soils, is synthesized in the 1: 5 million m a p produced by D'Hoore (1963) in collaboration with soil scientists working in the continent.

All occurrences which can be connected with latérites are grouped under the following heads:

A . Crude mineral soils. 1. R o c k s a n d rock debris.

A b . Undifferentiated ferruginous crusts a n d limestone crusts. B . Little evolved soils.

1. Lithosols (skeletal soils) a n d lithic soils. Definition: Soils without differentiation of genetic horizons, with coarse elements, a n d wi th solid rock lying at a depth of at least 30 c m . B b . O n crusts.

J . Ferruginous tropical soils (fersiallitic soils). These soils frequently h a v e a concretionary, a n d somet imes even a n encrusted horizon, with a n essentially ferruginous cement at slight depth. These indurated occurrences are particularly developed o n rocks with a high content of ferromagnesian minerals (Je), and less intensively developed on crystalline acid rocks (Jc). T h e y are often absent o n original s a n d y materials (Ja).

K . Ferrisol. This category is subdivided into: non-differentiated ( K e ) ; soils on rocks with a high content of ferromagnesian minerals ( K b ) ; h u m i c ( K a ) . Since the presence of a crust (indurated latérite) is not a n invariable feature of these soils, or of the preceding soils, or of the ferrallitic soils which follow, its presence at slight depth is indicated b y a superscript.

L . Ferrallitic soils (sensu stricto). T h e following are distinguished: 1. Predominantly yellow-beige soils (7.5 Y R or m o r e yellow) o n : soft sandy sedi

m e n t s (La); m o r e or less argillaceous sediments ( L b ) ; undifferentiated (Le). 2 . Predominantly red soils (5 Y R a n d m o r e red): soft sediments (LI); rocks with a

high content of ferromagnesian minerals ( L m ) ; undifferentiated (Lu) .

53

Global distribution of latérites

3. Ferrallitic humic soils; undifferentiated (Ls). 4. Undifferentiated ferrallitic soils 'with a dark horizon' (Lt). 5. Yellow and red ferrallitic soils on various types of undifferentiated original mate

rial (Lx).

Crude mineral soils and slightly evolved incrusted soils are to be found throughout intertropical Africa, especially on old plateaux and Tertiary and Early Quaternary outliers. These old crusts are prevalent on Miocene and Pliocene sedimentary clay-sand formations. They are to be found in the north towards the Sahara submerged beneath aeolian deposits.

Recent or subrecent ferruginous crusts seem to be restricted to the ferruginous tropical soils which are prevalent in Africa, where they form a band spanning the continent from west to east in the northern hemisphere, approximately between the 750 m m . and 1,200 m m . isohyets, and occur as significant patches in the southern hemisphere in Angola, the southern Congo and Mozambique. These incrusted soils, which are often eroded, are found mainly on materials with a low content of alkaline-earth minerals. A 'stepped' subhorizontal relief is characteristic. They support a typical savannah vegetation.

Between these two belts of ferruginous soils there is a zone of lateritic soils, which are often incrusted where the climate is tropical. Where the climate is equatorial, without a pronounced dry season, concretions are often well defined, but incrustation is rarer. These soils develop on a wide range of materials.

The relief is more or less accentuated hill relief. The natural vegetation is mixed forest, often degraded by m a n .

AMERICA

W e are indebted to the Inter-American Institute of Agricultural Sciences and to F A O for the first attempts to correlate and m a p the soils of South America. In Central America and Mexico attention has only recently been turned to the question.

In North America, Thorp and Reed (1949) have noted certain scoriaceous ferruginous occurrences in Nebraska which can be compared to fossil latérites. Similarly, the red yellow podzolic soils can, to some extent, be likened to lateritic soils.

For South America, Bramao and Lemos (1960) indicate the following cartographic units which can be related to latérites on a 1: 10 million m a p .

Red yellow latosols and red yellow podzolic soils. This heading comprises the red yellow latosols, hydrol humic latosols, red yellow podzolic soils and rego-latosols of Brazil, the lateritic soils of Peru, the red yellow latosols and hydrol humic latosols of Ecuador, the red yellow latosols and associated soils, and red yellow podzolic soils and associated soils of Colombia, the lateritic soils, yellow and red lateritic soils of Bolivia, the latosols and associated lithosols, and red yellow podzolic soils of Paraguay, the latosols and red podzolic soils of Argentina, the red yellow podzolic soils and red earth latosols, and brown-red lateritic soils of Venezuela.

Pale yellow latosols, sheet (nappe) latérites, yellow-red soils, podzolic soils and

concretionary latosols. This heading groups the regolatosols of Brazil and the latosolic regosols of Ecuador.

54

Global distribution of latentes

Arenaceous latosols. This heading groups the sandy latosols and yellow-red latosols of Brazil, the sandy latosols of Paraguay, and the sandy regosols, yellow-red latosols, and red earth latosols of Venezuela.

Low humic latosols, terra rossa. This heading comprises the terra roxa legetima and pararía of Brazil and Paraguay.

Rubrozems, comprising the rubrozems of Brazil, and the grey-black subtropical forest soil, the lithosols, and the regosols of Argentina.

Yellow-red latosols are found in the main beneath humid tropical forest. Pale yellow latosols are found in particular beneath equatorial forests and especially in the A m a z o n . They are frequently associated with sheet latérites and concretionary latosols. Arenaceous and sandy latosols are to be found mainly beneath tropical savannah.

Camargo and Bennema (1962) have since defined the characters of these principal headings.1

In Central America indurated latérites are extremely rare, but lateritic soils are c o m m o n . A few small patches have been reported on the east coast of Nicaragua.

In South America the most distinct manifestations of incrustation are to be found in Brazil and the Cuianas. This is particularly the case of the 'Canga' of Brazil (Marbut, 1932). Latérites cover the remains of the ancient peneplains from Bahia to western Ceara, to eastern Piauly, to northern Minas Geraes and south and central Maranhas. These latérites extended to Goyaz and the northern parts of Sâo Paulo, and further still to the west across Mato Grosso (Prescott and Pendleton, 1952).

Finally, very thick bauxitic latérites have been identified in the three Guianas : British Guiana, Dutch Guiana and French Guiana.

ASIA

India

Latérites have long been k n o w n in India, where they occupy large areas of the Deccan peninsula. Oldham (1893) notes that 'high level' latérites cap the summits of hills and plateaux on the highlands of central and western India. ' L o w level' latérite is found in long bands along both coasts of the Deccan peninsula.

O n the western coast it is difficult to distinguish between these two types of latérite. O n the eastern coast, on the other hand, the detrital origin is more apparent, and the faciès can be traced from Cape Comorin to Orissa and thence northward through Midnapur, Bardwan and Birbhum to the flanks of the Bajmahal hills.

Most of the information concerning India was brought together by R a y -chaudhuri (1941). In the last draft of the pedological m a p of India this author classifies latérite formations under the following heads: red gravelly soils, late-rites, and lateritic soils. The first category could equally well be termed red ferruginous gravelly soils, indicating the outcropping of gravels by a symbol on the m a p .

1. See Qassification, p* 111 et seq.

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The latérite soils could also be described as ferrallitic and subdivided in accordance with their topographic position into high level latérites, hill latérites, and low level latérites.

The latérites and lateritic soils are ferruginous and slightly affected by late-ritization (latosols).

South-East Asia

The main results have been rearranged by Dudal and Moorman (1962), who distinguish the following categories.

Red yellow podzolic soils which predominate in the most humid parts of South-East Asia, on non-basic material, on mature and decaying relief forms. They occupy more than 50 per cent of the surface of Viet-Nam. In Indonesia they are an important group of the non-volcanic regions and are found in the west of Java, Sumatra and Borneo. Their distribution is similar in Malaysia and in adjacent regions.

These soils develop where annual precipitation is in excess of 1,500 m m . , but where the dry season is marked and the temperature is above 10° C . They support the tropical forest plantations of the lowland regions or herbaceous savannahs.

Grey podzolic soils are often associated with sheet laterites. They are found on old, acid alluvium of light to medium texture. They are essentially confined to a tropical climate of monsoon type. A few patches are, however, to be found in Borneo and Sarawak where the climate is equatorial, in which case annual precipitation is generally in excess of 1,500 m m . and well distributed throughout the year. O n the other hand, the climate is drier in the north-eastern districts of Thailand. These soils develop on flat to slightly rolling relief, under open forests of Dipterocarpus sp., and, in more humid regions, under dense ombropbil forest.

Grey podzolic soils are found mainly on the vast Mekong terraces in northeastern Thailand, in south-west Laos, in Cambodia and in Viet-Nam. They are also to be found on the terraces of old rivers such as the Chao Phya and the Ta Chin in central Thailand and on the river terraces of the lower Irrawaddy in Burma. A few small areas have been reported in the most humid areas of central Viet-Nam, south Thailand, Sarawak and Indonesia (Borneo, Bangka and Billiton).

Dark red and reddish-brown latosols. These soils develop on basic materials (basalts, diabases, diorites, andésites, granites and black micaceous gneiss). They are found in well-drained positions on rolling hill relief from sea level to approximately 1,000 m . in accordance with latitude, setting and local climatic conditions. They form at mean temperatures in excess of 22° C . and annual precipitation of the order of 1,000 m m . to 3,000 m m . The dry season is usually indistinctly defined, but m a y be as long as three or four months, which corresponds to an increase in the base saturation of the soils. The vegetation ranges from ombrophil forest to savannah forest.

These latosols are very well developed on low-lying sectors of volcanic formations in Indonesia (West Sumatra, Java, Bali and the Moluccas, the Philippines,

56


south Viet-Nam, east Cambodia, and on the basic formations of the high plateaux of east and west Burma. They have also been identified in central Laos, in south-eastern Thailand and in central Malaya.

Red yellow latosols. These soils are found on original or transported acid materials, especially on old coastal and river terraces in north-western Ceylon and northeastern Thailand, where they are fossil forms. O n residual materials they develop most often on igneous, metamorphic or sedimentary rocks. The relief is rolling to mountainous. They are found from sea level to more than 1,000 m . where the climate is tropical and annual precipitation is between 600 m m . and 3,000 m m . The dry season is indistinctly defined in Borneo, but more marked in Viet-Nam, north-eastern Thailand and north-western Ceylon. The vegetation is ombrophil or savannah forest.

The range of these soils is widest in Borneo, north-eastern Sumatra, Sarawak, Brunei and Mindanao. They are found at high levels in southern Burma, Thailand, Malaya, southern Viet-Nam and north-western Ceylon. In south-eastern Borneo and the islands of Bangka and Billiton they m a y be associated with sheet laterites and bauxite formations.

Finally, some slightly humic gley soils and grey hydromorphic soils frequently exhibit indurated occurrences, which have been described as sheet laterites, at depth. They are to be found in poorly drained depressions on generally acid alluvial or colluvial materials where annual precipitation is in excess of 800-900 m m .

Large areas are to be found on the lower terrace of the Mekong in Cambodia, Laos and Viet-Nam. They have also been identified in central and north-eastern Thailand. The latérite occurs at a depth of between 100 c m . and 200 c m . on average.

AUSTRALIA

As in India, laterites have long been known in Australia (Darwin, 1844). It is, however, Edgeworth who , in 1887, first recognized latérite as such. The distribution and interpretation of latérite has been dealt with in many studies, often from a geological standpoint. Thus, the m a n y 'desert sandstones', and ferruginous sandstones in Cape York peninsula and the Northern Territory are indurated laterites (Davidson, 1905; Woolnough, 1918).

The importance of laterites in Western Australia was recorded by Simpson (1912). Walther (1915) also recognized latérite near Adelaide, in the Blue M o u n tains of N e w South Wales, near Brisbane and in the vicinity of Darwin.

Laterites are prevalent in the Northern Territory (Jensen et al., 1915) and in Queensland (Saint-Smith, 1921; Jensen, 1926).

Most of these indurated superficial formations are mapped as such on the geological maps of many regions. They are usually old formations, whose genesis bears no relation to present-day climatic conditions. They emphasize the traces of an ancient peneplain relief. Thus, the laterites of Queensland were probably Pliocene (Bryan, 1939). Hanlon (1945) regarded the ferruginous bauxites of N e w South Wales as Miocene.

The manual of Australian soils (Stephens, 1962) notes several large soil groups which can be related to laterites in the wide sense.

57


Lateritic podzolic soils. These are usually fossil soils of Pliocene age found on old tabular surfaces. The indurated horizon, which often outcrops in arid regions, has a nodular, pisolitic or massive latérite faciès formed by the action of a fluctuating ground-water table which has now disappeared.

These soils are to be found in all States, notably in the most humid regions, but also, in fossil form, in more arid regions, They support mixed forest vegetation, forest savannah and heather.

Yellow earths. The presence of a variable amount of ferruginous nodules in the B horizon is a frequent feature of these soils, which can therefore be to some degree related to latérites. They develop on a wide range of rocks, from sandstones to igneous rocks. They have been identified in the coastal area of N e w South Wales and in the more humid regions of Queensland and Brisbane as far as the Gulf of Carpentaria.

Krasnozems. These soils are to be found in eastern Australia, from Tasmania to Queensland, especially in the coastal tropical zones. A few small patches are also to be found in the more humid areas of southern Australia, in Western Australia and on Norfolk Island.

Lateritic krasnozems. Lateritic krasnozems are small peripheral formations found in conjunction with typical krasnozems in the coastal and subcoastal districts of Queensland.

Lateritic red earths. These soils are restricted to the tropical and subtropical regions of Australia, and are not found in the more humid temperate regions of southern Australia, where their place in the terrain is taken by lateritic podzolic soils. These red earths are largely associated with ancient surfaces, probably of Pliocene age.

Finally, calcareous lateritic soils (ancient lateritic soils invaded by calcium carbonate as a result of new pedogenesis) can also be related to latérites.

These soils are encountered sporadically in the Northern Territory along the southern edge of the Barkly Tableland, at the northern approaches of the Mac-donnell Ranges, in Western Australia between Kalgoorlie and Coolgardie, and in southern Australia, on the Eyre Peninsula.

RELATIONS W I T H E N V I R O N M E N T A L FACTORS

The first attempts at interpretation, which were made long ago, dealt principally with the effect of the various environmental factors. Mention m a y be made of the work of the following authors : Kôppen, Lang (cf. Lacroix, 1913) and Richtofen (1886) on the role of vegetation; Holland (1903) on the role of bacteria; Campbell (1917) and Harrison (1910) on the role of ground-water; Maclaren (1906) and Lacroix (1913) on the effect of the alternation of dry and wet seasons ; Simpson (1912) and Lacroix (1913) on the effect of topography.

In the light of present knowledge it seems possible to attempt a classificatory synthesis of the effect of the various environmental factors on the genesis of contemporaneous latérites.

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CLIMATE

The two fundamental indices to be considered are precipitation and temperature. Consideration of all indurated formations or only of indurated formations deriving from lateritic soils yields different values. Maignien (1958) notes that although a lateritic medium is conducive to incrustation owing to enrichment of the soils with sesquioxides, incrustation can equally well affect any other material containing sesquioxides. The conditions in which crusts develop in soils are therefore far less strict than those of lateritic alteration.

The confusion between these two processes (incrustation and lateritization) explains the sometimes contradictory data to be found in pedological literature. Pendleton (1943) is of the opinion that for latérite to form (as an indurated occurrence), rainfall must be sufficient to develop and maintain forest complexes. Crusts cannot develop in savannah climates. However, in Black Africa the zones in which incrustation is most prevalent have a climate of the Sudan type, and are therefore mainly savannah (Maignien, 1954). Humbert (1948) recognized that the development of indurated latérites is rendered more difficult in an area of permanent humidity. Maclaren (1906) was of the opinion that periods of dryness are conducive to the formation of crusts, but this was disputed by Scrivener (1930), who noted the presence of the latérites of Malacca in regions where there was no alteration of dry and wet seasons. Campbell (1917) and Humbert (1948) do not regard the regular alternation of dry and wet seasons as being indispensable if humidity and desiccation are irregular conditions affecting the soil.

Mohr and van Baren (1954) took a monthly precipitation of less than 60 m m . with allowance for evaporation and transpiration as characteristic of a dry month. Aubreville (1949) set this limit at 30 m m . Simpson (1912) thought that the semi-arid conditions of "Western Australia were decisive for the formation of indurated latérite. The number of examples could be increased.

These conflicting results have arisen from the study of objects which are not all related to the same type of pedogenesis. Thus, the conditions in which crusts of the sesquioxides of iron develop differ greatly from those affecting aluminous crusts. In the former case complex-forming organic compounds or the redox potential play the decisive role, whereas in the latter genesis is intimately related to deionization conditions in the medium.

If attention is confined to 'lateritic soils ' it will be noted that most climatic studies are concerned with the effect of climate on certain specific characters of the profiles, and especially on variations in the Si02/Al203 ratio, which is less than 2 for latérite, and lower the more far-reaching the process of lateritization. The Si02/Al203 ratio is, however, more dependent on local drainage conditions than on atmospheric climate. For example, the SiO/Al203 ratio is usually slightly below 2 in African soils on granites in the equatorial forest regions, where mean annual temperature is of the order of 25° C . O n comparable formations in Madagascar, where precipitation is comparable, but mean annual temperatures are 18°-20° C , the ratio often falls below 1.0. It is therefore difficult to solve the problem. Greater precision will be reached by comparing identical facts in relation to experiments in experimental pedogenesis. The most satisfactory approach is to treat the soils as a whole, and not to deal with more or less arbitrarily selected morphological or chemical characters.

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Temperature

There are few studies concerning the thermal conditions of latérite formation,

although this information is essential. Crowther (1930) has demonstrated that

the Si02/Al20g ratio increases as temperature rises when humidity is constant.

Robinson and Holmes (1924) consider that the ratio is below 2 when mean

annual temperature is above 16° C . Baver and Scarseth (1930) have confirmed

these results in Alabama. Segalen (1957) noted that the Si02/Al203 ratio can

fall below 2 with mean annual temperatures of less than 14° C on basic rocks

under certain conditions of high humidity in Madagascar.

Most contemporary lateritic soils develop at mean annual temperatures of

around 25° C . However, mean annual temperature is 18°-20° C . on the high

plateaux of Madagascar, where the lateritic soils are extremely deep. Denisoff

(1959) has noted latosols at temperatures of 16° C . at an altitude of nearly

2,000 m . in Ruanda Urundi. Prescott has noted the same thing at Kivu at

temperatures of 15° C . and an altitude of more than 2,000 m . O n the other hand,

there are regions where these mean temperatures are common and there is no

lateritization. The concept of temperature should be related to other indices

for which there are annual means. It might be better to consider the energy

balance. This was mooted by Scaetta (1939), but does not seem to have been

followed up.

Precipitation

The figures to be found in the literature are extremely variable, again for the

reason that soils which are not comparable are brought together. Most studies

deal with the relations between the Si02/Al203 ratio and precipitation (Segalen,

1957). Mohr and van Baren (1954) criticize this approach and note that the follow

ing contradictory conclusions can be reached : (a) a negative correlation between

precipitation and the Si02/Al203 ratio (Martin and Doyne, 1927); (b) no cor

relation between these two quantities (Vine, 1949); (c) positive correlation

between precipitation and SiOa/Al203 (Glangeaud, 1941).

In fact it is mainly a question of defining the limits of lateritization in relation

to the minimum of precipitation. Observation of contemporary profiles tends

to demonstrate that the figure for precipitation m a y vary in relation to a soil

in a tropical climate, or a climate with a pronounced rainy season followed by

a dry season of some duration, or a soil in an equatorial climate with more

extended precipitation.

In the first case, at least in Africa, the limit lies approximately towards the

1,200 m m . isohyet, and m a y possibly be reduced to around 950-1,000 m m .

in an equatorial climate. The concept of atmospheric precipitation is certainly

an imperfect one, since its effect varies in accordance with the nature of the

rocks. Thus, in West Africa lateritization can occur on basic rocks with a pre

cipitation of 1,100 m m . whereas on quartz-rich granites this limit is raised to

1,250-1,300 m m . annually. It therefore seems that, when precipitation is the

same, climates in which the seasons are ill-defined are more aggressive than

tropical climates. It also seems that monsoon rains m a y be more active than

trade-wind rains. In fact, the relations between these two factors have a greater

effect on the phenomenon than do temperature or humidity.

Latérites always correspond to climates in which the rainy period is a warm

60


season. This concept of climate is important. It applies to semi-humid tropical climates and to equatorial climates. B y contrast, subtropical climates in which precipitation occurs in the cold season do not seem to be conducive to lateri-tization even if the temperature is above 20° C .

At the opposite extreme there does not seem to be any upper limit to precipitation. Latérites with indurated horizons are to be found where annual precipitation is in excess of 4,000 m m . In lower Guinea, for example, where the climate is tropical, latérite is well defined at an annual precipitation of more than 6,000 m m .

Incrustation, for its part, is intimately related to precipitation in so far as it affects the water regime of the soils. In relation to ferruginous and also m a n -ganiferous crusts, immobilization of the cementing materials is strictly related to the redox factor—the reduced forms of iron and manganese are more soluble than the oxidized forms, and they readily form complexes with the products of biological activity, to give rise to soluble components with the normal p H of the soils. Active mobilization of iron and manganese in a humid medium gives way to abrupt immobilization in a dry medium. These optimum conditions are combined in semi-humid and humid tropical climates, where the spread of ferruginous crusts is greatest, and extends beyond the 750 m m . isohyet in relatively well-drained regions, and sometimes even the 540-500 m m . isohyet in poorly drained regions.

The problem is different in relation to aluminous crusts. Laboratory studies demonstrate that the genesis of gibbsite, the hydroxide of aluminium most frequently encountered in bauxitic crusts of pedological origin, is intimately related to excellent drainage conditions and intense leaching of cations and segregated silica by warm waters. These conditions are to be found where annual precipitation is at least 1,200 m m . in a tropical environment, on sites conducive to internal drainage, and therefore in irregular terrain, always considered in relation to the nature of the rocks in course of alteration, since some structures are more conducive to drainage than others, and some rocks have a lower silica content than others.

Conversely, an equatorial climate with semi-stable humidity is conducive to the neosynthesis of clay. Aluminous crusts are rare at the present day and are restricted to a few specific rock faciès.

In conclusion, incrusted ferrallitic soils (ferruginous, or ferro-aluminous, or aluminous) are formed below the 1,200 m m . isohyet in accordance with the precipitation pattern and the amount of water reaching the soil, and non-incrus-ted soils are formed in accordance with the composition of the percolating solutions and drainage conditions. Various incrusted or concretionary ferruginous, and sometimes manganiferous soils, which differ from lateritic soils, are formed between the 750 m m . and 1,200 m m . isohyets.

VEGETATION A N D F A U N A

The role of vegetation has also long attracted the attention of research workers. Going back as far as 1900, mention m a y be made of the names of Kôppen, Lang and Richtofen. Glinka (1927), followed by Erhart (1935), was of the opinion that latérite could form only beneath forest vegetation, and became indurated after disappearance of the forest cover. The effect of disappearance of forest is stressed in many studies (Blackie, 1949; Aubert, 1950; and recently

61


Alexander and Cady, 1962). There is little dispute on the phenomenon, and discussion has been centred mainly on the number of years required for induration or outcropping. Conversely, other studies seem to indicate that reafforestation has an effect on the disappearance of indurated horizons (Rosevear, 1942).

Once again the problem is poorly stated. If one sticks to the facts, it is found that the extent of present-day lateritic soils corresponds to that of the forest, taking as a definition of forest wooded areas with close crowns. The height of the stand is not a factor, since lateritic soils in course of formation are to be found under ombrophil forests and also under xerophytic forests, where the crowns are no more than 10-15 m . high.

It seems that the role of forest m a y be particularly that of protection. Close crowns filter the sun's rays and protect the soil against over-powerful insolation and excessive drying out.

Theoretically savannahs do not give rise to lateritic soils. Nevertheless they are often the site of latérites, but in this case they are a forest degradation faciès (Aubreville, 1949). Leached tropical ferruginous soils are formed in the main under the Sudanese savannahs, which are probably climatic (Maignien, 1958).

O n herbaceous savannahs the herbaceous layer promotes the formation of a surface humic horizon of some depth which is conducive to the mobilization of the sesquioxides of iron and therefore leads to concretion. D'Hoore (1954, 1963) has demonstrated that iron has been mobilized to a greater extent and to greater depth beneath herbaceous savannahs than beneath forests. It would also appear that in some cases herbaceous vegetation can attack kaolinite to assimilate silica (D'Hoore, 1954; Fripiat et al., 1954). Ash with a high content of this element increases the silica content of the upper soil horizons (D'Hoore, 1954). It is for this reason that phytoliths are sometimes present in considerable quantity in the surface horizons of lateritic soils (Delvigne, 1963 ; Riquier, 1960).

The evolution of organic matter under savannahs reveals the processes of leaching, which are far more active than under forests. The soils become discoloured and yellow, at least at the surface, and the iron is segregated as concretions and even sometimes as crusts at depth.

If one considers present-day indurated formations, it is apparent that ferruginous crusts are far more widely distributed under savannah than under forest. Concretions are observed in the main under forests, and m a y be very abundant, even under ombrophil forests. W h e n hardpans are encountered, they are either sheet crusts related to fluctuations in static ground-water level along the edge of a drainage axis, or an incrusted bed at a fault or on a terrace in the relief. These occurrences are always limited in extent. Since the incrustation developed the forest has disappeared and been replaced by herbaceous savannah. The clearings with incrusted soils to be found in extensive forests are the traces of former levels.

Ferruginous crusts assume a variety of forms under savannahs, although extremely dense, hard, dark-brown pisolitic crusts predominate. This general tendency to incrustation is related to the ease of mobilization of iron under savannah. Incrustations are to be found at various depths, but not below 2 m . The depth of the horizon affects the appearance of the vegetation. As the crust approaches the surface, the trees become lower and branch near the ground. As this happens the various tree species become isolated in groves related to termi-taria. Extensive clearings develop and are invaded by graminaceous plants

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and sedges. These clearings have a distinctive appearance owing to the abnormal development of 'mushroom' termitaria.

Although vegetation would appear to play a considerable part in the development of ferruginous crusts and concretions, it seems to play only a restricted role in the formation of deep aluminous crusts, which seem to be affected mainly by the physico-chemical conditions of the alteration medium.

The reafforestation of crusts is an open question. Although there are observed cases of the reinvasion by forest of ancient incrusted areas in course of dismantlement, attempts to plant sub-outcropping crusts in course of formation have usually met with setbacks before it was possible to decide whether the factor involved was an unusually unfavourable medium or the selection of unsuitable species. Slash fires undoubtedly have a not inconsiderable effect on the disappearance of wooded cover on incrusted soils. Mention m a y here be made of an extremely interesting experiment conducted for more than thirty years near Bouaka in the Ivory Coast on concretionary red ferrallitic soils. Three plots, each of 1 hectare, were marked out in a forest region of long-standing cleared by m a n in the course of the last century. One was completely protected, another was burnt over at an early stage, and the third at a late stage. Forest species have established themselves on the completely protected plot and the gap is gradually closing. The plot subjected to early fire has the appearance of an herbaceous savannah and exhibits all the characteristics of a 'bowaF with an outcropping indurated level cemented by ferruginous solutions mobilized by graminaceous plants.

It therefore follows that there are reciprocal interactions between vegetation and the formation of lateritic and incrusted soils.

The protective effect of vegetation against run-off plays an equally important part. Run-off is always significant in the intertropical zone, whatever the nature of the plant cover. Sheet run-off, which occurs even beneath ombrophil forest, is the most frequently encountered form (Rougerie, 19586). This run-off is dependent more on the intensity of precipitation than on the porosity of the soil, which is often less than the rate of rainfall, which can attain 6 m m . / m i n . (Four-nier, 1960). However, even if vegetation does little to check run-off, its effect on erosion is far from negligible. It slows down the rate of movement of the surface water and reduces its eroding effect. This applies to forest vegetation as well as to herbaceous plants. However, since herbaceous plants are destroyed in the main by slash fires, the soil is completely denuded at the beginning of the rainy season and susceptible to erosion. The soft surface horizons of incrusted soils are gradually thinned, and indurated levels begin to outcrop. There have been m a n y studies of the rate of these phenomena. The annual erosion of highly cultivated soils m a y reach 5 m m .

Another quite significant effect of vegetation is its action on alteration processes in the superficial horizons of lateritic soils. Vegetation influences the content of coarse elements (quartz debris and fine gravel), which become concentrated as a result of the removal of fine material by run-off, and also produces some homogeneity in these materials by the mechanical action of roots. The part played by roots in the alteration of crust blocks has also been noted by de Chetelat (1938), who points out that animals can also be responsible for some alteration in situ.

The part played by the soil fauna in the alteration, textural selection and homogenization of the surface horizons of tropical soils is often considerable.

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Attention is now being directed to the role of termites in the formation of crusts (Erhart, 1951), the morphology of some crusts (Tessier, 1959) which fossilize old termitaria, and the formation of stone lines (De Heinzelin, 1952; Ruhe, 1954), where it is possible that the superjacent levels m a y be the work of termites. Chevalier (1949), has noted the role of termites in the formation of neosols on outcropping crusts. Similar eifects have been noted by Boyer (1956). Other animals m a y also play a significant part. This applies in particular to earthworms, which transform the upper part of certain soils in Central Cameroon and the east coast of Madagascar.

The effect of soil fauna on the pedogenesis of lateritic soils has, in fact, been little studied. This is a field for research which should be developed, without forgetting the part played by the microflora and by the soil bacteria, which have an influence on some pedogenetic processes (sesquioxide mobilization, alteration of silicates, clay leaching, etc.).

ROCKS A N D ORIGINAL MATERIALS

With the possible exception of some particularly pure quartzites, indurated latérites and lateritic soils are to be found on all types of rock. Nevertheless, this has long been a matter of dispute. The existence of lateritic clays derived from the alteration of reef limestones has recently once again been called in question in Haiti (Butterlin, 1961). In practice the question becomes one of the possible relations between latérites and subjacent materials. It has to be decided whether or not latérites are derived from the alteration of subjacent rocks. If so, what are the relations involved, and if not, what are the processes involved?

In general it would appear that the processes of lateritization (alteration and incrustation) are more intense and more widespread on basic rocks than on quartz-rich acid rocks. However, these tendencies are masked by other factors, especially geographic and temperature factors. Because of the ease of alteration of such basic rocks as basalt, norite, and amphibolite schist, it was long considered that these were the only rocks that could give rise to latérites. There was even mention of a volcanic origin for latérites in the literature. However, indurated and soft occurrences of latérites are to be found on such acid rocks as granites, granulites, gneiss, sericitic schists, and arcóse schists, although these phenomena are more restricted than on basic rocks. Sedimentary rocks can also give rise to latérites. Although siliceous rocks with a low content of bases exhibit reduced alteration processes, it is also possible to encounter processes of incrustation by ferruginous impregnation due to solutions which have derived sesquioxide in adjacent formations (Maignien, 1958). Indurated latérites and lateritic soils have also been noted on calcareous rocks (Stephens, 1946). However, unless these rocks occur under conditions of favourable drainage, evolution is towards vertisols, especially when the original material is argillaceous (Paquet, Millot and Maignien, 1961). The horizons of ferruginous and sometimes slightly m a n -ganiferous pellets also to be found in these vertisols are strangely reminiscent of nodular latérites.

The approach to the problem once again calls for a distinction between soils with ferruginous crusts, those with aluminous crusts, and lateritic soils sensu

stricto. Lateritic soils can develop on all rocks containing aluminosilicate minerals. It also seems that there is a fairly strict relation between the percentage of these minerals that can be classified as 'subject to lateritization' and the

64


quantity of dissolved quartz. More or less decayed quartz grains are found in many ferrallitic soils. There would appear to be a connexion between the intensity of this alteration and the quantity of ferromagnesian minerals present. In addition, the alterability of quartz is directly proportional to the magnitude of internal stresses within the rock since crystallization. Thus, the quartz of melanocratic gneiss m a y disappear almost entirely, whereas that of granulites is particularly well preserved. Stresses create fine diaclases which are conducive to the penetration of ferruginous solutions which loosen the mass and carry away fine elements. These mechanisms increase the surface open to attack and, therefore, the scope of hydrolysis.

White mica (muscovite) is highly resistant to lateritic alteration (Pécrot et al., 1962), whereas the clays of sedimentary rocks m a y be profoundly altered. The pioblem arises in relation to kaolinite, the alterability of which in tropical soils has been disputed. Some authors relate decrease in the Si02/Al203 ratio to destruction of kaolinite (Lacroix, 1913; Segalen, 1957). However, there are observations to indicate that this mineral is extremely stable when acidity and unsaturation are pronounced in a tropical humid environment. The only positive information on this alteration is supplied by electron micrographs which show some attacking of kaolinite crystals by the action of herbaceous plant complexes after forest (D'Hoore, 1954). However, these processes would appear to be reduced and not related to the scale of the enormous masses of gibbsite encountered in some lateritic soils.

Earlier studies tend to indicate that the relative proportions of kaolinite and gibbsite are related to the nature of the rocks. However, more recent data demonstrate that this is not so and that these proportions are essentially a function of drainage conditions and of the ionic content of percolating waters. The part played by the rock is to define the scope of transformation (Pécrot et al., 1962).

Is lateritic alteration responsible for ferrallitic soils? There are m a n y tropical soils which exhibit the morphology and analytic characters of lateritic soils, but which develop on formerly lateritized and more or less altered materials. This applies in particular to the blanket soils of Black Africa. The original material is in climatic equilibrium with the natural environment. N o alteration or transformation is apparent. This proves that the mineralogical composition of the original material is of limited importance in the composition of latentes. The specific characters of the materials are obliterated by lateritization.

Turning now to soils with aluminous crusts, it can be noted that although the indurated formations are strictly related to the process of lateritization as previously expounded, they also correspond to the specific conditions of the medium, namely accelerated drainage, deionization, and harsh desilicification. Only under these conditions can gibbsite, which is the most constant constituent of aluminous crusts, form. Such formation usually takes place on basic rocks (norites, diabases, etc.) in fairly irregular relief and with a very humid tropical climate. Nevertheless, periods of dryness, even of short duration, are not indispensable to the formation of gibbsite (Herbillon and Gastuche, 1962). In general there are strict relations between the composition of the subjacent rocks and aluminous crusts. In particular rocks with a high plagioclase content yield gibbsite most readily.

Cases of enrichment by alumina in the form of boehmite m a y be observed. In this particular case the relations with the subjacent material are less clear. Crystallization does not occur in situ, as in the case of gibbsite, but after more

65


or less prolonged transport. This is a factor which relates this type of crust to ferruginous crusts.

The conditions under which ferruginous crusts are formed in relation to the nature of the rock are far more extensive than those of lateritic alteration. They are to be encountered on a wide range of materials and, although there m a y often seem to be some relation between the horizons of a single profile and the encrusted horizon, the relations are more frequently distant. The soil in situ is merely a receiving material in which solutions enriched with iron at the contact with adjacent formations accumulate. Although excellent examples of ferruginous crusts are to be found in soils which develop on rocks with a high content of ferruginous minerals, ferruginous indurated horizons are also to be found in soils formed on materials with a low iron content, as a result of oblique or lateral accumulation. These crusts are absolute accumulation crusts as denned by D'Hoore (1954). In the last analysis it is the capacity of the terrain to supply iron which governs the intensity of incrustation in zones of reception. Iron moves in pseudo-solution with percolating waters. In their totality these partial migrations account for the generalized movement of iron down the relief.

RELIEF

There are two main questions to be considered when studying the relations between latérite and relief: what are the relations with different relief forms, and what are the relative positions of the various relief forms ?

In answering these questions it is necessary to distinguish between lateritic alteration soils with or without aluminous crusts and all soils with ferruginous crusts.

Relations with the various forms of relief

The pedological literature is united in the opinion that latérite occurs principally on flat surfaces or gentle and, at the worst, monoclinal slopes (Newbold, 1846; Oldham, 1893; Lacroix, 1913; Holmes, 1914; Campbell, 1917; etc.).

This characteristic is to be noted wherever latérite is found, but especially in India, Australia and Africa.

There are, however, certain discordant notes to limit the general applicability of this assertion. De Chetelat (1938) has recorded bauxitic latérites on hill relief in a schistose region of Guinea. Latérites exhibiting the Buchanan faciès are encountered on small charnokite hills in south-western Ceylon.

These apparently contradictory facts can be reconciled only if a distinction is once again drawn between lateritic alteration soils and incrusted soils.

Lateritic soils are never found, at least in their juvenile stage, in a low, poorly drained position. They are well-drained soils, in relation to both external and internal drainage. As already noted, they develop under conditions of high precipitation and forest vegetation. These factors affect the formation of a 'hill relief and especially of a series of convex slopes which facilitates run-off.

However, some pedogenetic evolutions related to the formation of hardpans m a y profoundly modify this geomorphology. These interferences are more or less pronounced, according to whether the crust is aluminous or ferruginous. Only lateritic alteration soils can, under certain conditions, give rise to aluminous indurated horizons. This type of incrustation, which is mainly gibbsitic, is found

66


o n the best drained parts of bill reliefs, i.e., especially o n the s u m m i t s a n d the slopes leading u p to t h e m . T h e y occur far less frequently at the foot of the 6lope. I n the majority of cases they are 'alteration horizons' of pain d'épices faciès, w h i c h h a v e evolved t o w a r d s a scoriaceous crust. O u t c r o p p i n g of these h a r d e n e d horizons as a result of erosion of the soft subjacent horizons b y ordin a r y erosion or h u m a n intervention fossilizes the f o r m s of encrusted relief. F o r e x a m p l e , in the D a l a b a region of G u i n e a , there are slopes w i th a gradient of m o r e t h a n 2 in 10 w h i c h h a v e b e e n entirely encrusted b y the alteration a n d transformation in situ of dolerite to bauxites. A l t h o u g h these crusts are far less h a r d t h a n ferruginous crusts, they are sufficiently resistant to the agents of erosion to p r o m o t e gradual inversion of the relief, since the adjacent soft formations are r e m o v e d m o r e rapidly t h a n the indurated formations. Superficially the partial dissolution of their c o m p o n e n t s produces a m a m m i l l a r y texture o n the surface. A l t h o u g h there is a resulting t e n d e n c y for the u p p e r parts of the relief to be levelled, these mechanisms are not very pronounced, and in general the relief of lateritic alteration soils and that of lateritic soils with aluminous crusts are not specifically related to subhorizontal forms, but rather to convex relief forms (Fig. 4).

Al2o3+Fe2o3

LU. Fe,0 2"3

Fe + Mn

Crust developing + 3m.

Fe + Mn

Alluvial plain

Presently fiydromorphic level

F I G . 4 . Distribution of cuirasses in the relief features.

A : H i g h plateaux (Guinea).

B : Niger Valley (region of K a n k a n ) .

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The problem is quite different for soils with ferruginous crusts. These formations are typically related to a subhorizontal topography, with gradients invariably less than 8-9 per cent (D'Hoore, 1954; Maignien, 1958). Ferruginous crusts m a y be found on all forms of terrain with gradients less than those just cited (plateaux, plains, alluvial terraces and structural ledges, whatever their extent). This distribution is related to the fact that, dynamically, the iron which incrusts certain horizons is related to the movement of soil water. In tropical regions percolating waters circulate obliquely or laterally at various, but never great, depths (a few metres at the most). The effect of water, whether underground or not, or suspended or not, is of prime importance, since the mobilization and migration of iron are affected by variations in redox potential, largely dependent on the level of water saturation and on the water regime of the soils.

The intensity of ferruginous incrustation reflects the capacity of a catchment basin to supply a certain amount of iron which accumulates in subhorizontal sectors which function as reception zones. Therefore any subhorizontal form is capable of becoming incrusted. W h e n incrustation has occurred, the normal processes of erosion will tend to expose these formations. W h e n exposed the crusts offer considerable resistance to erosion and fossilize the forms where they occur. These subhorizontal forms increase in number, and m a y sometimes extend almost throughout the terrain, as is the case in certain regions of Black Africa. This evolution does not imply that the whole terrain was originally subhorizontal, but that the successive subhorizontal forms have become incrusted and preserved in the course of geomorphological evolution, and have therefore progressively encroached (Fig. 5).

B y contrast to the development in ferrallitic alteration soils, oblique and lateral accumulations of iron condition the formation of concave forms. The long slopes encountered have gradients of 8-9 per cent above and gradually level out towards their foot to terminate in an incrusted ledge above a drainage axis. Stepped occurrences are typical of this form of incrustation.

This relief can form in regions of lateritic alteration and in those of aluminous crusts. The subhorizontal sections of these reliefs m a y be impregnated by ferruginous solutions, incrusted and fossilized. Since this type of alumino-ferruginous crust is more highly indurated than the purely aluminous type, fossilized forms are better preserved, and m a y increase in number, as in the case of ferruginous crusts. A succession of small, suspended, incrusted ledges festoons the slopes. These crusts do not necessarily correspond to successive surfaces of erosion, and are often contemporary.

In conclusion, the subhorizontal topography of indurated latérites is essentially related to evolution of the sesquioxides of iron. Lateritic alteration soils and soils with aluminous crusts are to be encountered on steeper slopes, but the existence of slightly irregular relief, which produces stable surfaces over long periods, is conducive to these types of incrustation (Mulcahy, 1960, 1961).

The relative position of crusts in the relief

Indurated formations occur in the terrain in various topographic positions. Blanford (1859) was led to classify Indian latérites as 'high level' and 'low level', to distinguish the indurated occurrences found capping the summits of hills and plateaux on the highlands of central and western India from those which developed at low level along the coast. It was soon noted that these

68


F I G . 5. Cuirasse formation through lateral leaching.

A : In a ferrallitic environment.

B : In a hydromorphic environment.

C : O n the edge of a valley.

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differences of topographic position corresponded to morphological differences. Thus, low-level crusts are usually more ferruginous and less thick than those of higher altitude reliefs. O n the other hand, low level crusts frequently contain allochthonous materials (sands and shingle), indicative of their polycyclic or detrital origin (Oldham, 1893). Campbell (1917) took into consideration the important part played by ground waters in the genesis of latérites in regarding low-level crusts as 'live' crusts, because they were still subject to the influence of these waters. H e regarded high-level crusts as 'extinguished' since the ground waters had disappeared as the base levels became lower.

However, this information only indicates general tendencies, and contradictory data can be noted in detail. Harrison (1933) noted live latérites in high altitude reliefs, and Ruhe (1954) states that high-level crusts are conglomeratic (presence of stones and pebbles).

The thickness of crusts is not always a function of topographic position. This arises in particular from the importance of lateral accumulation by water draining along faults. In these locations crusts are always thicker (sometimes more than 10 m . ) than in the centre of plateaux. A cross-section reveals a characteristic bevelled form (Maignien, 1958). It is no less true that lowering of the base level tends to deepen the alteration front of lateritic soils and increase the thickness of the crusts which develop. These phenomena are related to the position of the crusts relative to the drainage axes rather than to their absolute altitude. These differences are accentuated by the fact that the crusts of lower levels always have a higher iron content, and that, in general, ferruginous crusts are always thinner than lateritic alteration crusts. Comparison of thickness is therefore only possible for crusts of the same pedogenesis.

Slope-bottom crusts are often distinguished in addition to high-level and low-level crusts. They are in theory detrital crusts at the foot of formerly encrusted hills, and are formed by the consolidation of latérite fragments derived from landslips at higher levels.

This type of crust would seem to be far less prevalent than the literature suggests. The dismantlement of crusts and the transport of residual materials is a question that is open to dispute. Although these processes are clearly defined in arid regions, where they lead to the formation of 'causeway soils' {sols de

chaussée) and regs, the phenomena of sesquioxides dissolution gain the upper hand over mechanical phenomena when annual precipitation exceeds 500 m m . It has been shown by observation that, under these conditions, lateral transport of incrusted material sliding along the slopes as a result of undercutting of the subjacent soft formation is never very long and is of the order of a few tens of metres. The grading of increasingly fine products is restricted to a narrow fringe at the foot of steep slopes. Sesquioxides are rapidly dissolved and subsequently carried away by run-off and percolation. They are immobilized in the lower zones, where they give rise to new impregnation crusts. The fact that these are sometimes rounded in structure m a y be deceptive. In fact this structure is produced only by a type of precipitation in a medium relatively saturated in alkaline-earth bases or in a fairly argillaceous medium (Maignien, 1958).

In a humid climate hardpans are eliminated in situ by collapse of the mass (Fig. 6). Lateral transport of solid material is reduced, and the life of dissolved products is fairly brief and related to the climate. In a climate of the Sudan type (annual precipitation 500-1,200 m m . ) sesquioxides m a y be rapidly i m m o bilized and m a y incrust the soils of adjacent lower levels. In an equatorial

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Unconsolidated

ground

Slope Slope 5 per cent 2 per cent. Erosion 70 cm*

Cuirasse under dissection

B Subsidence

Ledge Cuirasse

Gravelly ground

Remains of former

cuirasse

FlG. 6. Morphogeny in cuirasse regions.

Bovrat

? ^ Relative accumulation Colluvia

of M compounds

Absoluts accumulation

of Fe compounds

Valley

F I G . 7. Different modes of sesquioxide accumulation in relation to relief features.

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humid climate sesquioxides remain mobilized and m a y be carried away towards inland basins or towards the oceans, where they are deposited. Therefore, the relative position of crusts across a given terrain is merely an expression of selection of different sesquioxides in relation to their capacity for mobilization. Differential leaching occurs. Manganese and iron migrate first and furthest in this order. Alumina is evolved far more slowly. As a result crusts of high reliefs, especially if the massifs are greatly dissected, have a higher alumina content than those of low reliefs, not because the phenomena of lateritic alteration were intially different, but mainly because iron has been eliminated more rapidly than alumina in the course of time (Fig.7). There is a relative increase of this element, even if the general balance reveals significant losses. It does not matter that comparable alteration phenomena m a y occur on surfaces of different age, if climatic conditions are identical. The specific conditions of immobilization of the sesquioxides of iron ensure that most ferruginous crusts occur along faults in the relief. They have been given various names in the literature, including sheet crusts and gallery crusts. These crusts mould the drainage axes and form indurated aureoles at spring heads. The importance and intensity of the phenomena are related to the quantity of sesquioxides evolved in the zone under consideration (Fig. 8).

1

2

Marshy area and formation of a cuirasse

Lowering of the base level of erosion

Cuirasse Grotto

FIG. 8. Migration of sesquioxides and formation of a cuirasse.

Finally, one last point must be noted concerning lacustrine latérites (Fermor, 1911), which develop in swamp zones. Incrusted beds have also been noted along mangrove swamp zones.

The present-day topographic position of crusts often appears to be the result of inversion of the relief. Incrusted zones have protected the zones in which they developed while erosion has lowered the surrounding zones. Old crusts occupy the highest positions, and are gradually reduced by lateral retreat of the slopes. The products of their dismantlement accentuate the incrustation of low-lying zones in course of lateritic alteration in humid regions, or contribute to the incrustation of non-lateritic soils on a variety of rocks in drier regions.

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AGE

Study of the topographic position of latérites in the terrain demonstrates that many crusts are old. This is particularly apparent in arid regions, where present conditions of pedogenesis do not allow of the formation of these occurrences. In this connexion it can be noted that the European iron ore formations are probably former altered latérites.

In Australia the indurated latérites of Queensland are probably the products of the two humid periods of Pliocene times (Whitehouse, 1940). In Ceylon most of the crusts are probably of Pleistocene and Pliocene age or even earlier (Fernando, 1948). Ruhe (1954) dates the latérites of the Ituri (Congo) as Middle and Late Tertiary. In the Sudan and in Niger the oldest crusts are Rissian. In the forest regions of Guinea the crusts of the upper surface are Pre-Kamasian, and therefore Palaeolithic (Schnell, 1949). In Senegal and Mauritania, the ferruginous crust which caps the formations of the Continental Terminal are Villafranchian. Michel (1960) is of the opinion that the first levelling surface of the basins of Senegal and Upper Gambia is Lower Cretaceous, while the second is Eocene and the lower ferruginous crust is Neogene.

Cross-comparison of the work carried out in Africa tends to indicate that the first major period of lateritization is of Tertiary age, and developed on a Cretaceous levelling surface. This extremely long period must have had an extensive geographical distribution, and it is highly probable that it extended into Europe. In Black Africa there are numerous remains of this surface, and they have been better preserved in sub-arid and subhumid regions than in humid regions. In the forested region they are encountered only sporadically. Leneuf (1959) dates the crust of M t . Orumboboka (Ivory Coast) at 42 million years, which makes it of Oligocène or Eocene age.

The slow evolution of old crusts has sometimes prompted the idea that lateritization is a phenomenon of the past. There are, however, m a n y known cases of latérite of present-day formation (Fermor, 1911; Simpson, 1912; Lacroix, 1913; Campbell, 1917; Marbut, 1932; Harrison, 1933; and others). The phenomena of lateritization have probably been permanently in play in intertropical humid regions since the Tertiary period, but at a fluctuating level of intensity corresponding to climatic conditions. The effect of the phenomena has been cumulative. Leneuf (1959) is of the opinion that 'in the most humid conditions of the Ivory Coast 20,000 to 77,000 years would be necessary to ensure theoretically complete ferrallitization of a calc-alkali granite for a thickness of one metre. In the less humid central forest zone 53,000 to 192,000 years would be required'. It is known that climatic fluctuations are sometimes quite significant in periods as long as this.

Since the processes of lateritization have affected thicknesses that are often quite considerable, it is conceded that the final product reflects merely the total of the mechanism specific to each climatic period. It is therefore very difficult to relate the objects studied to environmental factors. Nevertheless there is much observational material which demonstrates that lateritization is to be found on surfaces of different age.

In order to arrive at a better understanding of the distribution of latérites as a function of surfaces of erosion, it is necessary to consider the domain of lateritic alteration, namely tropical climates with mean annual precipitation in excess of 1,200 m m . In these climates, as well as equatorial humid climates,

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all surfaces, whatever their age, m a y become lateritic in the sense of alteration. O n the other hand the processes of incrustation occur only in tropical regions with alternating seasons. In the humid tropics incrustation is restricted to specific pedogenetic conditions.

In drier tropical climates, lateritic alteration soils with aluminous crusts are to be found, if they exist at all, only on old surfaces. They are therefore a reflection of former climatic fluctuations. The soils on recent surfaces are différent. In Black Africa, for example, they are ferruginous tropical soils, which m a y be highly incrusted as a result of the dismantling of old surfaces with a high sesquioxide content. The sesquioxides accumulate, as already indicated, in the lower levels and cement them. The predominance of old crusts indicates that the general evolution of the climate has been towards drier conditions. The absence of an old surface in forest regions does not indicate that these crusts have not existed, but that it is possible that formerly encrusted occurrences disappeared when climatic conditions became more aggressive, i.e., when there was development towards a hotter climate and increased precipitation. It ¡6 possible, by studying the different types of crust and their relative distribution in a given terrain, to reconstruct the palaeoclimatic history of the region.

It would seem to be well established that the phenomena which contribute to the formation of latérites need to be active for very long periods. O n the other hand the formation of ferruginous crusts seems to take place far more rapidly. These crusts are also highly sensitive to climatic variations. Nevertheless, caution must be exercised in relation to calculations, in which there is often confusion between incrustation and the outcropping by erosion of indurated horizons. Thus, for example, it has been estimated that the outcropping of ferruginous crusts which developed at a depth of between 1.5 and 2 m . in the Casa-mance region must have taken approximately three to four centuries.

It is equally possible for confusion to exist between the individualization of lateritic horizons and induration. Alexander and Cady (1962) note that the induration of soft material preconditioned by lateritization m a y take place in a few dozen years of exposure to air. This is, moreover, the essential characteristic of latérite as denned by Buchanan.

These points are of prime importance since they affect the utilization of soils. It is improbable that lateritization can be affected, at least on the human scale. O n the other hand it is possible to prevent the erosion of the soft horizons above crusts and to avoid the induration of preconditioned horizons by protecting them against insolation (i.e., by conserving the vegetation).

In conclusion, a few points must be made concerning the phenomena of hysteresis (Maignien, 1960). Some soils that have commenced a lateritic type of evolution m a y sometimes continue it when climatic conditions become less active. It has already been noted that lateritic alterations are encountered at the present day in a relatively dry climate, for example, in the Sudan to the north of Bamako. In this particular case evolution commenced under conditions of higher humidity some thousands of years ago, and is still continuing because protected from external fluctuations by a ferruginous crust which restricts desiccation. This very common occurrence in Black Africa tends to demonstrate that once lateritic alteration has commenced it is in no way dependent on the vegetation above it. This idea has previously been advanced by Claisse (1953) in the Ivory Coast.

74


Finally, it is possible to arrive at incorrect interpretations by confusing the intensity of the phenomena of lateritization with their duration. For example, in West Africa the oldest surface is generally interpreted as a sign of a very humid equatorial climate of forest type, since it supports a thick crust (more than 10 m . thick) with a very high content of the sesquioxides of aluminium. This interpretation is in conflict with the subhorizontal relief, and with the presence of significant quantities of gihbsite, which would lead one to presume the effect of a semi-humid tropical climate, similar to the present climate of the Sudan, but effective over an extremely long period. A more aggressive climate of equatorial humid type would have produced a hill relief and more pronounced kaolinitic processes (Maignien, 1960).

75

Origin of latérites

Several theories have been advanced to account for the origin and formation of latérites. Historically, three periods can be distinguished. The earliest hypotheses, dating from the first half of the nineteenth century, and based on descriptions of the materials and their mode of occurrence, relate to studies carried out in India. In the second period chemical and, to some extent, mineralogical analysis helped to widen the definition of the term latérite. This period started at the end of the nineteenth century and continued until very recently. Lastly, modern methods of experimental soil science have now made it possible to study the genesis and alteration of latérites.

Lake (1890) admirably summarizes the earliest ideas on latérite formation in India. There were three different hypotheses: (a) latérite is a residual alteration product (Babington, 1821; Benza, 1836; Clark, 1838; Wingate, 1852; Kelaart, 1853; Buist, 1860; McGee, 1880); (b) latérite is a detrital and sedimentary product (Cole, 1838; Newbold, 1844, 1846; Blanford, 1859; King and Foote, 1864; W y n n e , 1872; Theobold, 1873); some of these authors nevertheless recognize that latérites are to some extent residual products; (c) latérite is of volcanic origin (Voisey, 1833).

The starting point for each of these theories was the morphology and occurrence of latérites. Their slaglike appearance and development as horizontal masses over the basalt flows of the Deccan supported the volcanic theory. A sedimentary, perhaps even lacustrine origin was suggested by the fact that some latérites attained a thickness of 60 m . or occurred as sheets covering dissimilar rocks, or possessed high iron concentration even in occurrences over gneisses low in iron (Oldham, 1893). Holland (1903) found that chemical weathering alone could not account for the abrupt transition from latérite to subjacent weathered rock or for the absence of latérite in countries where the summers were warm but the winters cold. H e therefore suggested action by micro-organisms capable of separating silica from the alumina in silicates. Lateritization was a process to be added to 'the long list of tropical diseases against which the very rocks are not safe'.

Study of the rock chemistry produced a broader view of the problem. Glinka in 1899 (Glinka, 1927) and Holland (1903) suggested that the sesquioxide concentration was due to the removal of silica and bases. This hypothesis was taken up by mineralogists, especially Lacroix (1913). Several authors postulate the probability, if not the necessity, of sesquioxide accumulation from outside

76

Origin of latentes

sources (Maclaren, 1906; Simpson, 1912; Campbell, 1917). Vine (1949) suggested an aeolian origin. The majority of investigators, however, suggested that accumulation was due to movements of the ground water (raising of the water-tables, lateral movement of percolation water, or both).

The accumulation theories were synthesized by D'Hoore (1954), who pointed out that two processes could be distinguished : (a) concentration of sesquioxides by removal of silica and bases, or relative accumulation; (b) concentration of sesquioxides by accumulation either across the profile or between profiles—absolute accumulation.

Today, new analytical methods, progress in soil-constituent determination and the achievements of experimental soil science are making more refined studies possible and hypotheses are becoming daily more realistic.

It has become clear that the origin of latérite cannot be due to a single process. Several phenomena are involved, not all operating at the same level of differentiation. The convergence of certain faciès, in particular the hardpan faciès, has sometimes given rise to the idea that the problem relates to a single level. Yet there are fundamental pedogenetic differences between aluminous and ferruginous crust formation, even though one m a y sometimes interfere with the other. This means that the problem of latérite origins must be approached, on the one hand, in terms of lateritic soils, whether they have a hardpan or not, and, on the other hand, in terms of hardpan tropical soils, whether they are lateritic or not.

In the preceding chapters w e noted the physical, chemical and mineralogical characters of all formations which could be called latérites, together with their environmental conditions. A more or less exhaustive list of latérite components was given, together with an account of their distribution in the profiles. The next problems are, first, how these components originate, and, second, h o w they develop in the course of pedological history.

ORIGIN OF LATERITE C O M P O N E N T S

W e m a y briefly recapitulate these components: aluminium sesquioxides, principally gibbsite, more rarely boehmite; iron sesquioxides, particularly goethite and hematite; clays, mainly kaolinite, often mixed with a little illite, and a series of aluminous and aluminoferruginous amorphic products, to which there is an increasing number of references in the pedological literature; lastly, residual (inherited) or detrital (contamination) materials in varying proportions.

These constituents m a y occur in an already more or less individualized condition in sedimentary formations. W e shall deal with this problem later, for originally all the materials come from the alteration of primary igneous-rock minerals. W e must therefore postpone consideration of this point until w e deal with latérites themselves, 'since lateritic alteration can be defined as a process of total decomposition of the rocks, leading to the concentration of iron, aluminium and titanium oxides and hydroxides after the leaching of bases and silica' (Herbillon and Gastuche, 1962).

77

Origin of latentes

ALTERATION PRODUCT COMPONENTS

Alteration of original minerals (Bonifas, 1959; Précot et al., 1962)

Olivine. Olivine is the first mineral to alter. As a result of lateritic alteration the crystals turn into patches consisting of substances which are yellow in natural light and isotropic, but brown at the edges and in the former cleavages. At least a portion of these substances develops into geothite. It could perhaps develop partly into a mineral of montmorillonite type before turning into goe-thite. The conversion of olivine into chlorite cannot be attributed to this type of alteration.

Antigorite. The bulk of the antigorite develops into yellow to brown-yellow ferruginous products which give rise to cryptocrystalline goethite.

Magnetite. This mineral is thought to be very resistant to lateritic alteration. Nevertheless, in the residual matter magnetite is usually surrounded by a border of rusty-brown iron oxides which turn into hematite and even into goethite.

It is possible that the transformation of magnetite into maghemite is a process specific to tropical alteration (Masson, 1943).

Ilmenite. This mineral seems even more stable than magnetite (Harrison, 1933), but areas with a less metallic sheen and the characteristic crystal-skeleton faciès are sometimes observed at the edges of the crystals.

Chromite. This again is a mineral extremely resistant to alteration; we can nevertheless not exclude the possibility of partial alteration.

Felspars. Plagioclases (oligoclase-labrador). The most usual alteration consists in penetration of the plagioclase crystals by isotropical rust-coloured substances which settle as an edging or penetrate into the cleavages and cracks. These substances give rise to small gibbsite crystals which gradually invade the crystal as small accumulations of brown filaments at the edge of or inside the crystal. B y the time the process is complete all that is left of the felspar crystals is a web or skeleton of crystalline gibbsite. Gibbsite is the first alteration mineral with well-defined crystalline characters, but the substance which replaces the plagioclase matter is optically indeterminate and is not gibbsite.

Another type of alteration that can occur is the conversion of plagioclase into an abundant mass of isotropic ferruginous substances within which a small amount of gibbsite appears. This mass then gives rise to goethite and to kao-linite with an 'accordion' type of facies. At this stage the matrix is kaolinite and the original structure has collapsed.

Alkalifelspars. Albitized orthoclase can turn into gibbsite which crystallizes in the cleavages and then takes over the entire skeleton of the original crystals, leaving numerous spaces (pain d'épices facies). Alternatively the felspars appear to turn into kaolinite but without observable direct replacement of the crystal form.

Nepheline. This mineral alters into yellowish, confused bands, which might be halloysite. The alteration is very rapid and is manifested by the appearance of hollows, indicating complete hydrolysis of the crystal.

Amphibole. The minerals of this group rapidly turn into amorphous ferruginous substances which develop into goethite.

Pyroxene. These minerals alter equally quickly into ferruginous substances which turn into goethite, forming a web around patches of (a) isotropic yellow substances with a few gibbsite crystals, and (b) monocrystalline goethite.

78


Biotite. Biotite alters rapidly. First, the laminae swell and burst and at the same time iron is lost. There is no specific mode of alteration. The biotite turns into chlorite, but this in turn alters, for it is not present in the residual products. The whole mass is reduced to a fine, more or less kaolinitic powder, mixed with powdery goethite.

Seriate. Sericitic micaceous rocks turn either into kaolinitic or into gibbsitic rocks. It is not clear precisely what happens on the purely mineralogical scale.

Muscovite. Muscovite is very resistant to alteration agents. Destruction in acid shows that trioctohedral mica (biotite) dissolves approximately 106 times more quickly than dioctahedral phyllites (Gastuche, Fripiat and de Kimpe, 1962). In deep ferrallitic soils, muscovite can be found even in the surface horizons. Physical weathering occurs, pulverizing the mass into extremely fine materials which can give rise to hydromicas. The ilutes often found in strongly altered African soils m a y well be cryptocrystalline muscovite.

Quartz. Quartz does not seem to be greatly subject to alteration in the zone of primary occurrence. In horizons where the structure has been preserved (pain d'épiées), it is corroded, cracked and traversed by fissures filled with isotropic rust-coloured substances. Jagged-edged crystals of globular quartz occur also. The first stage consists of impregnation by ferruginous solutions along the lines of least resistance. These substances loosen the mass, which crumbles like a piece of sugar. The grains become finer and finer, sometimes disappearing completely, and can therefore be presumed to dissolve. Conversely, in other cases it seems that the dissolved silica can give rise to neoforma-tion quartz. In either case the probability is that these processes depend on the degree of monomolecular silica saturation of the soil solutions. At saturations of 20, 30, 40 ppm/litre the solutions are oversaturated in regard to quartz and other mineral forms of silica and are therefore in a condition to generate these minerals (Krauskopf, 1959). Conversely, the amorphization of quartz through mechanical loosening promotes dissolution (Wey, 1961).

Clay minerals. The chlorite, halloysite and montmorillonite observed in all the early stages of mineral alteration subsequently disappear in the zone where the structure of the completely altered rock has been preserved. These minerals can be assumed to give rise to gibbsite and goethite. They m a y also develop into kaolinite, but the development of the minerals themselves has not been directly observed.

Kaolinite has been the subject of numerous contradictory theories. Some authors (Clarke, 1924; Mohr, 1944; Waegemans, 1951a; Erhart, 1956) believe that it is stable during lateritic alteration. Others attribute the formation of aluminium hydroxides to its destruction (Van Bammelen, 1904; Robinson and Holmes, 1924; Harrassowitz, 1926; Martin and Doyne, 1927; Craig and Haláis, 1934; Tañada, 1951; Segalen, 1957).

Allen (1948 a, b ; 1952) believes that most basalt minerals alter directly into clay minerals : halloysite, kaolinite and nontronite, which in turn change into gibbsite and iron oxides and hydroxides through loss of silica. Similarly, Harrison (1933) and Eyles (1952) suggest that the iron and aluminium oxides and hydroxides in bauxites result from loss of silica by kaolinite in lateritic clays or in the lithomarge. Mead (1915), Campbell (1917), Fox (1923, 1936) and Sherm a n (1949,1950,1952) demonstrate or again assume the destruction of kaolinite. D'Hoore (1954) was able to photograph altered kaolinite crystals under the electron microscope. Bonifas (1959) assumes that gibbsite can be produced by

79


kaolinite. The mechanism of kaolinite transformation into fireclay in an acid medium has been demonstrated by Oberlin et al. (1961).

It therefore seems that kaolinite can be altered or transformed. These processes nevertheless seem to be fairly restricted and incommensurable with the final amounts of individualized gibbsite.

The complete alteration series of principal primary minerals, in descending order, is as follows (Précot et al., 1962).

Olivine -> calcic plagioclases -> calcosodic plagioclases -> pyroxene -> sodic-calcic plagioclases -> amphiboles -> biotite -> sodic plagioclases -> orthosite -> muscovite -*• quartz.

The authors describe the process as follows: 'The neosilicates, in which the silica tetrahedra are bonded together by highly soluble cations such as calcium and magnesium, are highly alterable. Once these cations have passed into solution, there is no further cohesion within the crystal. Olivine belongs to this category. As the chain becomes more complicated, that is to say as the silicic lattice consolidates through the multiplication of bonds among the tetrahedra, the mineral becomes more resistant to alteration; this occurs in single and double chain inosilicates (pyroxene and amphibole respectively), phyllosilicates (micas) and tectosilicate (orthoclase, quartz) where the bonds between the tetrahedra form three-dimensionally'. Murata (1946) observed the same sequence in the alteration of silicates in acid medium and further noted that the ions other than silicon are the weak points in such an attack. Measurements of the dissolution kinetics in acid medium show that ions in an octohedral layer dissolve more rapidly than those in a tetrahedral layer (Brindley and Youell, 1951; Osthaus, 1956; Gastuche and Fripiat, 1960; Cloos, GastucheandCroegaert, 1961).

The commonly observed alterations of all the most frequently occurring minerals in a lateritic medium do not mean that each primary mineral has its own alteration mineral (Bonifas, 1959). In the opinion of Correns and von Engelhardt (1938), the mechanism of alteration is hydrolysis. The silicates are completely dissolved in their constituent ions, leaving no crystal skeleton. The ions liberated by hydrolysis reform, under certain conditions of drainage, pH and ionic concentration, to produce a genetic medium more specific to one mineral than to another. There are no mineralogical affiliations between an igneous silicate and a clay mineral. O n the other hand, certain conditions giving rise to clay minerals are common in certain alteration media, such as a latérite. In the opinion of Correns and Schlunz (1936) and of von Engelhardt (1937), the felspars would dissolve completely, leaving no residue, the rate of alteration being regulated by the rate of silica and alumina dissolution. The dissolved elements would be carried away by the water. Under certain conditions, however, these ions become organized into the structures of gibbsite, goethite, boeh-mite and kaolinite. Such neoformations m a y occur outside altered crystals, but also on their sides or even within them, given the right conditions, which it is our task to define.

Origin of secondary minerals

Gibbsite. Although we have some information on the part played by iron hydro

xides (Betremieux, 1951; Fripiat and Gastuche, 1952), little is known about

80

Typical landscape of ferrallitic soils, with inselbergs (central Madagascar)

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Colluvial soil with crust debris, Guinea [see opposite]

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the development of aluminium hydroxides in soils. Some authors, including Mackenzie (1957), think that these are rapidly converted into a stable crystalline form. In the laboratory, however, the crystallization of trihydrates, particularly gibbsite, calls for conditions of high alkalinity rarely found in nature. In the soil, numerous other factors intervene. Some authors regard kaolinite as the most likely of all minerals to serve as a nucleus of crystallization. Others attribute a guiding role in the development of crystals to humâtes or other organic derivatives.

T w o very different natural media can give rise to gibbsite: A n alkaline medium : gibbsite forms in an hydrolytic medium at contacts with

the parent material. Stevens and Carrón (1948) measured 'abrasion' pH as varying between 7 and 10 for most minerals. Pedro (1961) recalls that the jpH of hydrolytic rock solutions, even those called 'acid', is always slightly alkaline. Jenny (1950) proposes a pH. of the order of 9 to explain the alteration of a felspar;

A n acid medium: in tropical soils, large amounts of gibbsite can often be observed in profiles developed 30-40 m . above the water table or parent material. This makes it difficult to account for its formation by alkalinity due to hydrolysis, since the pH of drainage water varies between 4 and 5.

Some authors have thought that the mineral synthesizes at an earlier stage when the pH is higher. It is difficult to see how this could be true of so general a phenomenon and there are many facts to show that it is not (Bonifas, 1959).

W e owe the solution of this problem to Herbillon and Gastuche (1962). At normal temperature and pressure, the presence of foreign ions in the hydrated shell of the cation entails a distortion which is already perceptible in solution. Crystallization will occur only in proportion to ionic loss.

'At low pH., the positively charged gel is surrounded by extraneous anions which have been firmly retained in the structure and crystallization is inhibited. Drying leads to the formation of a strongly hydrated and disarranged gel in which a large proportion of the aluminium is in fourfold co-ordination following perturbation due to the anions.

'At high pH. values, the negatively charged gel repels the few extraneous anions; poorly polarizable extraneous cations (Na and K ) do not appreciably disturb the structure. These gels possess a structure in which forms of Al with a 6 co-ordination predominate; crystalline trihydrate will emerge as a result of their ageing.' The determining factor in the crystallization process is thus the elimination of extraneous ions rather than a rise in pH.

Herbillon and Gastuche (1962) give the following rule:

'Whatever the intial pH. of precipitation of the gel, dialysis always, though not always with equal facility, induces the synthesis of crystalline trihydrates. Baierite accompanied by pseudobromite seems to derive from gels with a more disordered structure which have been rapidly precipitated in the zone of maxim u m insolubility. The duration of the period of induction preceding the emergence of the crystalline products is a function of the rate of disionization of the medium'. The phenomenon is thus endothermal.

'In the case of gels which have aged in a parent solution of pH 8, the action of kaolinite is to promote trihydrate crystallization and inhibit the formation of pseudobromite. W h e n the ageing occurs at a lower p H value of the parent solution, kaolinite no longer promotes crystallization. In a dialysed medium,

81


this action reappears to some extent and slightly accelerates the processes of crystallization, in the direction of baierite. '

To summarize: whatever the pH. of formation, alumina gels developing in a disionized medium crystallize into trihydrates. This disionization is accelerated by intensive leaching and by high temperature of the percolating water, the very conditions present in lateritic alteration media.

Alumina gels are present in the soils in varying amount, but escape methods of detection and can be revealed only by chemical analysis. At the 'pregibbsite ' stage, which occurs at the beginning of depolymerization, the alumina gel displays exceptional fluidity; this accounts for its ready migration and could give rise to local gibbsite formations in the géodes.

The conditions for crystalline trihydrate generation from precipitated alumina gels due to mineral alteration are thus as follows : (a) good to excellent drainage conditions of disionization; (b) high temperature of percolating water; (c) intensive desilication. These are the conditions for the formation of bauxitic latérites.

Boehmite. This mineral is sometimes associated with gibbsite in hardpans. Alexander et al. (1956) mention it in relation to the African hardpans. Its absence from many determinations is due to coarse methods of analysis. The development of the X-ray method of determination shows that this is a sesquioxide common in nature. Harder (1949) and Frederickson (1952) postulate boehmite formation from gibbsite through the action of pressure and temperature : between 120° and 400° C , gibbsite would turn into boehmite and diaspore (Weiser and Milligan, 1934). This is a good way from conditions found in nature, although temperatures of 80° C . have been measured on hardpan surfaces (Mohr, 1954).

The alteration of felspars at relatively high temperature (280°-450° C.) and under pressure can yield gibbsite (Morey and Chen, 1955; Brindley and Rodoslovich, 1956). The same applies in experiments synthesizing clay minerals at 100° C . (Henin and Robichet, 1953). At very low temperatures down to zero, Havestadt and Frike (1930) obtained boehmite through the ageing of an amorphous gel. D e Lapparent (1936) and Sabot (1954) report that bauxites from decalcified clays are rich in boehmite and that the latter can therefore be produced from the desilication of kaolinite; but Bonifas (1959) showed that gibbsite could be generated within kaolinite. D e Lapparent (1935, 1936) suggests that boehmite can form in bauxites at the water-table level in the presence of humic acid. Keller (1952) agrees with this.

Wha t does seem clear is that boehmite crystallizes only after transport, since it usually filled cracks and small diaclases.

Willstàtter and Kraut (1923) and Kraut and H u m m e (1931), studying aluminium hydroxides precipitated by alkalis and ammoniacal liquor, distinguish a readily soluble C a or amorphous gel, a moderately soluble C |3 gel and poorly soluble C y gel, which change into one another through ageing within a few weeks.

Souza Santos et al. (1953) give the following system:

Spherical Cot gel -> fibres -> somatoids (C(3 gel) -»• somatoids (Cy) Amorphous Pseudobromite gel Pseudobromite Baierite

This sequence is observed only at pQ. values above 7.4 and the process acce

lerates as the pH rises. According to Papee et al. (1953), the pseudoboehmite

82


gel which appears at high p H values is the alumina gel par excellence. Herbillon and Gastuche (1962) give a more explicit account of these processes and mention that 'baierite accompanied by pseudoboehmite seems to derive from gels with a more disordered structure rapidly precipitated in the zone of m a x i m u m insolubility'—that is, at pH 6.7.

It thus seems that the conditions for boehmite formation are less strict in regard to disionization of the medium than in the case of gibbsite, but that a p H of about 6.5 is required. The forms are often highly soluble, but they can rapidly develop in the direction of immobilization in the form of somatoids, which makes them difficult to identify visually.

Goethite. This mineral is very widespread in lateritic formations. It is most often observed in the form of brown or red powdery earths, alms or concretions. Clearly defined crystals are very rare, even under the microscope. The state of crystallization, however, can be clearly determined by electron-microscope examination. Bonifas (1959) reports that goethite starts to 'crystallize' from isotropic ferruginous substances observed at all the first stages of mineral alteration (pyroxene, olivine, plagioclase). The gels are probably amorphous, but the composition and state of these substances has yet to be defined.

In the laboratory, precipitation by ferric hydroxide ammoniacal liquor from a ferric soil gives an amorphous product which yields goethite through ageing. This process, however, takes several months. There accordingly seems to be no particular difficulty in the transformation of a ferric hydroxide gel into goethite under the conditions of a lateritic medium.

Hematite. Hematite is often regarded as the principal constituent of ferruginous hardpans. Bonifas (1959), however, points out that if the hardpans contain appreciable quantities of oligite, their average constituent is, in fact, goethite.

There are several methods by which hematite can form in nature:

1. From the dehydration of goethite by heat and insolation (Mohr, 1944); although this transformation can be produced in the laboratory at 500° C . observation shows that hematite is present at the top of profiles which contain only very little of it or none at all in the subjacent horizons. Bonifas (1959) states that 'it seems we must exclude the possibility of relative accumulation of oligate through selective solution of goethite ' .

2. Within profiles, hematite seems to derive from the alteration of magnetite, chromite and ilmenite'.

3. Hematite can be a contamination product.

Fricke and Ackermann (1934) obtained hematite experimentally through ageing, at ambient temperature, ferric hydroxide gels produced at low temperature; otherwise they usually obtained a mixture of hematite and goethite.

Quartz. The quartz grains observed in lateritic alteration soils are usually residual or detrital material. Quartz can also be produced, however, from siliceous accumulations. D e Craene (1954) was the first to draw attention to the possibilities of 'quartz neoformations ' through diagenesis, either from silicates or as a result of addition or loss caused by percolating water. The siliceous horizons found in the alteration products of dunite at Conakry are products formed in the course of lateritic alteration (Bonifas, 1959). The appearance and composition of these horizons suggests the precipitation of a silica gel contaminated

83


with iron oxides and crystallizing into quartz on ageing. These observations, of which there are many other examples, throw new light on recent findings concerning silica variability: in natural water containing silica we have true solutions of monomolecular Si(OH4), which are not affected by variations of p H or by the presence of various cations, except those of alumina. Moreover, these molecular silica solutions are undersaturated as compared with amorphous silica, for the silica tenor is less than 120-140 p p m . at 25° C . O n the other hand, tenors of 20, 30 or 40 p p m . make them oversaturated as compared with quartz and other mineral forms of silica and therefore capable of further increase in silica tenor (Krauskopf, 1959). Proceeding from these data, Millot (1961) points out that 'colloidal solutions cannot play a part: first because silication necessitates an epigénesis which colloids cannot produce, but also and mainly because natural colloidal solutions do not exist. If the solutions are clean, not heavily charged with cations and not heavily charged with silica, quartzifica-tion occurs. This is the case with surfaces which have a short water history. Each quartz grain grows by itself and the process is that of the regular growth of macrocrystals. If the solutions are impure, heavily loaded with cations and more loaded with silica, the process is disordered. This occurs with solutions which collect at depth or come from water tables. In limestones, the disordered form of quartz known as chalcedony will form: here we have cryptocrystalline growth of a highly confused and imperfect kind'.

The first case seems compatible with well-drained lateritic soils in humid tropical climates. It could well be that quartz neoformations are produced, although such effects cannot be factually proved by analytical methods. For the second case, however, w e have evidence in the silica horizons found at the contact with alteration products. These processes must have given rise to the 6tratified silica horizons found in the Marly formations underlying alumina phosphate crusts at Thiès, Senegal. The cardinal fact here is the silica content of the water circulating in lateritic ground-water tables: 15-30 p p m . has frequently been noted.

Anotase. This mineral is fairly often found in lateritic alteration products. It is regarded as the form of Ti02 which is resistant to low temperature and is consequently the most likely titanium mineral to occur in lateritization products (Eyles, 1952). It can be derived from the alteration of titaniferous silicates (sphene, biotite, augite), ilmenite or titanomagnetites (Bramlette, 1936).

Chlorite. Although not a constituent of lateritic hardpans, chlorite frequently appears as a transitional alteration product. It is both a metamorphic and an alteration mineral. Bonifas (1959) states that, in the case of dolerites, some samples suggest that chlorite formed during the first stages in the alteration of the rock. In the less altered part of the rock, partial transformation of biotite into chlorite is observed. It is difficult, however, to attribute all the chlorite of the alteration crust to the transformation of biotite, which is not abundant. According to Harrison (1933), pyroxenes could equally well turn into chlorite. The latter would be produced within the pyroxenes in the process of weathering. The hydrolysis pH of these minerals, which is high (9 to 11), is compatible with our present knowledge of the origin of chlorite (Millot, 1949, 1953; Grim, 1951). Bonifas has no doubt that chlorite is due to lateritic alteration and that all the requisite conditions for its development are present in the hydrolysis zone of

84

Origin of lateritea

dolerite minerals. These conditions are difficult to define, but it can be noted that the rock seems isolated in an environment of kaolinitic argillaceous earths and we can deduce from this that chlorite genesis is connected with that of kaolinite.

Sericite. Like chlorite, sericite is a more or less elusive transitional, secondary mineral. Leneuf (1959) states that sericite scales have been detected microscopically in thin sections of plagioclases from altered rocks as well as in certain fresh rocks, both at the surface or at depth, in the forest regions of the Ivory Coast, and therefore in a humid equatorial climate. 'This does not seem to be a manifestation specific to lateritic media. Nevertheless, sericitization would be accelerated by a moderately high temperature under favourable conditions of humidity.'

Halloysite. This mineral is frequently found in alteration products. Lacroix (1913) regards it as an alteration of nepheline. Bonifas (1959) determined it in the first stages of alteration in the syenites of the Iles de Loos (Guinea), without being able to say which mineral it had derived from.

Allen (1948) mentions halloysite among the alteration products of basaltic rocks in Oregon. Segalen (1956) reports finding it at the base of a ferrallitic profile on volcanic rocks in Madagascar.

Bates (1952) found that halloysite 4 H 2 0 could form only under conditions of very high humidity. Variations of humidity could result in a close mixture of halloysite and kaolinite without the necessity for a transformation of halloysite into kaolinite through dehydration. To effect such a transformation, renewed solution and Tecrystallization would be necessary, rather than simple growth at particle level. This is not the opinion of Hauser (1953) who regards halloysite as a form intermediate between a silica-alumina gel and a clay mineral structure.

Montmorillonite. The presence of this mineral in latérites has very often been questioned. It is nevertheless constantly reported, but as an extremely elusive product developing under conditions of slow drainage in a cation-rich alkali earth medium (Hardon and Faveyee, 1939; Hosking, 1940; Nagelschmidt et al, 1940; Edelman, 1947; Millot, 1949; Grim, 1953). It has been described as occurring in the hydrolysis zone of basic volcanic rocks which have undergone lateritic alteration (Sherman, 1950; Segalen, 1957; Bonifas, 1959; Précot et al.,

1962). Leneuf (1959) recognizes a kaolinitic and montmorillonitic phase in the alteration of amphibolic granites or granodiorite in a hydromorphic zone. Nye (1954, 1955) noted the same effect on gneiss.

In general, montmorillonite forms in a medium of high p H (at least 7) containing numerous cations, including those of magnesium. Attempts at synthesis confirm these findings. It is important to note that gibbsite is invariably absent when montmorillonite is present (Leneuf, 1959; Précot et al., 1962).

Kaolinite. There is much discussion about the genesis of this mineral in lateritic formations (Mohr, 1954; Koster, 1955), for it is difficult to see how it could form in situ in the midst of silicates. Lacroix (1923) showed that the felspars from the Iles de Loos syenites could undergo not only direct alteration into gibbsite but also another type of alteration, again pseudomorphic, to kaolinite.

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Gordon and Tracey (1952) make similar observations in their study of the Arkansas bauxites, where gibbsite deposits have formed above the water table and kaolin deposits below it.

Laboratory investigations have shown that kaolinite is a characteristic mineral of relatively acid environments (Noll, 1936; Norton, 1939; Henin and Robi-chet, 1953). The studies of D e Lapparent (1936), Ross (1943) and Millot (1949) on sediments, of Edelman (1947) on soils, and of Lovering (1952) on hydro-thermal alterations confirm this. Frederickson (1952) and Keller (1957) maintain that a further condition for kaolinite formation is the addition of hydrogen ions and the loss of divalent and iron cations.

All these suggestions are based on different hypotheses: that kaolinite is formed directly from silicates or through the medium of collasols or gels; or else that it forms directly from ionic solutions. The idea of clay mineral neo-synthesis from colloidal precipitates or amorphous gels derives from Mattson's theories and hypotheses (1931). The theory that kaolinite crystals grow from ionic solutions derives from the work of Correns (1940). One fact is certain: the structure of silicates, like that of felspars, pyroxenes, amphiboles and peridots, is completely different from that of clay minerals. Total destruction of the former is thus essential. Hauser (1952), Lovering (1952), Oberlin et al. (1958) and Gastuche, Fripiat and de Kimpe (1962) conclude that gels or amorphous compounds form and provide the necessary material for kaolinite crystallization. Bates (1952) and Siffert (1962) believe that reorganization starts with the ions. Siffert maintains that 'clay silicates can derive from particles in solution : Si(OH)4

molecules and metallic cations'.

Essentially aluminous dioctahedral clays (kaolinite) are fairly readily produced at normal temperature and pressure by simple mixture of components in a state of extreme dilution, provided the pH of the environment is suitable. The principal problem is to maintain the 6 co-ordination of alumina at a pH.

at which it normally has 4 co-ordination, that is, a />H between 4.1 and 6.7. Siffert achieves this by introducing aluminium in the form of a complex ion (the oxalic complex anion of aluminium [A1(C204)3]

3-). It thus takes control of the pH of precipitation. This seems to prove that the ions which take part in building kaolinite are basic ions of the type Al(OH)++. The clays would be nothing more than basic salts (of the Feitknecht type), but silicated with certain cations. The formation reactions occur in several stages. First, a monomeric ion of the type (O Si O R O H ) forms, R being a cation. At a later stage, polymerization occurs, an effect which has not yet been elucidated.

Herbillon and Gastuche (1962) mention that the predominance of 4 co-ordination between pH 4.1 and 6.7 is connected with the presence of highly pola-rizable anions (like chloride), the anion radius being incompatible with the regular distribution of the six bonds. If a rapid disionization process starts in an acid medium, for example by dialysis, a gel will immediately start to form on the positive side of the isoelectric point (Van Schuylenborgh, 1950), where, despite the momentary imperfection of the crystalline characters, the 6 co-ordination of aluminium is already assured. In the basic zone, the gel is on the negative side of the isoelectric point; that is, the anions are repelled from the micella in process of formation and the only impurities are cationic and, therefore, poorly polarizable.

The gibbsite stage is preceded by a milky, followed by a somatoid 'pregibb-sitic' stage. This explanation eliminates a difficulty mentioned by Henin and

86

Origin of latérite*

Cailère w h o point out that what makes kaolin synthesis particularly difficult is the fact that in an acid m e d i u m an alumina gel will turn spontaneously into boehmite and not into gibbsite.

H o w does the silica-alumina interaction work? W e y and Siffert show that monomelic silica displays no affinity for crystallized hydroxides and phyllites. This is very important, for it proves that a crystallized gibbsite cannot fix silica to give kaolinite. It follows that the grafting of silica on the octahedral layer is effected at the very m o m e n t of the formation or disorganization of the latter. All these results show h o w important cations are in the building of phyllitic crystals (Millot, 1962). It is possible that a montmorillonitic stage, even though fleeting, is an important element in the synthesis of kaolinitic minerals. During its destruction, the montmorillonite would yield silica tetrahedra which would attach themselves to the octahedral layer in process of formation.

The conditions for kaolinite formation can thus be listed as follows : (a) from precipitated gels: acid med ium, moderately silica-rich but highly disionized, permanently humid; (b) from particles in solution: the presence of a complex, stabilizing the 6 co-ordination of alumina, in an acid, humid and silica-rich m e d i u m ; certain products of the decomposition of organic matter must in this case influence these mechanisms by complexing the aluminium.

This would explain kaolinite formation both at depth in 'alteration crusts' and in horizons near the soil surface. The explanation can be applied to the intensive kaolinization observed on all profiles in the humid environment of dense equatorial forests.

Finally, great importance must be attached to leaching, drainage and waterlogging, for these phenomena basically dictate the lateritic alteration and separation of the derivative products.

Influence of drainage. Harrison (1933) distinguishes two types of latérites, each capable of being associated with the same rock: high-plateau latérites composed almost entirely of hydrated alumina and iron oxides and formed under heavy, more or less constant rain and perfect drainage conditions, on the one hand; and low-plateau latérites containing secondary hydrated alumina silicates and occasionally secondary quartz, formed under less intensive and persistent rain and under imperfect drainage conditions, on the other. In the low-plateau latérites, primary lateritization is characterized by loss of silica and bases from the parent rock and leaves a residue of gibbsite and ilmenite. It is followed by resilication, gradually producing a vast mass of clay latérite.

Gordon and Tracey (1952) attribute great importance to the influence of ground water which determines both the kaolinitic and the gibbsitic alteration of the Arkansas syenite.

Hardier (1952) mentions the porosity of the rock, the free circulation of water, abundant rainfall and alternating wet and dry seasons, topography, movements of the water table and weather.

Allen (19526) points out that it does not matter whether the minerals form above or below the water table ; what does matter are the conditions and intensity of drainage or leaching, p H , the chemical activity of the environment and all other conditions likely to promote the formation of one mineral rather than another.

Bonifas (1959) notes that the hard and intermediate layers of latérites are alternately dry and engorged with water and that this promotes dissolution and

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reprecipitation. This would partly account for the relative accumulation of alumina in the hardpans, through the solution of part of the iron : some iron is carried along when the water circulates sufficiently, but some also passes into the surface water. In the Konkouré region (Guinea), the kaolinitic clay layer is permanently situated in the water table, but above the hydrostatic level there is a pain d'épiées layer, that is, a layer of ferruginized gibbsite.

Précot et al. (1962) point out that under conditions of excessive drainage, a gibbsite layer can be seen to form around the altered rock. In the Hawaiian Islands, Sherman (1952) and, later, Bates (1960) noted that the intensive rainfall promotes the formation of this mineral species. O n the other hand, when the percolating water remains rich in salts, as happens when drainage is poor, gibbsite does not occur. Good drainage conditions also make for the transport of silica and so-called mobile elements.

Influence of ionic content of percolating water. For any given rock in any giveD climate, mineral neoformation depends on the water circulation. This brings about modifications of pH and of ionic concentration and ultimately modifies the environment. The influence of the rock is decisive in so far as it provides the constituent elements of the secondary minerals and the elements present in the environment. This has frequently led pedologists, in discussing lateritic alteration, to take into account the different groups of rocks. Since the residual products retain the structure of the parent rock, Lacroix (1913) distinguishes two types of alteration in Guinea : (a) the abrupt alteration of gabbros, syenites and diabases, without transition, typified by the transformation of felspars into gibbsite and of ferromagnesians into colloidal ferruginous products more or less rich in alumina silicates, and the alteration of constituted peridotites and ferromagnesians, yielding ferric colloidal products and a little alumina; (b) the gradual alteration of mica schists, gneisses and granites, yielding kaoli-nite and colloidal alumina silicates which gradually turn into lateritic clays.

Harrison (1933) drew the following conclusions in regard to Guinea. Under tropical conditions, the destruction of basic and neutral rocks at or near the level of the aquifer, under more or less good drainage conditions, is accompanied by almost total loss of silica, and of calcium, magnesium, potassium and sodium oxides, leaving an earthy residue of alumina trihydrate (gibbsite), limonite, a few fragments of unaltered felspars, sometimes secondary quartz and various stable minerals present in the rock at the outset. Acid rocks such as aplites, pegmatites, granites and gneisses change gradually into pot clay or more or less quartzy kaolin. In the first case, resilication can occur, with transformation into clayey latérites; in the second case, desilication sometimes occurs, with the formation of surficial concreted bauxite masses. Harder (1952) also studied bauxite deposits in relation to different parent rocks. Such connexions m a y be frequent, but they are not specific. In Madagascar, Lacroix (1923) noted that gabbros, diabases and syenites underwent lateritization as in Guinea, whereas granites and gneisses displayed sometimes kaolinitic, sometimes gibb-sitic alteration. Mohr (1954) and D e Lapparent (1939) make similar observations. I have personally been able to observe kaolinitic alteration of diabases in Guinea, but in permanently wet, moderately drained horizons, and, conversely, pseudo-morphic alterations of granites into gibbsite in a very well-drained environment.

Précot et al. (1962), studying the alteration of the Kivu volcanic rocks, noted that in the case of alumina-rich minerals the first stages of alteration generally

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produced highly charged alumina-silica gels or allophanes. These then developed towards the kaolin stage. In this particular case, the pH. and the degree of calcium saturation of the percolating water diminished with distance from the volcano. The conditions of desaturation and acid pM. coincide with the emergence of kaolin of the halloysite type. Bates (I960) observed similar formations in the first alteration zone of the Hawaiian rocks and believed that the absence of kaolinite was connected with the absence of mica. Précot et al. (1962) note that the the occurrence of micas in ash coincides with the transformation of halloysite into fireclay. In soils derived from basalts, large amounts of kaolinite are observed accompanying muscovite. Under conditions of excessive drainage, gibbsite appears round the altered rock. O n the other hand, this mineral is not found in the case of trachyte alteration, when the slow dissolution of sodic-potassic felspars produces some cation concentration in the percolating water.

In deep alteration horizons, despite important variations of organic matter content in the surface horizons, it has been found that the character of the colloid fraction does not alter at all but is always determined by the character of the percolating water and the rate of drainage.

It thus seems that the character of the residual products of lateritic alteration is mainly dependent on the environment. This, in turn, depends on the relationship between the character of the rocks, the hydrolysis processes and the water régime:

1. Readily hydrolysable basic rocks displaying a silica deficit readily yield gibbsite if the drainage is good. If drainage is moderate and humidity permanent, these rocks give kaolinite; if drainage is poor, the neoformations tend towards phyllites of the type 2/1.

2. So-called acid rocks are hydrolysed more gradually. Silica excess makes for kaolinization under normal drainage conditions. Excessive drainage gives rise to gibbsite (Madagascar). Where drainage is poor, the cation property of the rocks promotes kaolinization after a brief montmorillonitic stage.

I shall pass over the conditions of lateritic hydrolysis, which have been m e n tioned in the chapter on environmental factors. The problem of the water régime in lateritic soils, however, calls for detailed comment. It depends primarily on climate. A distinction must be made between tropical and equatorial climates : that is, the rhythm of the seasons must be considered. Tropical climates produce alternating periods of extreme wetness and extreme dryness in the soil, thereby making for an environment favouring the neosynthesis of gibbsites. In equatorial climates, the soil retains some moisture throughout the year. This permanent moisture, in an acid medium, promotes kaolinization.

The contrast appears in the vegetation. Forests, characteristic of the more humid climates, mitigate fluctuations of climate from one part of the year to another and from one year to another.

The water régime can, however, be conditioned by other factors as well. Certain rocks, or rather certain textures of rock, make for good drainage. This applies, for example, to the texture of dolerites. In general, it is found that basic crystalline rocks are more permeable than acid crystalline rocks; this, combined with the silica deficit of the former, makes for gibbsite separation.

Another factor is the presence of a deep impermeable level preventing drainage (clay illuviation, rocks resistant to alteration). This phreatic water table can be either permanent or temporary. Suspended or oscillating, more or less

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short-lived water tables (Rougerie, 1958) appear in forest soils. All these conditions favour the formation of type 1/1 clay if the rock is desaturated and acid and of 2/1 clay if, for one reason or another, the percolating water is cation-enriched.

Finally, it is the interaction among all these factors that determines the composition of the alteration products of lateritic environments. A soil containing gibbsite is not more lateritic than a soil containing kaolinite, if we connect lateritization with hydrolytic processes. The only factor to be considered is the rate of hydrolysis. Here again, two factors are frequently confused : the vigour and the duration of the reactions on which the character of the residual matter depends.

Redistribution of lateritic alteration products

Study of the origin of latérite components shows the factor of prime importance to be the circulation of water through the material in the process of developing. The rate of migration of the liberated elements depends on this.

Frederickson (1952) believes that the mobility of an element in the soil is related to its 'ionic potential', that is, the ratio of ionic radius to valency. The lowest values will occur in the case of small, heavily charged ions which will combine with oxygen to give soluble anions. The highest values will occur in the case of alkalis or alkali earths which will form soluble cations; the intermediate class of 'hydrolysats ' corresponds to the poorly mobile elements such as iron and aluminium. These differences in solubility lead to differential migration of the elements affected, which become redistributed not across the profiles but among them. The dissolved cations and silica migrate towards the low parts of the relief where they accumulate to create a specific environment which conditions new neosyntheses. Iron and alumina accumulate in situ and then become partially segregated, the iron moving more easily than the alumina. According to the conditions in the environment so created, some elements will be trapped in more or less stable associations : gibbsite, kaolinite and ferruginous hardpans. This results in a temporary blockage of these elements at preferential levels; hence the distribution of latérites in a terrain. It does not, however, alter the fact that on the geological time scale, lateritic alteration corresponds to a general dissolution of the terrain, which literally 'melts' in situ.

Edelman (1946), Edelman and Schuffelen (1947) and Mohr and Van Baren (1954) explain how the distribution of clay types in the mountain regions of Java depends on alternate leaching and accumulation of silica: lateritic soils are associated with leaching, whereas black montmorillonitic soils are associated with accumulation at the bottom of the soil. A similar explanation would account for the frequently reported laterite-gibbsite succession in the top parts and laterite-kaolinite succession by resilication in the bottom part (Harrison, 1933). The cumulative effect of the leaching of certain alteration products and the redistribution of the residual matter often leads eventually to pronounced displacements of large quantities of material. The shearing effects producing these movements can be observed in alteration crusts formed in this way. Such movements would account for the polyhedral structure often found in kaolinitic soils (Sabot, 1952).

The total effect of this collapse is to mix the material and thereby help to homogenize it. These movements of large quantities of material, reinforced by

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creep effects on the slopes, are partly responsible for the stone lines found within the profiles. There is a remarkable parallelism in this respect between the distribution of these formations and the relief. The hypothesis is further confirmed by the occurrence of bent veins. Added to these general movements, which contribute to the normal development of the relief, are the limited types of redistribution already mentioned, namely: redistribution connected with biological activity—the action of animalcules, particularly termites, and the 'whirlpool ' effects produced by roots which draw material along with them and return it again. The cumulative effect of these local phenomena is considerable and affects the top 2 m . of most lateritic alteration soils.

PRE-EXISTING COMPONENTS IN T H E R O C K

M a n y sedimentary rocks are composed of minerals which can take part directly and without alteration, except in regard to redistribution, in the formation of indurated latérites. These formations are frequent on old continental platforms in tropical environments. They consist of clay-sand and sand-clay horizons, often poorly consolidated, composed of quartz grains, corroded or split, and of kaolins more or less impregnated with iron. They account also for the eroded and reworked products of old lateritic alteration soils, such as the Tertiary sands of the Lower Ivory Coast, Togo and Dahomey ; the clay-sand formations of the Terminal Continental and the Intermediate Continental, of Senegal, Mauritania, north-east Mali, Niger, north Dahomey, Chad and the Central African Republic, as well as the sedimentary structures of the Congo depression. Comparable horizons are probably fairly numerous in other parts of the world : for example, the siderolitic horizons in Western Europe, the Graulehm of Western Germany and so forth. Most of these faciès show that at certain epochs lateri-tizing climates prevailed throughout the world.

A m o n g the components of these formations, the most important element is iron which is readily mobilized and tends to concentrate at the most advantageous points, where it forms indurated ferruginous crusts. This accounts for the pisolitic ferruginous crust, less than 1 m . thick, which has developed on the Villafranchian in the majority of the Terminal and Intermediate Continental sedimentary formations throughout Central Africa.

The conditions for iron mobilization in these rocks are often less strict than those required for lateritic alteration. The iron mobilizes all the more rapidly because it is already separated and the rate of mobilization increases with the temperature and humidity of the soil. In the tropical African climates, where the mean annual temperature is above 25° C , the mobilization processes become perceptible above the 500 m m . isohyet. Even in subarid soils (200-500 m m . / year), free iron accounts for more than 65 per cent of total iron (Bocquier and Maignien, 1963). At a precipitation rate of about 750 mm./year , the processes of ferruginous crust formation appear and become stronger, the greater the amount of precipitation. These conditions are well below those inducing lateritic alteration which does not start, in the same regions, until precipitation is of the order of 1,200 mm./year. It can be seen that the soil crust development, or rather the formation of latérites in the Anglo-Saxon sense, is not specifically connected with lateritic alteration but can occur in very different soils.

A m o n g the formations yielding material which contributes to the formation of indurated latérites, old crusts must be mentioned. This question has already

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been briefly mentioned in connexion with the part played by relief in forming hardpans. Fossil, aluminous, ferruginous and manganiferous hardpans constitute a source of separated constituents which can, under certain conditions, be remobilized and migrate through a terrain to form new indurated horizons. As in the case of sedimentary structures, these mechanisms are closely bound up with the biological activity of the soil and, more particularly, with the decomposition of the organic litter.

While the remobilization of iron and manganese in old hardpans presents hardly any problem, the question of aluminium remobilization is open to discussion. The data given above show that the mobilization of aluminium is much less rapid than that of iron. It can, nevertheless, be considerable, if w e allow for the cumulative effect built up over a long period of time. Aluminium, iron and manganese thus take part in cycles of different duration and intensity. These cycles are more or less out of phase with one another, owing to the different degrees of solubility of the elements in question. In combination, these mechanisms contribute to the slow removal and redistribution of the lateritic constituents which are shifted from the higher to the lower levels of the relief.

In the light of these data, the course of hardpan formation processes can be schematically shown as follows (Maignien, 1958) :

Alteration of rocks (Lateritic alteration — ferruginization)

Separation of constituents Losses towards/ -J. the oceans ¡S Mobilization <

. + . Accumulation and immobilization •*-

(Cementing, impregnation, accumulation)

Induration and hardpanning

. + Disaggregation — reworking

I Alteration — solution

I New removal

Losses towards / the oceans x

W h e n sesquioxide liberation exceeds drainage losses to the oceans, hard-panning is increased. If mobilization and leaching effects predominate, either there is no hardpan formation or the old hardpans disappear. W h e n a relief is subjected to leaching but the drainage in the reception zones is deficient, the hardpan slides down towards the lower levels.

The possibility of hardpan formation in tropical soils is thus regulated by the balance between the factors governing sesquioxide evolution; but hardpan formation is not specifically linked to lateritic alteration processes.

ACCUMULATION OF LATERITE CONSTITUENTS

The accumulation of latérite constituents can result either in loss of the less soluble materials (relative accumulation) or in the addition of constituent

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material (absolute accumulation). These differential movements can occur on the profile scale even when there is no contribution from without ; in this case, they are due solely to vertical movements of the soil solutions. They can occur also when lateral movements bring in material from outside. In practice, these distinctions are too academic, for there are few examples in which only one of these processes occurs. Generally they interact, but each occurs in a different degree and at a different rate.

ACCUMULATION IN THE PROFILE W I T H O U T OUTSIDE CONTRIBUTION

Newbold (1844), Glinka (1899) and others had no doubt that indurated latérites formed from the products of rock alteration could develop without any contribution from outside. Hanlon (1944) supported this opinion and found that certain latérites possessed the same amounts of iron and aluminium as the subjacent rock. This could only mean that there had been no contribution from outside the profile, for the two principal constituents possess very divergent chemical characteristics. Alexander et al. (1962) report an identical case in Guinea, where the indurated product results from relative accumulation of sesquioxides after loss of silica and bases. The principal constituents are reorganized in situ to form a skeleton. The fact that the induration properties are often poorly expressed when the parent rock is poor in iron supports this hypothesis. W e must nevertheless once again distinguish between aluminous accumulations on the one hand and ferruginous or manganiferous accumulations on the other.

Aluminous latérites

Study of lateritic weathering shows that under certain conditions minerals can be altered directly in situ into well-crystallized gibbsite. The crystals form a coherent skeleton and give rise directly to an indurated aluminous crust. The latter is more or less impregnated with iron sesquioxides, although it is impossible to tell exactly what proportion is due directly to alteration of the rock in situ and what proportion is due to secondary enrichment. This type of aluminous hardpanning is common. It displays partial dissolution effects, giving the material a scoriaceous texture. Sometimes the nuclei of fresh rocks are fixed in the hardpan mass. It thus seems that aluminium is mainly a residual lateritic element. The aluminium proportion can nevertheless vary according to the mineralogical nature of the altered rocks. Bonifas (1959) notes that whereas alumina and titanium remain in situ or are evacuated only in small amount during the alteration of dolerites, syenite and hornblende alteration is accompanied by a considerable intake of these elements. During the alteration of dunites, aluminium is partially removed.

Alumina certainly migrates at varying rates, depending on environmental conditions, and when it migrates it is certainly carried by water. Yet it is hard to believe that the gibbsite found in contact with fresh blocks could have come from the leaching of higher parts of the profile. There must have been in situ

transformation followed by relative accumulation.

The filling-up of small cracks with alumina trihydrate, forming aluminous pisoliths, is hardly ever observed except in the top horizons of a profile. This surface concentration could be achieved only by capillary action or from outside

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through a cross-leaching channel. Study of the water balance in tropical soils seems to exclude the first hypothesis. Such concentrations could occur only in the vicinity of a phreatic water table where fluctuations of level were slight. O n the other hand, the surface accumulations can be much more easily explained by lateral movements of the soil solutions. Nevertheless, these types of accumulation are rare compared with relative aluminous accumulations, which are far and away the commonest.

Other latérites

The other indurated horizons are connected with movements of iron and/or manganese in the soil. Some of the iron in these hardened latérites comes directly from the in situ alteration of rock minerals. Bonifas (1959) points out that there is a considerable increase of iron during the alteration of dunites and serpentines. In other cases, however, iron is partly lost. Except in some instances which have not been fully explained (lateritic red soils), iron mobilizes very readily and migrates over considerable distances along with the soil solutions.

O n the profile scale, it is possible to explain the formation of horizons enriched in iron and/or manganese solely by vertical movements, either descending, when iron is leached from the higher horizons and accumulates at depth, or ascending, due to the capillary rise of iron-enr>hed solutions in the alteration zones ; the segregation and redistribution in situ or through several horizons can be explained by the action of a more or less temporary, fluctuating water table.

Enrichment through vertical leaching. It has long been known that indurated, iron-rich horizons formed through the leaching of surface horizons and accumulation at depth. Not until the works of Mohr (1932), followed by those of Pendleton (1936, 1943, 1953), appeared, however, was there a clear account of this process. In the opinion of these authors, latérite is an illuvial horizon. Maig-nien's works on tropical cuirassé ferruginous soils supported this view in regard to ferruginous concretioning and hardpanning. D'Hoore (1954) mentions the various possibilities of hardpan formation connected with sesquioxide migration. Transport will not occur unless conditions of mobilization and transport are present simultaneously. The different forms of iron in soils are not all favourable to its displacement: (a) the ferric ion is almost insoluble at the pH. of tropical soils ; (b) the ferrous ion is appreciably soluble, but stable only in a reducing medium; (c) iron in its colloid form can be displaced, but is very sensitive to electrolytes ; (d) because of its electropositive charge, iron vigorously attaches to clay and can be leached together with it; (e) ferric and ferrous ions are able to associate with certain substances passing through soils, giving electronegative complex ions which do not attach to clay and are less sensitive to electrolytes; these combinations are essential in the processes of iron, and perhaps of aluminium migration; (f) under certain conditions, iron can migrate in the form of carbonates.

The difference in solubility between ferric and ferrous ions and the ease with which they form organic complexes explains why attention has been devoted mainly to the 'absolute' accumulation of iron (Betremieux, 1951; Maignien, 1958). Bonifas (1959) does not believe that iron migration occurs on a scale larger than that of the profile, but this opinion is disputed.

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T w o groups of substances take part in the formation of pseudosoluble complexes : silicic acids, the presence of which is directly connected with the alteration of the rock; and organic products resulting from the biological activity of the soil.

Ferrisilicic and ferrosilicic complexes were studied in detail by Demolon and Bastisse (1938, 1942, 1944). These products were found in soils through electrodialysis. They form in concentrated media, but can undergo strong dilution. In the opinion of Bastisse (1946, 1949), the organic anions which m a y conceal iron and manganese and the metal hydroxides in general are primarily the polyacids and acid-alcohols. A m o n g the latter, the hydroxy acids combine with iron and manganese to give fairly undissociated and highly stable complexes. These acids are present in plants and in their decomposition products (for example, lactic acid).

The reduction of ferric into ferrous iron in the soil always depends on a microorganism; but the precise mechanism by which this reduction is effected is as yet unknown. Bromfield (1954) believes that it is connected with dehydroge-nation. Bacteria capable of reducing iron include: Eicherichia coli (Halvorson and Starkey, 1925), Bacillus polymyxa (Roberts, 1947), B. circulons, Aerobacter

corogonas (Bromfield, 1954), Staphylococcus aureus, B. mycoidae, B. mesentericus,

B. subtilis, etc. (Kalakutskii, 1959). This reduction is nevertheless not a specific function of these organisms.

The fermentation of crude organic matter (plant material) and of certain definite products (for example, glucose) can give rise to products which reduce iron and make it soluble (Betremieux, 1951; Islah and Elahi, 1954; Mandai, 1960).

The humic and fulvic acids seem to have a solvent power over ferric iron through complexation and reduction (Baba and Yamamoto , 1957; Beres and Kiraly, 1958; Ponomareva, 1949).

Several authors have studied the action of litter extracts on soil (Bromfield, 1950, 1956; Schmitzer, 1954; Lossaint, 1958, 1959; Motomura, 1962). These extracts are capable of dissolving the iron and carrying it away. The mobilization would be due to transition products of the polyphenol, fatty acid, chelate type. The greater part of the reducing activity would be due to products with a low molecular weight; those with a high molecular weight would tend rather to inhibit it (Bromfield, 1956).

Once complete or partial reduction of the iron has been achieved, as well as its solution, the element can migrate within the profile. There are various opinions in regard to the form under which it does so. Soil scientists seem to be less and less inclined to favour the ionic form, at least so far as lateritic soils are concerned. M a n y authors, on the other hand, envisage the existence of complexes similar to co-ordination complexes (Bremer et al., 1946; Betremieux, 1951; Bromfield, 1954; Schnitzer, 1954; Beckwith, 1955), while others (Atrinson and Wright, 1957; Kawaguchi and Matsuo, 1959) think that the fundamental component is a chelate.

Lossaint (1959), going to the root of the matter, believes that various influences come into play, such as, for example, reducing power, complexing power, chelating power, the possibility of a protective soil and so forth; now one, n o w another of these will predominate, according to environmental conditions.

To summarize, we can say that iron becomes more labile in soils in proportion to the reducing conditions in the environment. The latter m a y be temporary

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or permanent. They are bound up with the soil climate. Iron mobilization can nevertheless vary greatly according to the soil type. Tropical ferruginous soils present extremely favourable conditions for it (Maignien, 1962); lateritic red soil, on the other hand, seems to retain iron fairly strongly. With kaolinite, iron forms relatively stable bonds which have been studied by Fripiat et al.

(1954). These authors reached the following conclusions: (a) the iron oxide covering natural kaolinite surfaces occurs in two forms which behave differently; (b) there are two types of kaolinite-iron oxide combination with different structure and properties; (c) the type of combination occurring depends on the original kaolinite; (d) the nature of the edges of the kaolinite films is probably the cause of these differences.

The above-mentioned authors distinguish ordered and disordered complexes. The former possess a structure resulting from partial accumulation of kaolinite pseudohexagons following the C-axis and their co-ordination through the agency of the oxide; the curve connecting the specific surface of these combinations passes through a m a x i m u m towards 12 per cent of Fe 2 0 3 ; if the content is higher, surface saturation occurs with the formation of extremely small particles of pure oxide. The second type forms from disordered aggregates of kaolinite particles combined through the agency of the iron oxide; the specific surface of these combinations increases as a linear function of the Fe 20 3 content, without any saturation effect appearing.

The ordered complexes form from kaolinite which has undergone neutral treatment; the disordered complexes from kaolinite which has undergone acid treatment.

D'Hoore (1954) found that the value 12 per cent Fe 2 0 3 seemed to correspond to the oxide saturation of the clay surface of Congolese soils. Above that percentage, concretions formed.

The rate of iron hydroxide fixation on tropical clays thus depends on the soil-forming environment. If the latter is strongly altered and leached, the clays will be of kaolinite H-type. The isoelectric point of the separated hydroxides is lowered where there is a tendency to concretioning. W h e n the leaching processes do not affect the hydrolysis of alkali earth minerals, there is a tendency for M kaolinite to form and for the separation of an hydroxide for which the isoelectric point has a relatively high pH.. Absorption phenomena predominate.

According to environmental conditions, a more or less considerable proportion of iron can migrate together with the clay; it must be noted, however, that although clay eluviation is very restricted in lateritic red soils, it can nevertheless be very pronounced in certain lateritic yellow soils.

The displacement of the materials mobilized depends on the movements of the water, which are mainly gravitational and therefore downwards. Absorption by roots nevertheless plays a by-no-means negligible part in removing material from the deep levels and restoring it to the soil surface. Another factor is the action of soil animals (termites and worms), bringing material from the deep layers up to the surface horizons and homogenizing it. Nye (1955) mentions soils in Ghana colonized by Hippopera nigeriae in which the amount of material brought to the surface can be estimated at 500 kg. per acre per year. Kolhmanns-Perger (1956) gives values of 2.1 tons per hectare per year for the soil matter excreted by worms in north Cameroon.

The depth of vertical transport of iron and/or manganese sesquioxides due to gravity is variable, depending on rainfall and on the type and topographical

96


position of the soil, factors which determine the presence or absence of a temporary phreatic water table.

Maignien (1958) gives the following values: Lower Cassamance (Senegal) : ferruginous accumulation immediately above the

lowest level of the phreatic water table (between 4 m . and 6 m . ) ; Cap Roxo (Portuguese Guinea) in slightly lateritic red soils, depth restricted

by the sea level to about 8-10 m . ; Leached tropical ferruginous soils, between 75 c m . and 250 c m . under the argil

laceous illuvial-horizon ; The High Plateau of Fouta Djallon (Guinea), on poorly drained ferrallitic soils,

hardpanning towards 150-200 cm. , at the perched temporary ground-water level ;

The forest part of Guinea : ferrallitic soils on gneiss and granite, concretioning and hardpanning towards 150-250 c m . in the relatively argillaceous levels.

In general, concretioning or induration of clay soils occurs at less great depth than the hardpanning of light soils. In the latter, the indurated horizons again occur at a greater variety of depths.

The precipitation and immobilization of iron occurs when the iron-complex-ing structure (whether chelate or soil) is destroyed. This destruction can be produced by a change of pH, of Eh , of dissolved ions, or by oxidation destroying the 'protector' soil or restoring Fe2+ to Fe3+. It is therefore mainly a question of physical-chemical reactions, although oxidation can in certain cases be effected by micro-organisms.

The fixation of the liberated hydroxide is promoted by the presence of preexisting hydroxides.

These different results corroborate the observed facts, namely: Iron never precipitates in an organic-rich medium; the ferruginous minerals in

such a medium are heavily corroded; The upper limit of the hardpans is more distinct than the lower limit; towards

the top, the physical-chemical modifications (pH, etc.) associated with well-drained, loose horizons promote abrupt precipitation of the more or less complex sesquioxides ; at the base, the deposits are progressive and diffused, for they are due to the construction of organic complexes and the consequent gradual liberation of the oxidized forms;

A n initially indurated horizon, even if aluminous (fossil latérites, for example), displays secondary immobilization effects; the superposition of these phenomena contributes to the complexity and heterogeneity of the indurated formations :

All variations of texture in the direction of an increase in the coarse elements content promote immobilization (oxidation through aeration); this phenomenon is marked in heterogeneous alluvia; the coarsest horizons are converted into sandstones, puddings, conglomerates and breccias with ferruginous cement; the aeration of the medium influences also the formation of ferruginous deposits along the roots; similarly, in soils where induration has already started, the sesquioxides are deposited mainly along cavities and channels in the ground;

Aeration inducing oxidation and consequently immobilization of iron and/or manganese can be due to the accelerated loss of water of percolation or to lowering of the ground-water level; this occurs as a fringe effect in drainage axes or transverse faults and also in alluvial deposits, forming the 'gallery'

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hardpans, indurated halos at the head of springs or erosion gulleys and 'sheet' hardpans ;

A reduction of the organic matter content and of the biological activity in the soil again promotes immobilization; these processes are frequent when savannah populations replace the forest ; it can happen also when the mineralization of the organic matter is accelerated; for example, after the soil has been worked or drained;

The sudden onset of the dry season and consequent water deficit can restrict biological activity and promote immobilization; this accounts for the more frequent occurrence of indurated soils in tropical climates with well-defined seasons than in the almost continuously humid equatorial climate;

The concentration of roots in certain definite horizons makes not only for the accumulation ofironsesquioxides but also for their immobilization, by drying out the soil at selected sites; the concretioning of savannah soils is due to this effect which is particularly clearly marked under herbaceous vegetation after forest.

Enrichment by capillary rise. It was long ago recognized that indurated latérites formed through the upward movement of water under the influence of evaporation. This opinion was held by Maclaren (1906) who concluded that there was a pallid zone of reduction and solution of iron moving upward by capillarity to enrich the overlying latérite. Harrassowitz (1926) produced data showing that iron content increases upward to the overlying indurated horizon, thus corroborating Maclaren's hypothesis. Simpson (1912) suggested that latérite crusts were like surface efflorescences where iron concentrated through capillary rise following evaporation.

Numerous studies, however, have shown that capillary rise is much less important than was thought and is restricted to favourable materials and sites. Under optimum conditions, it reaches a m a x i m u m of 2-2.5 m . (Mohr and Van Baren, 1954; Baver, 1956). It is significant only at the contact with a zone of temporary or permanent saturation.

There are thus two questions, the first relating to capillary rise and suction through evaporation, the second to the mobilization of sesquioxides in aqueous media.

If w e study the water régime of tropical soils, w e find that during the rainy season water-saturated horizons form. During the dry season, on the other hand, there is intensive desiccation of the surface profiles, or at least of those which are not under forest and are not situated in a region of humid tropical climate. Under these conditions, the factors governing the movement of the soil water are : in the rainy seasons, those which regulate the flow in saturated soils, in other words, downward percolation; in the dry season, those which influence the movement of water in non-saturated soil, mainly upward movements.

The transition periods, which are often very short, can be ignored. W h e n the soil is not water-saturated, the capillary potential operates alone. Hallaire (1953) showed that in cycles of alternate drying and wetting, water was discharged through evaporation: it rises, but the discharge cannot exceed a certain value, otherwise a surface crust forms which tends to create a balance between hygroscopicity and the atmosphere and the water diffuses only as vapour. The discharge is closely dependent on the rate of evaporation and the gradient of the latter is much less steep than that of capillary rise. The soils therefore dry rapidly at the surface and the rise of soil solutions is thus restricted.

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Further, the crusts display discontinuity which reduces discharge, for it tends towards a value such that at each level m a x i m u m evaporation will equal m a x i m u m diffusion from the subjacent horizon and capillary diffusion in the indurated horizons is always very slight because of their coarse texture. The result is that capillary rise is restricted and affects only a narrow fringe directly in contact with a saturated horizon. The water balance observed in lysimeter experiments confirms this interpretation. The periods of m a x i m u m evaporation at the surface occur during the wet season. Where there is an annual rainfall of the order of 650 m m . downward percolation occurs. In Senegal this affects one-third to one-quarter of the precipitated water, depending on the type of soil.

The second question is whether such restricted capillary rise can transport sesquioxides. It has already been mentioned that iron can migrate over long or short distances within the soil only in certain combinations (complex, chelate or soil protector) which sufficiently protect it against variations of the media through which it has to travel. Most of these combinations are connected with biological activity and, in particular, with the decomposition of organic matter and these phenomena can occur only in the surface horizons. It is therefore difficult to see how the effect can originate at depth, away from these biological influences.

In the dry season, on the other hand, when atmospheric evaporation is most intensive, the restitution of organic matter to the soil is reduced, along with biological activity. Oxidation processes predominate and it is difficult to see h o w the conditions for mobilization could be maintained.

Lastly, it now seems to be well established that the indurated horizons develop at depth in the soil. If we are to keep the hypothesis of enrichment through capillary rise, w e are entitled to ask why it stops at such deep levels and does not continue to the surface, as the first explanations seemed to indicate. Nevertheless, iron accumulation through capillary rise can play some part, limited to the fringe of the ground-water table, and partially enrich certain latérites and nodules (latérite sheets and galleries). This process, however, is not essential to the occurrence and development of hardpans, even if we allow for the cumulative effect built up over several decades. In most cases, the influence of a phreatic water table is to segregate products, a phenomenon which will be discussed later.

The presence of latérites at the surface is merely a consequence of the removal of loose surface horizons by erosion.

Enrichment by a fluctuating water table. Numerous hypotheses to account for latérite formation are based on the assumption of a fluctuating water table at the top of which iron is precipitated by oxidation (Campbell, 1917; Marbut, 1932; Pendleton, 1943; Mohr, 1944).

Campbell postulated two stages in the weathering of rocks, one due to increase of oxygen, the other to increase of vadose water. To make this clearer, he defined three zones in the soil: (a) a zone of non-saturation comprising all the profile above the reach of vadose water; (b) a zone of intermittent saturation comprising all the profile from the m a x i m u m upper limit of the capillary fringe to the lower limit at which atmospheric air can penetrate ; (c) a zone of permanent saturation comprising the lower part of the profile from the lowest level of the water table to the capillary fringe.

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•Origin of latentes

The zone of intermittent saturation is enriched by iron carried by solutions rich in humic acids and by ferrous compounds rising from the zone of permanent saturation. The latter could be precipitated by oxidation when the water table or the capillary fringe recedes. The fact that oxygen could penetrate the soil only with difficulty would restrict the depth at which induration could develop. The recession of the water table following the disappearance of a base level would thus not increase the thickness of the latérite; hence, different late-rites would not be able to form one under another, but successively on different forms of the relief where the water table lay near the soil surface.

This hypothesis has been repeatedly advanced by later investigators, yet it is difficult to reconcile with our present knowledge of rock alteration.

Mohr (1944) and, later, Mohr and Van Baren (1954) tried to adapt Campbell's hypothesis to the new data in order to explain the development of latérite in volcanic ash. They assumed that during hydrolysis of volcanic glass and primary minerals in the ash, silica-enriched water would dissolve progressively more calcium and magnesium to a depth such that the alkali earth concentration in the solution would become sufficient to cause precipitation of the silica which would cement the ash. A hard horizon would thus form, restricting permeability and producing a fluctuating water table. At this level, kaolin, gibbsite and iron oxide would separate, the kaolin precipitating in the lower (pallid) part and the iron in the upper part, thereby fixing the upper and lower limits respectively within which the water table could move. With time and the accumulation of clay, the hydrostatic level would gradually rise, the iron being regularly remo-bilized and redeposited at a higher level where it would gradually concentrate.

Alumina would be subject to the same mechanism but at a slower rate and would accumulate below the iron-rich level. The ferruginous horizon could harden immediately on exposure.

This explanation contradicts the ideas of Nye (1955) who maintains that iron dissolves in the top part of the latérite to enrich the underlying layers when normal erosion lowers the zone of saturation.

Although the precipitation of the iron contained in the zone of fluctuation can well be explained as due to transition from the ferrous to the ferric state, such a mechanism is not applicable to aluminium. It therefore seems that enrichment due to fluctuations of a water table can apply only to iron. Even here, it is not clear how the iron can accumulate in the top part of the fluctuating water table when the movement of the latter is due to gravity.

According to Wentworth (1955), who argues from the Ghyben-Herzberg hypothesis, at the end of the rainy season the ground water would be composed of several successive layers which would not mix and would correspond to the successive rains which had fallen throughout the year.

Since the lower levels would be richer in dissolved materials than the upper levels, it is difficult to see how accumulation could occur at the surface of the impoverished water tables. These data, used for practical purposes in Hawaii, nave been borne out by lysimetric studies performed by Bambey in Senegal (Charreau, 1961), who has shown that drainage is effective only after heavy rain. H e observed successive waves associated with the precipitations, each humidified zone rolling back the one before it. If we accept these mechanisms (Alexander et al., 1963), it could be suggested that the enrichment of certain soil levels by a fluctuating water table would be reduced to two principal effects : (a) accumulation of the material dissolved in the upper horizons through

100

Origin of latérite»

absorption and/or precipitation, leading to absolute accumulation; (b) accelerated differential dissolution of the different elements with partial losses, local translocations and segregation; the more soluble elements would be slowly eliminated and relative accumulation of the less readily mobilizable materials would occur.

This explanation is opposed by the results obtained by Betremieux (1955) who found experimentally that in the presence of a high water-level deposition was retarded and biological activity occurred at depth, with consequences on the leaching of the soil.

The present position seems to be that we can admit the operation of segregation mechanisms due to fluctuating water tables without appreciable overall enrichment. These segregations produce local concentrations causing the appearance of an indurated skeleton specifically composed of iron and sometimes also of manganese oxides. The skeleton takes varied forms, depending on the texture of the parent material, the commonest being the alveolar forms frequently found in latérite sheets. The skeleton imprisons loose, discoloured, often more argillaceous materials which can be swept away on exposure to the air.

The formation of mottled clay must be attributed to these mechanisms. In this particular case, which is often generalized, the determining factor is not a true phreatic table but a semi-permanent vadose water level. The characteristic mottling of these formations results from the same mechanisms of segregation affecting iron and manganese, together with intensive hydrolysis effects which contribute to the genesis of kaolinitic neoformations. W h e n the base level recedes, the mottles can harden to give concretions, sometimes producing a true hardpan, but rarely displaying the alveolar appearance characteristic of sheet latérites. The development of mottled clays has been particularly well studied in the Congo (Leopoldville) by Waegemans (1949, 1950, 1951, 1952, 1954) and by Waegemans and Vanderstoppen (1950).

The development of mottled clay horizons is closely associated with forested regions of humid equatorial climates. These formations are much rarer under tropical climates. They are practically never found in the lateritic alteration soils of Madagascar.

ACCUMULATION B Y OUTSIDE CONTRIBUTIONS TO THE PROFILES

The formation of latérite crusts rarely occurs simply on the profile scale. Observations show a close connexion between the occurrence of an indurated horizon and the succession of soils along slopes. The dissolved lateritic elements are m o bilized and leached in the topmost formations and are carried down by water of percolation to accumulate in the soils below.

Moreover, study of the constituent sesquioxide balance shows that the lateritic materials cannot derive from the development of a single original material in situ. These elements for the most part come from neighbouring soils through lateral transport. The movement of sesquioxides across a terrain is due to oblique leaching of the soil solutions. For this reason, the notion of a soil chain (catena) is of great interest in connexion with the mechanisms of hardpan formation.

The theoretical and practical conditions for the migration and concentration of sesquioxides depend on the general relief. There is a balance between the normal relief and the extension of the induration effects. The latter are connected,

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Loth on the profile and on the terrain scale, with the presence of a source of hydroxides, a mechanism of transport and a reception level. The various data given in the preceding chapters provide a basis for interpreting the geographical distribution of latérites. D'Hoore (1954) and Maignien (1958) studied numerous examples embodying these effects. It seems that existing or potential hardpans often owe their origin rather to the dismantling of fossil-indurated horizons situated at higher levels owing to inversion of the relief than to separation in situ through alteration of the constituent materials.

The migration of sesquioxides through vertical and cross-movements causes hardening both of the soils in situ and of the allochthonous materials. Where the development material consists of poorly alterable products (quartz cobble and sands), no transitional horizon is observed between the indurated horizon and the one below. The iron cementing the formation has been brought in from outside by water, either through cross-leaching 01 by a fluctuating water table.

The importance of contributions brought in by laterally circulating water is shown by the shape and distribution of the indurated layers. In cross-section, they present a characteristic bevel shape. Crusts bordering ledge-shaped reliefs are always thicker than those which develop on plateaux where crusts are sometimes lacking in the middle. Feuer (1956) gives good examples of this from Brazil.

The importance of migrations due to cross-leaching is worth noting. Maignien (1958) reports that in Guinea the possible distance of such transport is restricted to the distance over which conditions for the solution of sesquioxides are maintained. O n the Fouta Djallon plateaux (Guinea), crust formation on sandstones and cherts is effective at distances of 500 m . to 3,000 m . , depending on the quantity of material brought in by lateral transport.

W h e n the crust has invaded the whole terrain it is difficult to estimate the distance of transport, for intensive separation of sesquioxides through hydrolysis of the rocks in situ, particularly in a region of lateritic alteration, is superimposed on the lateral movements. Conversely, these effects are much better dissociated in drier regions, particularly in tropical ferruginous soils. The crusts occurring at the bottom of slopes and the gallery and sheet crusts owe their origin mainly to contributions from without.

Lateral and cross-movements affect principally iron and manganese. Aluminium is much less mobilizable and migrates much more slowly. The concentration of lateritic constituents at different levels in a terrain and at different depths in the soils thus derives from their different rates of mobilization and migration. Manganese, an element which mobilizes with extreme facility, is for the most part carried out of the profile. The iron mobilized has a short life and is therefore readily deposited. Aluminium behaves like a residual product, with the result that, aluminous latérites are almost invariably of the 'relative accumulation' type, whereas ferruginous and manganiferous latérites are mainly of the 'absolute accumulation' type.

Natural occurrences are in practice often more complex and it is possible to observe ferruginous (therefore absolute) impregnations in relative aluminous crusts. It is therefore extremely difficult to be sure how much is due to in situ

alteration and how much to contributions from without.

The rate of formation of a ferruginous crust due to outside contributions can be approximately calculated. Maignien (1958) states that in a century a sloping basin of 1 k m . 2 could account for the formation of a crust 1,000 k m . 3

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in area and 1 m . thick. These figures can be compared with those given for the rate of lateritic alteration, of the order of some tens of thousands of years, and show how far the former can mask the latter.

D E V E L O P M E N T OF ACCUMULATION HORIZONS

The constituent elements of latérites accumulate, become immobilized and then develop to produce more or less indurated horizons. This means that w e have two series of mechanisms to study, the development of the microstructures of these horizons and their induration.

D E V E L O P M E N T OF MICROSTRUCTURES

According to Kubiena (1950, 1956), lateritic soils or latosols are characterized by an earthy (erdig) structural relief which distinguishes them from other tropica] soils with a Lehm microfabric. The fine elements become organized into a porous structural edifice of spongy texture and generally bright-red colour. Eichener (1927), Kubiena (1948) de Craene (1954) and D e Craene and Laruelle (1956) postulate the derivation of this Erde from a Lehm through flocculation of colloids derived from the alteration of primary minerals, mainly through dehydration.

Numerous African latérites have been studied in thin section by Alexander et al. (1956) who draw attention to the evidence of mechanisms entailing the migration and segregation of the major constituents. The following micromor-phological faciès can be observed.

1. Conservation of the original rock structure (compare pain d'épiées faciès); the form of the original minerals and the cleavage lines are preserved by a finely crystallized sesquioxide skeleton. There has been in situ transformation through pseudomorphs of felspars, amphiboles, pyroxenes, and so forth, into gibbsite, goethite and kaolinite, emphasizing the principal traits of the altered minerals' structure.

2. Side by side with these in situ transformations, we find a series of phenomena implying a certain reorganization through localized and restricted movements. Sometimes secondary enrichments, too, are brought about through contributions from without and much longer migrations. Such mechanisms account for the oriented kaolinite films which are impregnated with iron in varying proportions and show no structural connexion with the original minerals. Such a facies can result only from more or less pronounced movements of the components. These clay films, like the goethite covers of varying thickness, border and line the pores and cavities. In many cases the iron is immobilized by being adsorbed on the clay. It can be remobilized and then recrystallized if the kaolinite is destroyed.

3. In other cases, where the rock is particularly rich in goethite, minute spherical aggregates, a few microns in diameter, can be seen under the microscope. These consist of clay materials impregnated with iron which have assembled in more or less dense clusters according to the amount of sesquioxides present.

4. Many latérites display well-separated and minerally segregated materials. These materials are either true pisoliths of regular concentric structure or pseudo-pisoliths composed of a more or less thick cortex of concentric films isolating

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some nucleus (quartz grains, rock debris, and so on). The pisoliths display cracks, often radial and partly filled with secondarily crystallized sesquioxides; the cracks indicate a certain amount of contraction in the course of crystallization. Hanlon (1944) believes that these pisoliths form in or directly above a water table. This hypothesis, however, is disputed, since many pisolithic horizons appear to be outside the influence of even a fluctuating water table. Moreover, the hypothesis implies the presence of an extremely aqueous medium. It is possible that the origin of true pisoliths m a y be due to extension of a poorly drained zone producing vast marshes (Tessier, 1954). True pisoliths, however, occupy very limited space. They are observed mainly in rounded, but not in ovoid, form. According to Schade (1910), these differences must be connected with the degree of purity of the materials mobilized in the process of development. The structure would be radial in the case of a pure substance in solution; it would be simply concentric if other materials such as colloids and crystalloids were present.

5. Lastly, in addition to these different faciès, aggregated materials are observed, of varying size and composition. Some of these products fill up the cracks and pores; others cannot be related to known cavities. These materials can be composed of pure goethite, hematite or crystallized gibb-site; others are amorphous. Still others are unidentifiable mixtures of elements of the particle size of clays, the majority of them impregnated with iron sesquioxides. These textures of varied appearance correspond to a reorganization of the constituents, implying transport, whether localized or not. Observation under the microscope tells us more about these processes than direct visual observation.

Although we still know little about the development of latérite microstructure, the little w e do know shows the importance of more or less localized sesquioxides and clay movements and this knowledge can be used to differentiate lateritic materials which harden from those which do not.

LATERITE INDURATION

It was long thought that the induration of latérites was due to the development of constituent sesquioxides, precipitated, concentrated and crystallized as a result of desiccation. W e have already referred to the weakness of this hypothesis.

The analytical facts show that a mere concentration of these materials would not in itself create the conditions for induration. Many lateritic alteration soils display no indurated horizon containing large amounts of iron or alumina or both, while other hardened soils m a y contain much smaller quantities of these materials.

O n the one hand, the relative proportions of sesquioxides in the indurated horizons can vary. In some cases, the iron content exceeds 80 per cent, with less than 5 per cent of alumina, while in other cases the alumina content can be as high as 60 per cent and the iron oxides as low as 4 per cent. Nor is the kaoli-nite content dependent on the degree of induration. M a n y Guiñean crusts contain more than 20 per cent of combined silica in the clays.

It does seem, however, that iron plays a key part in the induration processes (D'Hoore, 1954; Alexander et al., 1956; Maignien, 1958). It is not the absolute value of the iron that counts but its arrangement in the profile in relation to

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the other constituent materials. This is w h y the Fe 2 0 3 content in roughly comparable indurated crusts is approximately inversely proportional to the insoluble matter content.

Moreover, there are many degrees of induration, ranging from almost loose, barely coherent products to the most hardened blocks, difficult to break with the hammer. It is difficult to define these degrees of induration by physical methods because the indurated horizons are frequently not homogeneous and induration affects only certain parts of them (the more or less enclosed, indurated skeleton imprisoning loose materials, hardened nuclei in an earthy matrix and so forth).

Simple methods are usually the best for estimating the degree of induration resistance to penetration by a spade, to breakage by hand, to hammer blows and so on.

Maignien (1958) distinguishes four degrees of induration: slightly indurated crusts which break in the hand; moderately indurated crusts which can be dug with a spade; indurated crusts which can be smashed with a hammer ; and strongly indurated crusts which cause the hammer to rebound.

Aubert (1954) distinguishes a 'shell' stage which can be broken by hand and easily worked with a spade and a 'crust stage' which can be broken only with a hammer.

If iron seems to be the predominant element in latérite induration, it must nevertheless be pointed out that there are uniquely aluminous crusts which owe their induration to alumina crystals forming a coherent fabric. Numerous scoria-ceous bauxitic crusts are merely alteration horizons with a pain d'épiées faciès partially leached of iron. The occurrence of these materials, however, requires large quantities of alumina and very long periods to develop.

With iron, on the other hand, the induration processes are much more rapid. In general, the more indurated latérites seem to contain the largest relative amount of iron.

In most cases, induration seems to be a function of iron content, but the latter, which is sometimes very small, does not necessarily correspond to all the iron present. Quantities vary considerably according to the nature and proportion of the material present and according to the environment. A high iron content does not necessarily mean induration. This is an axiom proved by numerous published examples. Further, a single profile m a y contain two horizons of comparable iron content, one indurated and the other not. Nor has it been demonstrated that a lateritic alteration soil will necessarily develop towards a degree of induration with time, even if the iron-enriched horizons are exposed. The assumption that induration will necessarily occur is still very c o m m o n and is at the root of a number of false interpretations. The causes of induration depend rather on the way the components are arranged than on their content in absolute values.

According to Alexander et al. (1956), the ferruginous skeleton of indurated latérites displays a higher degree of crystallinity and, above all, a greater continuity of the crystalline phase than do the loose materials associated with them.

Study of crusts in thin section has shown that the crystalline phase of iron oxides is distributed over a continuous fabric. This holds good even though the shape of the assemblage can vary greatly from one sample to another. In most cases the mineral in question is goethite, although hematite is sometimes dominant.

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Origin of latentes

The result is that all the conditions permitting segregation and crystallization of iron into a continuous assemblage lead to the formation of a skeleton which ensures the induration of the latérites. This implies the presence of environmental conditions such as to stimulate at least local movements of the iron, promote the development of crystallinity and permit the development of some degree of continuity of the crystalline phase.

The transformation of a loose material into indurated latérite can affect only a small part of the mass and involve only a small proportion of the iron present. For example, Buchanan's latérite is merely a kaolinitic material impregnated with iron which hardens rapidly on exposure; Alexander et al. (1956), studying a comparable sample from the Ivory Coast, report that the principal change corresponding to induration seemed to be a reorganization of the constituent minerals. The chemical, mineralogical and micromorphological differences between the loose material and the indurated material are not pronounced. A reduction of kaolinite content corresponds to an increase of crystallinity and continuity of goethite, and this seems to be the principal cause of induration.

Certain latérites, however, although similar to Buchanan's in chemical composition and mineralogy, never harden on exposure.

Alexander et al. (1962) connect this abnormal behaviour with the results obtained by Fripiat and Gastuche (1952) who showed that kaolinite possessed marked properties of absorbing and immobilizing iron. Kaolinite, a c o m m o n constituent in lateritic loose materials, occurs isolated in pockets, imprisoned in the cortex of crystallized goethite at the time of induration. This type of induration cannot occur if the kaolinite is impregnated with ferruginous solutions before the goethite crystallizes; in such a case, there would be more continuity of the crystallized phase and hence no induration.

The results obtained by the above-mentioned authors show that in hardened materials the percentages of kaolin are almost always lower than in the loose subjacent material, although this is not an absolute rule. It is impossible to be certain whether this reduction is due to destruction of kaolinite or to iron brought in from outside. In many cases, however, it seems that kaolin is altered (Jackson et al., 1948) and that the alteration is accompanied by a liberation of iron which crystallizes into goethite (Sivarajasingham et al., 1962). The disappearance of kaolinite would diminish the capacity of the whole to immobilize iron and would consequently make it more likely that the continuous phase of goethite would develop.

The excess of iron over clay materials could result from other processes — the leaching of kaolinite or the illuviation of iron.

These mechanisms would be difficult to demonstrate except by studying complete profiles and chains of soils. D'Hoore (1954) and Frankart et al. (1960) tried to tackle the problem by using the electron-microscope. They found that the presence of kaolinite crystals with corroded edges produced alteration of the mineral, with corresponding increase in the iron content. This was a case of relative accumulation. Conversely, the presence of well-preserved pseudo-hexagons in a hardened medium would indicate absolute accumulation of iron from outside the profile under study.

The causes of latérite induration can vary. They are connected with the conditions of immobilization of materials in solution and with the environmental conditions in which the processes occur. These conditions and mechanisms of

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immobilization have been dealt with in the preceding paragraphs (lateritic neosyntheses, accumulation, etc.) and will not be repeated here. I shall mention only that the induration of latérites can occur directly in soils, without exposure, or only after exposure, whether natural (erosion) or artificial (openings of roads, quarries and so on). In the first case, the induration is connected either with certain alteration faciès (gibbsitic pain cFépices crusts) or, more often, with absolute iron accumulations, and seems relatively unaffected by external climatic fluctuations. Conversely, climatic fluctuations seem to be important in the formation of crusts which harden after exposure. Atmospheric agents remo-bilize the sesquioxides and redistribute them in small channels and pores to form continuous films which gradually cement the exposed horizons and make them coherent.

Under the influence of natural agents, the exposure of sesqaioxide-enriched horizons will lead to fairly intensive and pronounced induration. The exposure of the horizons in which the sesquioxides have accumulated is due to the removal of loose, surface horizons by erosion. Such exposure does not occur suddenly but gradually. The plant populations, particularly those of the herbaceous vegetation, will be partly preserved with the result that in the rainy season there will be intensive mobilization of sesquioxides which have already separated (reduction, complexation, chelation). This mobilization, followed by leaching, will lead to the impregnation of the subjacent horizons and the deposition of more or less continuous films in small hollows, imparting rigidity to the whole structure. The phenomenon is therefore one of gradual redistribution, connected with the rhythm of precipitation of previously segregated materials. Such conditions must successively bring about conditions of mobilization, then of immobilization and crystallization, the former being predominant in the rainy season and the latter in the dry season. Tropical climates will therefore be more likely to promote the formation of crusts than equatorial climates.

Alexander et al. (1962) studied in detail the induration of a soft laterite artificially exposed to the air and found that induration appreciably increased. The original sample could be cut with a fingernail. After fifteen years of exposure, the sample could be broken with a gentle blow of the hammer . This slight induration corresponded to a migration of iron within the sample, towards the outer canaliculi and cortex.

In the above case, the organic matter could not be a factor in the mobilization and segregation of the iron. The sample has merely been subjected to alternate wetting and drying each year over a number of years. This led the authors to think that the succession of periods of wetting and drying, not merely from one year to another, but from one rain to another, was the essential condition for induration, rather than drying alone. They further thought that it implied humidity in the zone of segregation equal to or greater than field capacity over considerable periods. The presence of a true ground-water table is therefore unnecessary, although it can contribute to the wetting-drying cycle.

Laterite induration thus seems to depend on: (a) a more or less continuous arrangement of the constituents, implying some degree of migration; (b) the state of crystallization.

The degree of induration depends heavily on the immobilization of sesquioxides. W h e n immobilization occurs through precipitation, induration is almost immediate and there is no desiccation stage. At most, there is slight retraction of the whole formation, producing cracks which become secondarily enriched.

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Origin of latentes

If immobilization is due to flocculation, colloidal films are deposited on the walls of the pores and canaliculi.

Induration does not necessarily occur, but only when desiccation causes the groundmass to age; it then crystallizes more or less rapidly and to some extent the constituent minerals become arranged one upon another.

Induration generally precedes this ageing which merely stabilizes it. W e can assume that desiccation leads to a more pronounced attraction of the molecules, with adsorption on surfaces already in situ. The successive deposition of matrices packed one upon another produces resistance superior to that of uniform accumulation.

These two cases correspond to specific soil-making environments. The former is connected with temporary hydromorphic effects; the latter is connected with oxidation mechanisms in a well-drained environment. In reality, most cases are less clear-cut and every possible transition between the two extremes is observed.

LATERITE DEGRADATION

If latérites develop at more or less great depth in soils, it m a y be asked how they can disappear. Study of neosol formation on exposed crusts and of the development of the relief in regions where crusts occur shows that these horizons can be recaptured by the agents responsible for the weathering of rocks and can then take part in a new cycle of development. Just like any other surficial litho-logical formation, crusts are destined to disappear at some time or another. Even on the pedological time-scale, the idea that crusts are irreversible is unacceptable (Greene, 1950). O n the human time-scale, lateritic horizons do seem to be relatively stable, yet detailed studies show that ferruginous cements, which usually constitute the internal skeleton of the crusts, can develop rapidly if environmental conditions permit. Vegetation can become established on the crusts and help to loosen the hardened materials to varying depths. Erosion sweeps away formations already in process of breaking up, at varying rates. All this makes for the development of an appreciable relief.

Latérites disappear either because they are exposed or because the soil-forming conditions change.

Exposure and demolition of crusts

In the great majority of cases, the exposure of indurated horizons is due to erosion by water in one form or another. The loose surface material is swept away more or less rapidly, exposing the hardened horizons.

These processes can be subdivided into: (a) geological erosion, contributing to the normal development of the relief; (b) accelerated erosion due to human intervention.

H u m a n intervention has been overstressed as a reason for the exposure of crusts. Although its effect is sometimes very pronounced, it cannot alone explain the exposure of all hardened horizons. Normal erosion alone has been responsible for the exposure of Tertiary crusts and has, in addition, caused inversions of clearly defined reliefs (Bonnault, 1938). Crust formation protects the looser subjacent horizons against erosion. It acts as a protective cover on gentle slopes where there is nothing to enable the relief proper to table or mono-

108


clinal structures to develop (cliffs, cuestas, monadnocks, scarps, and so forth). W h e n the protective cover disappears, the true relief emerges in the subjacent formations.

According to Fournier (1960), Africa and South America are the most eroded parts of the globe, followed by Asia, the whole of North America, Central A m e rica, Australia and, lastly, Europe.

It has been calculated that the time required for the outermost film of the earth's crust to lose an average of 1 m . by ablation is as follows: 16,666 years in Europe, 5,126 in Australia, 2,857 in North and Central America, 2,300 in Asia, 2,000 in South America and 2,000 in Africa.

These figures are in themselves sufficient to explain the exposure of crusts. H u m a n intervention, however, can accelerate the effect. Fournier (1956) measures earth losses in tropical climates as 5-50 m m . of thickness per year when the slope is more than 3 to 5 per cent. A simple calculation shows that a few centuries would be enough to remove 1-3 m . of loose earth such as covers the majority of crusts in process of formation.

Regressive evolution

Once the crusts are exposed, they fall victim to normal weathering effects. First, disaggregation mechanisms set in. Erosion causes the crust to break up as the base of the loose formations underlying the indurated horizons is sapped away and the mass moves down the slopes (Fournier, 1956). The larger blocks crumble and fall to form chaotic masses at the foot of the elevations. This, combined with mechanical wearing-down of the debris by torrential waters, produces gravel and fine material.

A n important point is that these materials are never carried very far away. Most of them remain in situ where they form sheets on nearby slopes. Such m a terial is hardly ever transported even a few kilometres and certainly not hundreds of kilometres away, as some authors maintain. As a result, the iron sesquioxides become readily soluble. Morphological and mineralogical examination of indurated formations in process of destruction reveals mainly physical disaggregation and cement-leaching effects (Alexander, 1956). There is comparatively little rehydration and resilicification.

It can happen, when the environmental conditions are modified, that soils in process of hardening develop in reverse, the indurated horizons beginning to disappear in situ. The first stage is the development of herbaceous vegetation, attaching itself to the smallest cracks where a certain amount of earthy m a terial collects. Gradually an organic horizon builds up, rapidly dissolving the cement. The crust is surface-loosened and forms a juvenile soil, often enriched with earth material brought by termites. Shrubs and then trees can next be planted. The woody roots complete the dismantling process and the soil horizon deepens. The new ecological conditions, especially the increased humidity, produce a chemical alteration. The finer the crust debris and the closer the mixture of the loose and humic mass, the more active will be these chemical processes. The soil solutions contribute to the solution first of iron and then of alumina sesquioxides. The disappearance of the cements leads to the formation of gravelly soils.

Regressive evolution of latérites thus frees sesquioxides, sometimes in large amounts, which migrate with the water of percolation and can thus take part in the formation of new crusts at lower levels.

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According to the ecological conditions of the environment of degradation, the products derived from the destruction and dissolution will take part in a variety of processes: (a) accumulation of slope debris or crumbling in situ; (b) development of beaches of residual gravel on nearby slopes; (c) formation of crusts at the bottom of slopes and of sheet crusts, mainly ferruginous ; (d) transport, through drainage, to the oceans or into depressions.

The hardening of soils thus contributes to a succession of cycles affecting the constituent elements of latérites. T h e result is the formation of different indurated faciès in the terrain. B y studying these, w e can reconstruct the history of their formation and therefore the stratigraphy.

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Classification of latérites.

Correlations

The problem of classifying latérites, if only for the purpose of refining the definitions, soon attracted attention (see the first chapter) and is still central to research on intertropical soils.

At the outset, views differed widely, but the divergence is becoming steadily narrower and we m a y hope that this will facilitate efforts at correlation in the near future.

Early attempts at classification related exclusively to samples of indurated latérites and were based on chemical and sometimes also mineralogical criteria.

Lacroix (1913), for example, distinguished : (a) true latérites, with an hydroxide content of more than 90 per cent; (b) silicate latérite, with an hydroxide content of 50-90 per cent; (c) lateritic clays, with an hydroxide content of 10-50 per cent.

The relationships between latérites, indurated horizons and soils, however, were obvious and attempts at classification were aimed at narrowing down these relationships on the basis either of morphological characters or of genetic criteria.

Pendleton (1936), using an extremely strict definition, distinguishes between 'latérite soils', which have a latérite horizon in their profile, and 'lateritic' soils containing only a poorly developed horizon of soft, lateritic character, which will turn into true latérite if the right conditions are maintained for long enough.

Kellogg (1949) derived the word latosol from the term 'lateritic soil', but gave it a broader meaning.

In 1949, Kellogg restricted the term latérite to four principal forms of ses-quioxide-rich material which was already indurated or could become so on exposure : (a) soft mottled clays that change irreversibly to hardpans and crusts when exposed; (b) cellular and mottled hardpans and crusts; (c) concretions or nodules in a matrix of unconsolidated material; (d) consolidated masses of such concretions or nodules.

Martin and Doyne (1927), on the other hand, relied on certain chemical characters, especially the silica-alumina ratio. Strongly lateritic soils would have a Ki ratio of 1.3. These findings were extended to all lateritic soils, whether crusted or not, and widely used in the French and Portuguese classifications. The value of this ratio has lately been disputed and it now seems to be no more than an index datum.

Ill

Classification of latérites. Correlations

Since 1950, several studies have been published (D'Hoore, 1956; Maignien, 1958; Fridland, 1961) suggesting that the problem of lateritic alteration soils is not necessarily bound up with that of crust formation. The latter phenomenon is secondary to the former and is not specifically connected with lateritic soils. Other pedogenetically different soils can display induration processes. These results have led to the introduction of a new nomenclature: the terms plinthite and cuirasse are used for indurated formations and the terms oxisols, kaolisols, ferrallitic soils, and so forth, for lateritic alteration soils.

Several attempts at classifications of indurated horizons have been proposed, based on the mechanisms of their formation.

D'Hoore (1954) proposes:

1. A broad subdivision based on the present state of development of the accumulation zone and comprising three large categories: (a) zones in which accumulation is still continuously in progress; (b) zones in which accumulation has stopped but which have remained intact; (c) reworked old accumulation zones and the alteration products.

2. Further subdivisions of each of these main categories, according to the mode of concentration: Group A. Absolute accumulation, negligible relative enrichment. Group AT. Absolute accumulation, marked relative enrichment. Group R. Relative accumulation, negligible absolute enrichment. Group Ra. Relative accumulation, marked absolute enrichment.

3. The zones of absolute accumulation (A and Ar) are classified according to the nature of the recipient material in which they form. Groups R and R a are classified according to the material from which they are derived.

4. Groups A and Ar can be classified according to the range of translocation phenomena which have given rise to them.

Accumulation in the profile and in the terrain could thus be distinguished according to the following system: (a) vertical transport (ascending, descending); lateral transport, within a site, terrain, natural region, continent, etc.

Groups R and Ra can be further subdivided according to whether accumulation is superficial or deep.

5. Further classification would be based on morphological and analytical data. This classification takes into account only induration processes at an extremely low level and does not emphasize any relationship with the types of soil in which they have occurred.

Maignien (1958) attempts to classify soils with crust horizons (à horizon cui

rassé) on the basis both of the genetic and of the morphological characters of the soil in situ.

His first criterion is the characterization of the soil in situ according to the major ranks (orders and groups) of the French classification of tropical soils (Aubert, 1954, 1961).

Maignien's second criterion is based on the origin of the sesquioxides : (a) free sesquioxides produced in situ by the development of the soil; (b) sesquioxides of external origin, reaching the soil as pseudo-solutions ; (c) accumulated sesquioxides in an already indurated form.

The third criterion relates to accumulation processes: relative accumulation; and absolute accumulation.

The fourth criterion relates to the various factors regulating the intensity

112

Classification of latérites. Corrélations

of the accumulation processes: (a) drainage and migration characteristics (percolation, oblique leaching, sheetflood erosion) ; (b) physicochemical and biological properties of the accumulation medium; (c) composition of the original material.

Maignien specifies that these different criteria are not independent of one another, but are more or less interconnected. This reduces the number of cases considered to fifteen, of which he gives the morphological, chemical and mineralogical characters.

It is difficult to separate the classification of latérites from that of tropical soils. Without entering on the long discussion which this problem alone would require, I think it important to indicate the principles and characters on which the main classifications in current use are based.

There are two types of classification, synthetic and analytical (Manil, 1956).

Synthetic classifications can be divided into three main groups: (a) classifications based on genetic factors (the U . S . S . R . system) ; (b) classifications based on soil genetic processes (the French and Portuguese systems); (c) classifications based on the properties of pedogenetic factors or processes (the British and Australian systems).

Analytical classifications are based mainly on morphological characters, with a bias towards soil-genetic considerations (the U . S . A . , Belgian and F A O systems).

The system used by the Service Pédologique Interafricain (SPI) must be considered separately since it consists of a m a p legend and therefore defines cartographical units rather than classification categories.

U . S . S . R . S Y S T E M

This system is based on genetic factors. According to Gerasimov (1962), w h o uses the term latérite in the broad sense to include all tropical soils, a primary distinction must be made between tropical latérites and subtropical lateritic soils. The differences, which are not qualitative in character, are connected with thermal-erosion and soil-formation factors; they relate only to the degree of development of the process and the thickness of the resultant products. The boundaries between these two large categories are gradual and consequently conventional.

Another important division of lateritic formations is based on the humidification factor. Here, the following types are distinguished: (a) extra-laterites and lateritic soils or allites, which are proper to permanently humid, tropical countries and to subtropical countries; (b) mixed latérites or typical lateritics and lateritic soils which are allito-ferrites or ferrito-allites proper to tropical and subtropical, semi-humid regions.

Great importance must be attached to the petrographical peculiarities of the parent rock subjected to a lateritic process. In particular, the influence of basic rocks leads to the formation of special soils, very rich in newly formed minerals (allophanes).

These soils are the allophanites. They are subdivided according to biochemical conditions into the following : (a) humic allophanites, proper to arid tropical regions (regur soils and smolnitzes); (b) lateritized allophanites, proper

113


to semi-humid and h u m i d tropical regions (terra roxa) and to h u m i d tropical regions (Hainan latérites, China).

T h e system as a whole, distinguishing between latérites and lateritic soils, can be presented as follows. Tropical latérites. Extra-laterites (allites); typical latérites (allito-ferrites and

ferrito-allites); ferrous latérites (ferrites); h u m i c allophanites (regurs); late-ritized allophanites (terra roxa).

Subtropical lateritic soils. Krasnozems and lateritic zheltozems (red and yellow soils); allophanites or tropical black soils (smolnitza 'tirs' ash soils, etc.).

Complete systematization of contemporary latérites and lateritic soils would entail also laying great stress on the transitional geographical-genetical subdivisions bridging the gap between the principal types mentioned above and other genetic types accompanying or bordering on t h e m . It is thus well founded to distinguish the yellow-red soils of south-east China, the yellow-brown soils of central China, and the red-brown soils of north India as transitional lateritic occurrences; similarly, the subtropical soils transitional between the m o r e developed lateritic soils (the krasnozems and zheltozems of h u m i d tropical regions), on the one hand, and the reddish forest soils of the northern belt and b r o w n soils of semi-humid subtropical regions, on the other, can be regarded as transitional types. T h e induration process accordingly does not appear at the highest levels of this classification.

L o b o v a and K o v d a (1960) give the classification s h o w n in Table 7 relating solely to the tropical and equatorial Asian soil belt.

T A B L E 7. Tropical and equatorial soils of Asia, classified according to climatic conditions

Very pronounced dry period Moderately pronounced dry period

Mildly dry period or no dry period

Clotty (grumous) regurs (black soils), 'black cotton soils'. Margallitic soils under herbaceous and arboreal vegetation.

Black, humus-poor, compact and clayey soils under savannah vegetation; associated with gilgai relief.

Red-brown savannah soils with carbonate concretions.

Red soils of tropical deserts under thorn bushes.

Red and yellow (clayey) tropical soils with ferruginous concretions under monsoon forests and secondary savannah.

Red-brown tropical soils of dry forests and secondary savannah.

Strongly lateritized yellow lateritic soils of equatorial humid forests (lato-sols).

Red moderately lateritized lateritic soils of tropical forests (ferrisols).

Slightly lateritized (relatively recent) brown soils: rubrosols and brunosols.

Podzolized lateritic soils of very humid equatorial forests.

Ground-water latosols of secondary savannahs.

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F R E N C H SYSTEM

The French system makes a clear distinction between lateritic alteration soils, called ferrallitic soils, a term proposed by Robinson (1922), and crust-formation processes which m a y occur in genetically different soils, such as tropical ferruginous, ferrallitic or hydromorphic soils. As a result, the presence of horizons containing sesquioxide-rich indurated materials (latérite) appears only at sub-group level.

The general characters of ferrallitic soils are much the same as those indicated by other authors, particularly Kellogg (1949) for latosols : Very low mineral content in the parent rock, apart from certain very stable

minerals ; Very high content of metal hydroxides (iron, aluminium, manganese, titanium),

resulting in low silica-sesquioxide and silica-alumina ratios in the colloidal fractions and (apart from primary quartz) in the soil as a whole; in general the silica-alumina ratio is never wider than 2 and m a y fall below that figure; at least one of the soil horizons contains separated alumina;

Colloidal fractions composed, at least in the more developed horizons, of iron, aluminium and titanium hydrates and hydroxides, associated in varying degrees with kaolinite and sometimes with a little illite;

Small or very small silt fraction;

Generally low organic content, particularly of humus ; absence of coarse humus. Only the humic ferrallitic soils, corresponding to humic latosols and hydrol humic latosols, are well supplied with, and often very rich in, humus; but even here the humus consists, apart from undecomposed plant residue, merely of organic matter with a very wide carbon-nitrogen ratio (between 8 and 12); base exchange capacity of the mineral fractions is low or very low and there is usually high stability of the aggregates, and, in many cases, concretioning, especially of the ferruginous fractions, in the form of pseudo-sands, concretions, hardpans or crusts (Aubert, 1954).

Ferrallitic soils fall into four main groups:

Slightly ferrallitic soils. In these, mineral decomposition has not advanced to the fullest extent; hence, the Ki ratio is near to 2 and there is often an appreciable reserve of alterable minerals. These soils accordingly often have a pseudo-psammitic texture, which in many cases appears specific. They comprise several subgroups: modal (terre de barre); ferrisolic (in the Belgian sense); pseudo-gley hydromorphic; indurated with a crust horizon (horizon cuirassé).

Typical ferrallitic soils, in which the dominant structure is often polyhedral. They are divided into subgroups according to the colour of the A and B horizons : red soils; yellow or beige soils; yellow soils on red horizons; indurated soils (cuirasses).

Leached ferrallitic soils. These soils are almost completely desaturated and their mineral reserves are non-existent. They often present a degraded structure, at least in their surface horizons. They can be divided into : soils strongly leached of bases at the surface but not leached of mineral colloids ; soils leached of bases, sesquioxides and clays; leached soils with a crust horizon (à horizon cuirassé).

115

Classification of latérites. Corrélations

Humic ferrallitic soils. These soils are characterized by a high organic matter content (more than 6 per cent to 20 cm.) . The following subgroups are distinguished according to the characters of the humic horizon : black soils, often approximating to andosols; brown soils, often approximating to tropical brown eutrophic soils; brown-red soils; very acid leached humic ferrallitic soils, often with a textural B horizon; high-altitude humic ferrallitic soils.

Turning now to crusted soils (sols cuirasses), we find that these again are mentioned among the tropical ferruginous soils and, more specifically, in the group of leached tropical ferruginous soils which present a textural B horizon. These are divided into four subgroups, according to the rate of iron accumulation at depth in relation to the processes of temporary blocking due to clay illu-viation: leached tropical ferruginous soils without concretion; with concretions; with crust, and with pseudo-gley at depth.

Lastly, w e have essentially ferruginous crusts (cuirasses) in moderately or slightly humidified hydromorphic soils with pseudo-gley at depth (sheet crusts).

Indurated horizons which have been levelled are regarded as the parent m a terial of rough erosion soils ; accumulations of old crust debris are regarded as crude mineral-intake soils.

P O R T U G U E S E SYSTEM

The Portuguese system in many respects resembles the French classification. O n the Angola soil m a p , for example, Botelho D a Costa and Azevedo (1960) distinguish, among others, the following soils:

Tropical ferrallitic soils with or without latérite, subdivided into brown-yellowish soils and red soils and corresponding to the tropical ferruginous soils of the French classification;

Paraferrallitic soils corresponding to the ferrisols of the Service Pédologique Interafricain (SPI) m a p and subdivided according to the type of relief where they are observed (normal or excessive);

Ferrallitic soils with or without latérite, comprising brownish ferrallitic, yellow ferrallitic, red ferrallitic, yellow-sandy ferrallitic and red-sandy ferrallitic soils.

The group covers the slightly ferrallitic and typical ferrallitic soils of the French classification, with the exception of soils transitional to ferrallitic, or paraferrallitic soils.

BRITISH SYSTEM

The British classification of latérites and lateritic soils is based on the ideas of Charter (1954) which were taken up by Brammer (1956) in his study of Ghana soils. The system takes into account also Kellogg's definition of latosols (1941).

Latosols are classified as climatophytic earths in which the characteristics of the soil are determined partly by climate and partly by vegetation. Latosols are subdivided, according to the direction of variation of their pïL in the different horizons, into ochrosols and oxisols, the latter being more acid at depth than the former. The term latérite is used in Pendleton's sense which was to a

116

Gaseification of latentes. Correlations

large extent adopted also by D u Preez (1949). In the definition of the Great Groups, attention is drawn to the possible action of phreatic water and a group of ground-water latérites is specifically distinguished. According to the descriptions of Oben and Quagraine (1960), this group could in certain cases be more or less equated with the leached, ferruginous tropical soils with crust.

In recent British work in Africa, the SPI key has been increasingly used.

A U S T R A L I A N S Y S T E M (Stephens, 1962)

Four Great Soil Groups are recognized in Australia as lateritic: krasnozems, lateritic krasnozems, lateritic red earths and lateritic podzolic soils. The term latérite is used in the Anglo-Saxon sense. The four groups belong to the acid-to-neutral category of soils and display no trace of the clay leaching proper to the pedalfers or soils in which chalk carbonate does not accumulate in any part of the profile. Even if there is carbonate in the matrix, it is constantly transported out of the profile.

All these soils have a differentiated profile (class). Apart from the krasnozems, all the lateritic soils are regarded as polymorphic.

Lateritic podzolic soils are characterized by a deep horizon of nodular, piso-litic or massive latérite, often, but not necessarily, resting on a mottled or whitened kaolinitic clay horizon. They terminate at depth in a pallid or a mottled zone. Sometimes there is an intermittent horizon of siliceous material called silcrete.

It seems that these soils can to some extent be equated with the leached, tropical ferruginous soils with concretions and crusts of the French classification. Since they are distributed in Pliocene-type formations, they are generally regarded as fossils.

The krasnozems have often been classed with the red loams and recently with the latosols. They are red to yellowish-brown, deep, friable and argillaceous ; with the exception of the organic A , their horizons are poorly developed. Occasionally, small manganiferous and ferruginous concretions can be found in the profile. This makes them very similar to the slightly ferrallitic red soils of the French classification often known as terres de barre.

The latérite krasnozems differ from the preceding categories in having an indurated horizon of variable thickness which m a y be blocky and pisolitic, fragmentary or diffused, or porous and vermiculate. This horizon normally rests on a mottled or whitened kaolinitic clay zone. Although no phreatic water has been recognized, it seems that these soils are of hydrogenic origin. They seem to be of contemporary age.

The lateritic red earths are deep red to reddish soils with a latérite horizon at depth resting on a mottled or whitened kaolinitic horizon.

The A horizon is generally sandy to silty, blackened by some organic matter. It gradually gives way to a bright-red horizon of fine texture and usually compact but sometimes vesicular structure.

The latérite horizon, of variable thickness, is nodular, pisolitic, vermicular or blocky. The subjacent clay horizon often shows signs of containing silcrete. These are probably fossil formations of Tertiary and Pliocene origin. Fluctuations of the phreatic water level are responsible for the development of the clay and the latérite horizons.

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U . S . A . S Y S T E M (USDA, Seventh approximation, 1960)

All lateritic soils, with or without crust, are classified as oxisols. These are defined as follows : ' G . Other mineral soils with an oxic horizon or with at least 30 cm. of plinthite surface forming a continuous phase and not indurated'1

The definition thus rests on two conceptions, that of an oxic horizon and that of plinthite. The former is mentioned tentatively and is not definitive.

Oxic horizons occur either below a soil surface (in virgin soils) or at the surface of certain soils. Such horizons possess the following further characteristics: 1. Polyhedral or blocky structure with numerous visible pores. 2. Little trace of the original structure of the parent rock, or none. 3. Not more than 15 per cent, at most, of particles smaller than 2jx, of which

90 per cent consist of a 1/1 poorly dispersible mixture of free sesquioxides and clays.

4. A least 12 per cent of a 1/1 mixture of free sesquioxides and clays. 5. Not more than 1 per cent of micas, felspars or ferromagnesian minerals in

the sand and silt fractions; the clay fraction must not show any traces of montmorillonite, illite, allophane or vermiculite;

6. The exchange capacity is less than 20 meq. per 100 g. of clay (ammonium acetate method).

Plinthite is a sesquioxide-rich, humus-poor, strongly altered mixture of clay, quartz and other components, usually in the form of red mottles, generally forming a foliated, polygonal or reticulate pattern. It forms after repeated wetting and drying. It m a y also consist of the hardened residue of loose red mottles. The lower boundaries of the formation are often diffuse and gradual, but can be abrupt at the contact with a lithological discontinuity. The plinthite formation gives rise either to isolated concretions or to more or less porous, scoriaceous hardpans or crusts as a result of differential hardening of the zones richest in Fe a03 , often in horizons which are over saturated with water at certain times of the year.

Oxisols can display a clay horizon under the oxic horizon provided the latter is sufficiently thick (at least 1 m . ) . Generally, however, there is no leaching of the clay, in contrast to ultisols.

Subtypes of oxisols

Aquox. Oxisols display one or more characters indicating current or previous hydromorphism : (a) non-indurated plinthite in the top 30 cm. (b) cellular surface soils ; (c) chroma of a least 2 in the top part of the oxic horizon if the hue is homogeneous or of at least 3 if there are distinct or marked mottles. These soils are generally rich in A1 2 0 3 and poor in Fe203 .

Acrox. Other oxisols, which have an oxic horizon in the top 125 cm. , have a KC1 p H higher than the water p H . These are silica-poor, maximum-alteration soils with very low silica-alumina ratio.

Udox. Other oxisols which are permanently humid and those in which at a certain level desiccation does not last longer than 30 days. The base saturation

1. Retranslated.

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of these soils in the 50-125 c m . level is less than 25 per cent. These are the most humid ferrallitic soils occurring under heavily shaded forest. At high altitudes, they are enriched in organic matter. The colour is dark brown or black (umbrudox soils).

Ustox. Oxisols which periodically have certain dry horizons and a base saturation below 50 per cent at the 50-125 c m . level in the oxic horizon. These soils support tropical forest vegetation or secondary savannah.

Idox. Oxisols which are habitually dry or which have a base saturation above 50 per cent at the 50-125 c m . level in the oxic horizon. These soils must have formed under more humid climatic conditions, but developed in a dry climate.

This classification needs checking against the facts, since it is often difficult to differentiate oxisols from ultisols and sometimes even from alfisols within lateritic-alteration soils. Moreover, the formation of plinthite is not specific to oxisols and the definition needs expanding.

Latérites can thus be found in two other soil types—alfisols and ultisols.

Alfisols. These are generally humid soils with a clay horizon. There is no soft, oxic or ash horizon. The base saturation in the B horizon is above 35 per cent. This type comprises also soils with a base saturation below 35 per cent provided there are whitish vertical streaks running from the A ^ to the B horizon.

A latérite horizon is observed in soils of the subtype ustalf. These are generally humid, but dry for three or more months. Their A horizons are blocky and friable in the humid state, hardened in the dry state. Here, w e have the special case of the group known as ultustalfs which possess the following characteristics : an exchange capacity T of the clay fraction below 40 meq . per 100 g.; plinthite usually present; kaolinite dominant; S/T ratio fairly high. These soils can be regarded as similar to leached tropical soils with concretions or crust.

Ultisols. These are very strongly base-desaturated soils with a clay horizon (S/T below 35 per cent); they often display plinthite. They have no oxic horizon and the alterable-mineral content is very low. The following can be regarded as equivalent to latérites :

Plintaquults : soils with plinthite (unhardened) at a depth not exceeding 1.25 m . These soils are grey with small red mottles, giving way with depth to a mottled clay. Hardening occurs if this horizon is exposed to the air. The plintaquults can be compared to tropical ferruginous soils and to hydromorphic ferrallitic soils or hydromorphic ferrisols.

Plintochrults : plinthite to a depth of at least 1.25 m . These soils are more coloured at the surface than the plintaquults. The plinthite takes the form of a mottled clay. This group seems to correspond to the slightly ferrallitic soils with hydromorphism at depth.

Rhodochrults: general hue brown-red; Fe 2 O s content of the order of 12-30 per cent of the clay content (as in the case of the oxic horizon). These soils are very similar to oxisols. They are characteristic of basic rocks, despite their surficial acidification. They can correspond to acid (fersiallitic) tropical red soils or to acid ferrisols.

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Classification of latentes. Correlations

Typochrults: the Red and Yellow podzolic soils of the U . S . A . -with the following characteristics : low value of the clay T (under 40 meq./100 g.) ; non-basic parent rock; low proportion of 2/1 clay; very low silt content; presence of iron-rich red mottles, recalling plinthite which produces concretions hardened through exposure. These soils can be compared to tropical ferruginous soils or slightly ferrallitic, leached and desaturated soils or to desaturated ferrisols and fersiallitic soils.

Lastly, the subtype umbrult seems to correspond to humic ferrallitic soils, in particular to certain ferrallitic brown soils.

B E L G I A N S Y S T E M (Sys, 1962)

This again is a morphological system. The criteria for differentiating the various

categories are as follows:

Types: major differences in the type of alteration and in the type and development of the horizons in the profile;

Subtypes: major differences in the degree of gleyification, humidity and soil temperature ;

Major groups : type and sequence of diagnostic horizon and succession of horizons in the profile;

Minor groups : minor differences in the degree of development of the diagnostic horizons and of the characteristic horizons transitional between the higher categories.

The classification of the lateritic soils rests essentially on the definition of the

diagnostic horizon as a ferrallitic B horizon. This is a tropical soil horizon situated between an A horizon and the softened

rock and displaying the following characteristics: 1. Usually a low or non-existent reserve of alterable minerals.

2. A colloidal ( < 2u,) fraction composed principally of kaolinites and/or oxides ; gibbsite often, but not always, present; large quantities of alumino-silicate gels sometimes present.

3. Silica-alumina ratio sometimes near to 2, but usually of a lower value.

4. M a x i m u m intensity of hues in the reds and yellows resulting from an accumulation of iron oxides.

5. M a x i m u m clay content due to a m a x i m u m of alteration, not to accumulation; this horizon can attain a thickness of 5-10 m .

Lateritic soils are classified mainly as kaolisols, but some of them appear as leached kaolisols.

The kaolisols are subdivided into five subtypes according to pedoclimate: (a) hygro-kaolisols (udox?): the kaolisols of low-altitude tropical forests; (b) hygro-xerokaolisols (ustox?): low base-saturation savannah kaolisols; (c) xero-kaolisols (idox, xerox?): dry-savannah kaolisols with a high base saturation; (d) humic kaolisols (humox?): mountain-belt kaolisols; (e) hygro-kaolisols (aquox?): hydromorphic kaolisols.

The major subdivisions are based on the degree of alteration, the silt-clay ratio and structure. These subdivisions are the ferrisols which display a high silt-clay ratio and/or the presence of coatings on the well-developed structural aggregates throughout the profile (structural B ) ; and the ferralsols which have a low silt-clay ratio and lack well-developed structure. The presence of the

120

Classification of latentes. Correlations

above-mentioned characteristics in the 20-70 c m . level is not confined to fer-ralsols.

The minor subdivisions are based essentially on the conceptions of 'central' or orthotypical, on the one hand, and intermediate or 'intergrade' cases, on the other.

SPI S Y S T E M (D'Hoore, 1963)1

This is not really a classification system, but a set of definitions of cartographic units.

The principal latérite and/or lateritic soils fall into three categories: tropical ferruginous or fersiallitic soils, ferrisols and ferrallitic soils sensu stricto. N o distinction is made in regard to the types of crust, which when level are indicated as lithosols.

Tropical ferruginous (fersiallitic )soils

Definition. A group of soils with an A B C profile, some of which have an A^ horizon and a textural B horizon, in which case they tend to have a nuciform or slightly prismatic structure. There is often extensive individualization of the free iron oxides, promoting their lixiviation out of the profiles or precipitation within the profile in the form of mottles or concretions. Their alterable mineral reserves are often appreciable. The silt-clay ratio (20/2 ¡A), determined by repeated dispersion, sedimentation and separation of the supernatant suspension, is generally greater than 0.15. The clay is largely kaolinitic, but often contains small amounts of 2/1 clay. Gibbsite is generally absent. The Si02/ A1 2 0 3 ratio is near to 2, generally slightly higher, while the Si02/R203 ratio is always less than 2. The cation exchange capacity of the mineral complex is low, but above that of ferrisols and ferrallitic soils of comparable clay (gra-nulometric) content. The cation saturation of the B horizon is generally above 40 per cent (N ammonium acetate pH 7).

Ja 1. O n sandy parent material. J B 2. O n rocks rich in ferromagnesian minerals. JC 3. O n acid crystalline rocks. J D 4. Not differentiated.

Ferrisols

Definition. The ferrisols display a profile very similar to that of the ferrallitic soils sensu stricto, often with a structural B horizon (which is sometimes lacking in the coarse materials), with shiny-surface aggregates. These are not necessarily clay films, nor do they occur always on profiles in the dry state. These films m a y be due to the presence of mixed alumino-siliceous gels. The alterable mineral reserve is generally low, but can exceed 10 per cent in the 50-250 y. fraction. The silt-clay ratio (20/2 ¡jt,)2 is generally wider than 0.20 on alluvium and sedimentary rocks and wider than 0.15 on igneous and metamorphic rocks.

1. SPI = Service Pédologique Interafricain. 2 . See above, 'Tropical ferruginous soils*.

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The clay fraction is made up almost entirely of kaolinite, free iron oxides and amorphous gels, sometimes with small quantities of 2/1 clays and gibbsite. The silica-alumina ratio is near to or narrower than 2. The cation exchange capacity of the clay granulometric fraction in the B horizon, generally greater than 15 meq./100 g., is intermediate between that of the tropical ferruginous soils and that of the ferrallitic soils sensu stricto. The saturation in the B and C horizons is less than 50 per cent (N ammonium acetate pH. 7).

The ferrisols must be regarded as very similar to ferrallitic soils sensu stricto.

They have been kept separate in the SPI list, which relates to cartographic units, partly because they represent a stage of development towards ferrallitic soils sensu stricto, partly because of their better agronomic qualities and their wide distribution.

K c 1. Non-difFerentiated.

K b 2. O n rocks rich in ferromagnesian minerals.

K a 3. Humic. These are ferrisols with larger organic matter content in the surface horizons. In the natural state, the Al horizon is thicker than 25 c m . and has an average organic carbon content of at least 2 per cent. The Ca saturation is less than 40 per cent. The surface texture is finely gritty, rarely massive. Dark surface colour (Munsell value approximately 3). These soils often occur at altitude.

Ferrallitic soils (sensu stricto)

Definition. These soils are often deep and the horizons poorly differentiated, with diffuse or gradual transitions, sometimes with an A 2 or a textural B . This B horizon can be lightly structured in the more argillaceous profiles, but the aggregates do not display the well-developed shiny surfaces described in connexion with ferrisols ; the structural elements are often very finally subangular polyhedral, more or less coherent and form a very friable porous mass.

The alterable mineral reserve is low or non-existent; the silt-clay ratio (20/2(i)1

in the B and C horizons is in general narrower than 0.25 and the clay minerals, of the 1/1 type for the most part, are usually associated with large amounts of iron oxides. They generally contain hydrous aluminium oxides, but although gibbsite—one of the crystalline forms of these—is frequently present, it is not essential. The silica-alumina ratio is often near to, but generally narrower than, 2. The cation exchange capacity of the clay fraction (granulometric) is generally less than 20 meq./100 g. and the saturation in the A and B horizons generally under 40 per cent (N ammonium acetate p H 7).

Dominant hue: yellow-beige (7.5 YR or yellower)

La 1. O n sandy loose sediments. Lb 2. O n more or less clayey sediments.

Lc 3. Undifferentiated.

Dominant hue: red (5 YR or redder)

LI 1. O n loose sediments.

L m 2. O n rocks rich in ferromagnesian minerals.

L n 3. Undifferentiated.

1. See above, "Tropical ferruginous soils'.

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Humic feTTallitic soils

Ferrallitic soils which in their natural state have A horizons rich in organic matter, similar to those described in connexion with humic ferrisols.

Ls 1. Undifferentiated.

Ferrallitic 'dark horizon' soils

Definition. Ferrallitic soils which display below the B horizon (which m a y be either textural or compact) a darker horizon than the overlying and subjacent horizons. This colour is often the same as that of the surface humic horizons. The 'dark horizon' m a y have a well-developed, medium to coarse polyhedral structure with thick, often black and shiny, covering, but it can also be structureless and powdery. Its appearance in the profile is often accompanied by an increase of at least unity in the value of the C / N ratio, which generally brings it up to 15. The average organic carbon content of this horizon is of the order of 0.7 per cent.

Lt 1. Undifferentiated.

Yellow and red ferrallitic soils on various parent materials

L x 1. Undifferentiated.

FAO SYSTEM

This is in fact the 1938 U S D A system, as extended and amended in 1949. It is used in South-East Asia and in South America.

In Asia

Dudal and Moormann (1962) describe the following soils which m a y include latérites sensu lato, though the list is not exhaustive. The classification is by groups.

Red-yellow podzolic soils. These are similar to the soils identified in the southeast of the United States of America. The nomenclature has been used in Indonesia (Dudal and Soepraptohardjo, 1957), Viet-Nam (Moormann, 1961) and Ceylon (Moormann and Panabokke, 1962). In Malaysia, a large number of these soils are called latosols (Owen, 1951).

In South-East Asia, these soils have been classified as lateritic, generally with a qualifying colour (red, yellow, yellowish-brown) (Mohr and Van Baren, 1954; Fridland, 19616; Joachim, 1935). The term latérite does not indicate the presence of indurated materials, but refers to the very low Si02 /R203 and Si02/Al203 ratios or simply to the red colour. It must be noted, however, that not all soils formerly called lateritic should be classed as red-yellow podzolic soils.

Under the seventh U S D A approximation, these soils are classified as ulti-

sols, subtype ochrults. Some of them, however, can be classified among the

alfisols (ultulstalfs).

Grey podzolic soils. This name has come into use since the study of the L o w Mekong soils (Dudal, 1960; Moormann, 1961). In Indo-China, they are called

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terres grises (grey earths). T h e comparable soils in Thailand are the korat series (Pendleton, 1953). These soils can be classed as ground-water latérites. T h e y are comparable with certain lateritic podzolic soils in Australia (Stephens, 1962). It is difficult to find t h e m a satisfactory place in the seventh approximation.

Dark red and reddish-brown latosols. These were originally classified as lateritic soils. T h e term latosol is extensively used b y D u d a l and M o o r m a n n to denote the soils formerly k n o w n as red earth (Mohr , 1948), Rotlehm (Vageler, 1938), sols latéritiques ( D a m e s , 1955) and terres rouges (Henry, 1931). T h e colour adjective enabled soils of different composition and fertility to be distinguished. These soils all belong to the oxisols of the U S D A seventh approximation.

Red-yellow latosols. These are the clearest equivalents of the above-mentioned latosols. T h e y s eem to correspond fairly typically to the udoxes, but the colour variations of the latter have no equivalent in the seventh approximation.

Low humic gley soils and grey hydromorphic soils. These are hydromorphic soils with a textural B and without a well-developed h u m i c horizon. T h e y often display indurated latérite formations at the bot tom of the B horizon or in the subjacent horizons, in the form either of irregular aggregates or of a continuous horizon (ground-water latérites); they are generally between 100 c m . and 200 c m . deep. These soils can for the mos t part be compared with the ochra-quult group ( U S D A seventh approximation), sometimes with the aqualts. T h e y are the equivalents of certain hydromorphic soils with a sheet crust or of leached tropical ferruginous soils with pseudogley at depth (French classification).

In South America

In South America (particularly Brazil), the first attempt at latérite classification w a s m a d e b y C a m a r g o and B e n n e m a (1962), w h o traced the latérites in the class of soils with a latosolic B horizon approximately equivalent to the oxic horizon of the seventh approximation.

These soils are subdivided into three categories, according to the values of the silica-alumina ratio of the granulometric clay fraction.

1. Latosols with a silica-alumina ratio narrower than 1.0, subdivided according to the following characteristics:

1.1. Mineral composition of the whole soil. 1.2. Colour of the latosolic B horizon. 1.3. T y p e of the Al horizon.

2 . Latosols with a silica-alumina ratio between 1.0 and 1.6, including the following (classification based o n the above characteristics) :

(a) Latosol roxo (dark red to dark brown-red) : A l 2 0 3 / F e 2 0 3 < 1.7 (for clay soils); M n 0 2 > 0.10 per cent; T i 0 2 = 4-8 per cent. T h e hue of the oxic horizon varies from 10 R 3 / 4 to 2.5 Y R 3 / 6 . C h r o m a never higher than 4 .

(b) Blackish-red latosol: Al203/Fe203: 2.0-4.6 for clay soils, 2.0(?)-2.6 for medium textures; M n 0 2 > 0.02 (?). The commonest hues for the oxic horizon are 10 R3/6 to 2.5 Y R 3 / 6 ; values below 3.5; chroma 5-7.

(c) Red and yellow latosols: A l 2 0 3 / F e 2 0 3 : 4 .6-8.0 for clay soils, 2.9-5.5 for m e d i u m texture soils. H u e of oxic horizon, 2.5-5-7.5 Y R ; values above 3 .5 ; c h r o m a 6-8.

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Classiñcation of latérites. Correlations

T h e distinctions within these three categories are based o n : the development of the A horizon (prominent, intermediate or w e a k ) ; high or l ow base saturation.

3 . Latosols with a S i 0 2 / A l 2 0 3 ratio wider than 1.6 (upper limit about 2.0) :

(a) Latosols with A l 2 0 3 / F e 2 0 3 ratio near to 2.0 in clay soils, near to 3.0 in m e d i u m texture soils;

(b) Latosols with A l 2 0 3 / F e 2 0 3 ratio near to 4.0 for clay soils;

(c) Latosols with A l 2 0 3 / F e 2 0 3 ratio wider than 4 .6 .

T h e concreted latosols are regarded as intermediate grades be tween the concreted lithosols, the soils with a textural horizon B , the ground-water latérites and other soils, if necessary.

In conclusion, it seems that the correlations are o n the whole fairly good at the group level, and that the t e rm latérite is coming to be used m o r e and m o r e as an adjective indicating indurated formations rich in sesquioxides or, even m o r e frequently, is being replaced b y s o m e less confusing term. M u c h w o r k remains to be done , however , particularly in regard to the yellow or red late-ritic soils without well-differentiated horizons. T h e w o r k started and developed b y V a n W a m b e c k e (1962) o n the structure of these soils seems to offer interesting prospects. W h a t is needed are simple criteria, easily observable in the field and relating to well-defined properties a n d pedogenetic characters which can be used as a basis for a n objective classification of latérites.

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Utilization of latérites

The problem of latérite utilization has often been discussed from the agronomic point of view. Originally, however, the chemistry and mineralogy of latérites were studied in connexion with mining research (iron and aluminium) and in recent decades such work has become important in the search for bauxite, iron and manganese deposits. The civil engineering aspect has also been the subject of numerous studies in connexion with road building and the construction of reservoirs. Lastly, the importance of latérites in hydrological studies of regions with a tropical climate is rapidly increasing.

Latérites contribute to the general economy of the warm, humid parts of the world. N o matter which aspect of them we consider, sooner or later we come up against the problem of latérites. Its scope is so wide that it cannot be dealt with in a few lines. This chapter will therefore merely bring out its most important aspects.

FERTILITY OF LATERITES1

The problem of latérite fertility is not strictly connected only with the intrinsic characteristics of the soil. All the factors of geographical environment play a part: first, climate; secondly, vegetation; lastly, characteristics intrinsic to the profiles.

CLIMATE

Latérites in the broad sense develop in subhumid and humid tropical and equatorial climates. These conditions influence soil fertility to a greater or lesser extent.

Favourable factors. Humidity generally in excess of 1,000 mm./year and high temperatures are eminently favourable factors for the development of vegetation. W h e n the humidity is well distributed throughout the year, the plants will be well supplied with water and their vegetative system m a y develop excessively. Tropical environments also promote m a x i m u m hydrocarbon synthesis.

1. General data supplied by B . Dabin.

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These factors accelerate the decay of organic matter, reducing the carbon cycle to a few years. Laudelout and Meyer (1950) believe that an amount of the order of 15 to 20 tons of dry matter annually reaches the soil under the Congo (Leopoldville) forests and is immediately mineralized, releasing cations which have been transported to the deep parts of the profile in quantities estimated at 80-105 kg. of calcium, 50-55 kg. of magnesium and 50-90 kg. of potassium per hectare per year.

Even when the pH. is acid, nitrogen evolves rapidly, producing high spontaneous fertility which is associated in turn with intensive microbic activity (Dommergue, 1963).

Unfavourable factors. Where precipitation is too well distributed over the year, the corollary is often lack of sufficient insolation, with the result that some plants suffer (for example, the oil palm). High rainfall promotes vertical and lateral leaching of the cations and colloidal elements from the top horizons, which often become very impoverished. Too great a concentration of rain leads to water-erosion processes (Fournier, 1956). The readily dispersable organic and mineral colloids are carried away to the sea, where they float in contact with salt wateT to produce vast mangrove swamps. This excess humidity promotes internal blocking and the development of anaerobiotic mechanisms in plain or low-altitude regions. The rapid mineralization of the organic matter brings about nitrogen losses and the nitrogen reserves rapidly disappear.

These conditions of humidity are the cause of numerous cryptogamic diseases, especially the 'melting' of seedlings.

Jf precipitation is poorly distributed (tropical climate), the more or less abrupt onset of extremely dry seasons restricts all growth during the period of drought. The high temperatures and dry winds cause a water deficit lasting many months ; hence the frequent necessity for irrigation even in regions where mean annual precipitation is high.

VEGETATION

At the outset, the vegetation is usually forest, which is rapidly degraded into savannah or herbaceous vegetation, sometimes both.

Favourable factors. The supply of organic matter to the surface is often very large, with important consequences on plant nutrition. W h e n the plant populations are not too degenerate, they provide excellent protection against heat and erosion. If the vegetation is sufficiently dense, total radiation reaching the soil is greatly reduced; Aubreville (1947) estimates this reduction at 58-75 per cent in the Central African Republic and at 81-85 per cent in Cameroon. In East Africa, Vageler (1933) found that the temperature in bare soil varied from 50° to 54° C , compared with 34° C . in a neighbouring soil under vegetation and 25° C . under forest. Beinaert (1941) found differences of 17° C . between bare soil and soil under forest in the Congo (Leopoldville).

This protective role of vegetation affects also the thermal amplitude, which is greatly reduced (Aubert, 1959).

The same applies to the water profile of the soil. The dense forests in the humid tropical belt preserve the surface soil horizon and the atmospheric layer in contact with it remains humid throughout the year, even during the dry-period (Jaeger, 1956).

127


All these conditions often make for very high fertility immediately after the ground has been cleared.

Unfavourable factors. The organic matter in latérites is restricted to a very thin layer which rapidly becomes deeper through cultivation. This organic matter is the source of numerous animal and plant parasites. Another disadvantage is that if the vegetation is dense, the cost of preliminary clearing before cultivation can be started becomes too high. Again, unless the soil is quickly covered again after clearing it will become too desiccated, there will be too much mineralization and erosion processes will develop. Lastly, the organic matter is often destroyed by fire.

FACTORS INHERENT IN THE PROFILES

W e shall deal with the main horizons in turn, in descending order.

Top horizon

Favourable factors. The top horizons are usually light and loose-textured. The fine structure promotes drainage when the soil is not degraded.

Unfavourable factors. These horizons are poor in organic matter and consequently in nitrogen. They are often leached of bases and colloids. Phosphorus is frequently blocked, either by the organic matter or by sesquioxides.

B horizon

Favourable factors. The B horizons are generally very thick and rich in colloids. They are usually highly retentive of water. The rather finally polyhedral, not very compact structure allows good penetration by roots.

Unfavourable factors. These horizons undergo much alteration. Their reserves of bases are low, owing to leaching. The pïL is very low, often under 5.5. The horizons are practically without organic matter and consequently without nitrogen. Their exchange capacity is very poor.

The B horizons are rich in sesquioxides (iron and/or alumina). They sometimes contain concentrations, sometimes crusts, sometimes 'stone lines' which restrict penetration by roots and create a discontinuity which affects the whole genesis of the profile, particularly its water supply.

In certain cases, a pseudogley is observed at the base of the B horizon, resulting from reducing conditions. The horizon is therefore practically sterile, particularly when erosion brings it to the surface. It is therefore likely to harden and restrict all plant growth. Lastly, its richness in sesquioxides sometimes leads to toxic effects: aluminous (Castagnol and Shan-Gia-Tu, 1940) or manganiferous (Martin, 1961).

C horizon (parent zone)

Favourable factors. In well-drained soil, when the parent zone is near the soil surface, the high rate of cation liberation through hydrolysis promotes a good supply of minerals to the plants.

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Utilization of latérite»

Unfavourable factors. W h e n the parent zone is near the soil surface in a poorly drained soil or when this horizon is too deep and out of reach for the roots, the environment is extremely unfavourable to all plant growth.

Another approach to the problem of latérite fertility is to review the various factors inherent in these soils.

PHYSICAL FACTORS

The structural stability of lateritic soils is generally fairly good. Combeau and Quantin (1963) obtained remarkable results in this connexion in the Central African Republic and Martin (1963) in the Congo (Brazzaville). One of the direct consequences of this stability is that land cultivated for one year must be left fallow for two years in order to restore the soil to the degree of structural stability which it had before cultivation. It is therefore important to k n o w the stability index before starting to cultivate.

Permeability is moderate to high in the surface horizons, sometimes low at depth.

Water retention is variable and depends mainly on the humus amount. Available water diminishes with depth, even in clay soils.

CHEMICAL FACTORS

Absorbent complex. The exchangeable cation content varies. Generally, it is very low in the B horizon. The spread of cation content is m u c h greater in the humic horizon and depends on the amount of organic matter. Surface increases are connected with biological contributions from deeper levels due to the forest. These act not only on the cations, mainly calcium, but also on the trace elements.

In an Ivory Coast ferrallitic soil where the root system is highly developed at the surface d o w n to 1.50 m . , Aubert (1959) found the following values (in ppm. %):

Surface 1 m . 1.2-1.5 m . 2.0-2.25 m .

Zn

3 3 4 6

Co

0.3 0.04 0.06 0.08

Fe

1.3 3.5 7 9.5

Cu

1.4 0.6 1.4 1.2

Mn

24 1 4.2 2.3

In certain cases, however, this enrichment can be offset either by a particularly humid climate—formation of very acid oxisols starting at the surface in Ghana (Brammer, 1956), formation of leached ferrallitic soils in the Ivory Coast (Dabin, 1964)—or by the effect of a slope (Dabin, 1964).

Saturation of absorbent complex. Saturation is very slight at depth in the B horizon. M a n y authors mention values under 15-20 per cent. At the surface, on the other hand, the degree of saturation varies greatly, mainly as a function of climate. Where precipitation is above 1,500 m m . and well distributed, the values measured are the lowest. They increase appreciably and even greatly in a tropical climate where there is less precipitation. In Lower Dahomey , for example, the degree of saturation associated with an annual precipitation of 1,000-1,100 m m . sometimes exceeds 90 per cent in the surface horizons (Fauck,

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1961). In such a climate, of course, there are also appreciable variations, depending on the nature of the parent rock and the age of the soils, young soils on rock rich in alkali earth being more saturated than mature soils or soils on acid parent material. This is one of the principal physical-chemical differences between deep red soils and yellow soils.

In certain cases, better saturation corresponds to an increase in the amount of organic matter which further increases the saturation capacity of these soils.

In the surface horizons, there is always a fairly clear relationship between the amount of organic matter and the amount of mineral colloids present. The organic/mineral colloid ratio rises in proportion to the saturation rate of the soil.

Mineral reserves. The mineral reserves are often extremely low or too deep to be reached by the roots, with the result that, if the soil is worked normally, deficiencies in major elements will occur. If for one reason or another, however, the parent rock is near the soil surface, the intensity of the hypothesis processes can effectively offset the loss of fertilizing elements through leaching. The equality of the original material is therefore of vital importance.

Nature of the mineral colloids. These are mainly kaolinitic clays and iron and aluminium oxides. Although these components have a favourable effect on the physical structure and permeability of the soils, their low exchange capacity, poor water retention and tendency to concretion and crust formation are factors that lower the fertility of lateritic soils.

PRINCIPAL MINERAL DEFICIENCIES IN LATERITIC SOILS

W h e n the soils are intensively leached, as usually happens when precipitation exceeds 1,500-1,800 m m . per year, there is often very considerable deficiency in Ca++, M g + + and K + . In the typical, less impoverished lateritic soils, potassium deficiency is the commonest and most marked. O n acid rocks, this deficiency is absolute, the potassium content being extremely low. O n basic rocks, calcium and magnesium are more likely to be deficient.

Deficiencies in phosphoric acid are particularly marked on granitic rocks. These deficiencies naturally depend on the needs of the plant. The banana, oil plant and pineapple need little phosphoric acid, whereas the cocoa bean, coffee shrub and other food plants require a great deal. Phosphoric acid occurs in lateritic soils in various forms, differing in solubility and assimilability; organic phosphorus, calcium phosphates, aluminium phosphates, iron phosphates originating from an inclusion or due to rétrogradation. These different forms can turn into one another according to the state of degradation of the soil and its ^>H. Organic phosphorus is not directly assimilable, nor is iron phosphate originating from an inclusion. The calcium phosphate content, on the other hand, is very low in an acid med ium and it therefore looks as though aluminium phosphate makes a considerable contribution to plant nutrition (Dabin, 1963). A genuine phosphorus cycle is established, so that the phosphorus takes more or less assimilable forms according to the state of the soil, making its determination very difficult.

Nitrogen deficiency is also frequent, owing to the intensive leaching of nitrates and rapid mineralization of organic nitrogen. Nitrogen deficiency is a

130


function of soil acidity in soils containing comparable quantities of organic matter.

In general, deficiencies in certain major elements and particularly in trace elements are not very pronounced in a natural environment. They become marked when production is forced through the addition of mineral fertilizers. Here, the effect is rather one of imbalance than of true deficiency. A n excess of potassium over magnesium, for example, produces blue spot disease in bananas.

In regard to trace elements, the critical deficiency therefore varies for different plants. Apart from the Australian and American works, particularly those of Sherman in Hawaii, the literature provides little information. The following conclusions, however, seem plausible.

Manganese. T w o aspects must be considered, deficiency and toxicity. Deficiency is likely to be appreciable mainly in very acid soils. Figures below 10 p p m . can. be suggested. Deficiency particularly affects pineapple and cotton.

Toxicity effects, on the other hand, are more spectacular. These occur mainly on basic rocks. The organic matter content is usually high and the p H nearly always above 6. Toxicity appears if the p H is greatly lowered following over-cultivation. Liming is sufficient to remedy this (Martin, 1963). Harmer and Sherman (1944), in Hawaii, recommend mulching, which prevents the soil from drying out and restricts the toxic liberation of divalent oxide.

Iron. The lowest iron content ( < 5 p p m . ) is observed in soils with a relatively high p H (above 6).

Copper. A copper content above 2 p p m . can be regarded as relatively good. Deficiency would appear with the values obtained mainly in hydromorphic soils.

Zinc. The right content is considered to be between 3 and 15 p p m . ; under 3 p p m . the content is low and can mean deficiency, particularly below 1 p p m . In the Ivory Coast, the values found lie between 1 and 2 p p m . , suggesting that most of the lateritic soils are deficient.

Molybdenum. Molybdenum deficiency increases when the p H falls; assimilation is easier at a high p H . The molybdenum content found is usually very low (0.01 to 0.06 p p m . , Ivory Coast), and this, combined with the acid p H , means a clear deficiency.

Vanadium. This trace element can to a certain extent offset a molybdenum deficiency. The values obtained are often below 0.05 p p m .

Cobalt. The cobalt content seems to depend fairly closely on the state of development of the organic matter. A content higher than 0.1 p p m . can be regarded as satisfactory; a content of 0.02 p p m . or undeT is low, but on the whole this is a rather rare occurrence.

To summarize, effects due to trace elements are often appreciable in the two extreme cases of lateritic soils on basic rocks and in a condition of poor drainage.

The problem of making latérites fertile derives from the facts outlined above and its solution depends on local conditions: the amount of organic matter must be maintained at equilibrium level in relation to the mineral substrate; this level can be raised by increasing the supply of bases (Ca and M g ) and raising the p H .

There are m a n y methods of producing these results : the addition of manure, green manure, mineral fertilizers, and so forth. These must be reinforced by

131


anti-erosion methods (belt cultivation, contour ploughing, etc.). But above

all, it must not be forgotten that lateritic soils belong to a forest environment

and are therefore mainly suitable for perennial crops. The introduction of annual

crops produces an imbalance which is difficult to overcome economically and

is justifiable only in the case of highly profitable industrial crops.

VALUE OF LATERITES AS A SOURCE OF ORES

W e owe to Bauer (1898) the recognition that latérites are a possible source of

aluminium. Since Bauer's day, many studies have been made by goelogists

to show that bauxite deposits often coincide with latérites.1

At present, numerous latérite occurrences are exploited for their ores.

Iron ores. The deposits are large, but low in iron and unprofitable. Profi

tability depends mainly on the proximity of ports. W e m a y recall the utiliza

tion of latérites as a source of iron by many African tribes, but this custom

is disappearing.

Aluminium. The utilization of lateritic bauxites is much more general and

of world importance. The majority of humid tropical countries possess large

reserves of these occurrences. The best bauxites occur in very old reliefs, generally

Tertiary. The value of the ore depends not only on its richness in A1 2 0 3 , but

also on the presence of combined silica. This raises special problems in regard

to dressing, which are not always satisfactorily solved.

Manganese. Here again many tropical deposits are of lateritic origin (in the

Lower Ivory Coast and Gabon, for example). The value of these deposits arises

from the fact they are readily accessible to open working and often very large.

Whether a system of working is economic or not depends mainly on whether

the dressed ore can be profitably exported or not. In general, the lateritic origin

of these ores entails the use of mining prospecting methods. It is necessary

to determine the most favourable sites for surface mineral accumulations.

In the case of aluminium, these are principally of the absolute accumulation

type, but in the case of iron and, particularly, manganese, which is deposited

in marshy faciès, the accumulation is more often relative.

The prospecting assumptions are therefore considerable; namely, that the

ore is present at the surface and not at depth, that it is associated with an ori

ginal material and that the occurrence is in a suitable type of relief.

UTILIZATION OF LATERITES IN CIVIL ENGINEERING

There is no need to emphasize the importance of latérites for various building

purposes. Latérite crusts were originally widely used for the construction of

monuments and dwellings. Certain African megaliths are of lateritic origin.

It seems that the use of indurated latérites as a building material has been,

and is still, very general in India and Thailand. The temple of Angkor Vat is

built of latérites, although the quarrier's art has been lost in Cambodia.

Civil engineering studies on the Atterberg limits of these materials are now

in progress, with a view to their use in road and earth d a m construction. The

1. See Bauxites, their Mineralogy and Genesis, M o s c o w . U . S . S . R . A c a d e m y of Sciences Press, 1958.

132


majority of the roads in the tropics are made of concreted latérite. This produces the regular waves known as 'corrugated iron', an effect connected with vibratory processes. It seems that practical methods of mitigating this disadvantage have been devised.

One of the main advantages of lateritic material is that it does not readily swell with water. This makes it an excellent packing material, particularly when it is not too sandy.

USE OF LATERITES FOR STRATIGRAPHICAL PURPOSES

Numerous ferruginous, red, interstratified formations in old sediments are more and more frequently being interpreted as characteristic of humid tropical climates. The petrographical characters of these formations enable us to reconstruct former terrain. This applies more particularly to the red Per-mian-Triassic sandstones of the Vosges (Millot, Perriaux and Lucas, 1961), the Carboniferous red series and the Cretaceous siderolithic beds of Europe.

Similarly, study of the argillaceous neoformations in cuvettes near old late-ritized massifs is a help in understanding the interactions occurring between these different types of formation. Here we have the phenomenon of biorhe-xiadasis (Erhart, 1956). The tropical forest acts as a filler. Bases and silica are carried into the depressions where they give rise at first to detrital and then to neogenetic deposits. Sesquioxides and kaolinite accumulate in situ. In the course of a new erosion cycle, these materials are eroded and are in turn deposited on the sedimentary neoformation. The resultant sedimentary sequences yield new information on climates, and climatic interpretations based on their study are becoming increasingly frequent (Millot, Ellouard, Lucas and Slansky, 1960 ; Millot, Radier and Bonifas, 1957).

HYDROLOGICAL PROPERTIES OF LATERITES

S tndies in this field are gradually developing. The results obtained in the case of small sloping basins show that rock consisting of lateritic alteration products forms an excellent storage place for water. Even exposed crusts possess high total permeability, because indurated horizons are often strongly jointed. In a rainy season, running water rushes into the cracks and impregnates the loose subjacent formations where it circulates laterally. Sometimes true subterranean watercourses form, carrying with them some of the rock material. This accounts for the networks of subjacent grottoes found in the indurated levels, with underground corridors between them and siphons, sometimes several hundreds of metres long. W h e n large amounts of loose structure are swept away, circular or linear crumbling can occur, levelling the line of water flow. The wendu of West Africa are a case in point. The stream waters converge towards these depressions to form small swamps where they abruptly disappear through a joint, emerging again at the surface in the channels of existing rivers and thereby causing an abrupt increase in the latter's discharge. This accounts for the frequent phenomenon of sloping basins without active collateral intake even where neighbouring basins of the same area display a more or less permanent watercourse (Maignien, 1958). These data are a guide to the utilization of water in lateritic country.

133

Appendix

In addition to the general report, several papera were submitted during and after the Sympos ium on Laterites held in Tananarive, Madagascar, from 21 to 29 September 1964. T h e following are succinct summaries.1

1. Formation, classification et utilisation de certaines cuirasses de bas de pente, by D r . C . Sys, University of Ghent (Belgium).

T h e purpose of this paper is to explain the origin of the crusted horizons which are found on the lower slopes of valleys.

T h e soil surface is a leached clay ferralisol which, at depth, is subjected to the fluctuations of a phreatic nappe. Under these conditions the iron transported by the clay is separated into its component parts. T h e iron oxides separate and become concentrated in an ¡lluvial horizon. Induration results from the ferruginous or manganic erosion of the illuvial horizons. It thus appears that the crusts formed by vertical leaching m a y originate in different ways. The host material m a y be ferrallitic or fersiallitic. In the first case the aluminium is a residual material, while the iron originates from outside the accumulation area. For these reasons it would seem that the classification of crusts should be based more on the morphological and mineralogical characteristics.

2. Latérite in Indian geology (a sketch on the concepts of origin), by M . K . R o y Chowdhnry , V . Venkatesh, M . A . A n a n Dalwar and D . K . Paul. Geological Survey of India.

This is a bibliographical study of the laterites of India, as based on the following acceptance of the word : 'Rocks predominantly rich either in alumina or combined silica (kaolin) that are associated with a latérite profile or lateritic weathering are designated bauxite and lithomarge respectively. Similarly, a rock very rich in manganese or iron is referred to as lateritic manganese or iron-ore if found in a latérite blanket'.

3. Latérite as a source of industrial minerals in India, b y M . K . R o y Chowdhury , V . Venkatesh, M . A . A n a n Dalwar and D . K . Paul, Geological Survey of India.

A study of the deposits of the main ores of lateritic origin in India (bauxite, manganese, iron and nickel).

4. The occurrence of latérite in Amazona, by F . C . Camargo. The report is divided into two parts: 'Type and age of the laterites in the A m a z o n Region' (4.1); ' T h e ecosystem of soil and forest vegetation in the humid tropical regions' (4.2).

In the first chapter a latérite still in formation and a fossil latérite, both developed in alluvions, are compared. The writer m a k e s clear that they are two typical examples

1. These papers may be consultée! at Unesco, Place de Fontenoy, Paris.

134

Appendix

of latérite (crusted) formed as a result of the lateral displacement of iron and aluminium by the underground water tables. It would not appear that these phenomena could have occurred as a result of simple vertical displacements, since no latérites are to be found in the centres of the plateaux and peneplains.

In Brazil, the most frequently occurring types are fossil latérites. F r o m the pédologie viewpoint the term 'laterite-soil' should be dropped because it

can be applied correctly only to the 'concretional lixo-soils', or the soils resulting from the decomposition of fossil latérite.

In the second part of the report, the problem of rotation in lateritic surroundings in the A m a z o n forests is discussed.

5. Genesis of the latérite, by G . D . Sherman, F . S. Schultz and J. L . Walker, University of Hawaii.

The genesis of latérite horizons is dependent on the chemical and mineralogical characteristics of the iron oxides. Induration results from the dehydration of the colloidal iron hydrates. The indurated horizons thus formed are relatively stable. O n the other hand, aluminium and titanium oxides do not result in similar conditions in the formation of continuous indurated layers. Segregated gibbsite is rarely produced at the surface.

Latérites m a y be classified as follows: 1. Residual latérite formed in place under free drainage conditions. The products of

rock weathering are leached in different ways. In these conditions of extreme oxidation the iron alone remains at the surface. The aluminium also moves downward.

2. Latérite formed by accumulation of iron oxide. Such latérite can also be formed by oblique leaching. The areas from which the iron has been leached are enriched with gibbsite.

3. Transported latérite. This is the remoulded product of old indurated layers that have been broken up.

4. Ground-water latérite, which is developed over a permanent water table near the surface.

5. Fossil latérites, which m a y be secondary formations resulting, in particular, from the addition of silica or carbonate.

6. Histoire des sols ferrallitiqu.es de Madagascar, by J. Riquier et F . Bourgeat, O R S T O M Centre, Tananarive.

Madagascar's ferrallitic soils are very old, complex soils. The profile is formed from an earlier profile subsequently subjected to successive phases of pedogenesis as a result of climatic changes. Four phases are recognized: 1. The hydrothermal rock weathering phase in a very hot and very humid climate (Miocene,

Pliocene). 2. Main formation phase of the red surface horizon in climates with alternating drier

seasons (Upper Pliocene, Pleistocene). 3. Phase of more intense ferrallitization due to a more humid period (Middle Quaternary)

and development of the upper horizons under the effects of vegetation. 4. Phase of degradation due to slow desiccation and the appearance of m a n (Recent

Quaternary). Finally, to illustrate the in situ studies m a d e during the symposium, the O R S T O M Centre in Tananarive has drawn up an 87-page report dealing with the 'Presentation of some ferrallitic soil profiles and the study of the pedogenetic surroundings in the neighbourhood around Tananarive'. Twenty-one profiles are described with their analytical and mineralogical data.

135

ferrallitiqu.es

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