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Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185 Comunicações Geológicas (2014) 101, 1, 63-69 ISSN: 0873-948X; e-ISSN: 1647-581X

Lithological control on the mobility of elements during chemical weathering Controlo litológico na mobilidade dos elementos durante a meteorização química M. R. G. Sayyed1 Recebido em 26/12/2012 / Aceite em 12/09/2014

Disponível online em Dezembro de 2014 / Publicado em Dezembro de 2014

© 2014 LNEG – Laboratório Nacional de Geologia e Energia IP

Abstract: Rock weathering is controlled by variety of factors. The lithological control on the chemical weathering has been investigated with special reference to the mobility of major elements. The various rock types occurring in the Vasai-Virar area, north of Mumbai (India) have been studied for their chemical weathering under all other controlling factors remaining essentially constant. In order to evaluate the mobility of major elements, the chemical analyses of samples of weathered and fresh rocks were carried out. Binary and ternary plots, by considering the behavior of single element oxide or a group of elemental oxides during progressive weathering, give interesting information. These chemical studies revealed that crystallinity of rocks, intensity of fracturing and jointing in the rocks and their depth of exposure to various weathering processes, to great extent, have controlled the chemical composition of the weathered products. Irrespective of the type of rock, the compositions of weathered products seem to have been governed by the mobility of elements during weathering. The rocks during weathering, despite their different mineral and chemical composition, have shown a general depletion of alkalies (K and Na) and alkaline earths (Ca and Mg), Fe ++ and Si on one hand and enrichment of Al, Fe+++ and H2O

+ on other hand. The magnitude of depletion or enrichment of these elements however has been different chiefly due to their chemical composition.

Keywords: Chemical weathering, elemental mobility, geochemistry, lithological control. Resumo: A meteorização das rochas é controlada por vários factores. O controlo litológico na meteorização química tem sido investigada em especial no que respeita à mobilidade dos elementos maiores.Os diversos tipos de rochas que ocorrem na área de Vasai-Virar, a norte de Mumbai (India) têm sido estudadas no que respeita à meteorização química considerando essencialmente constantes todos os outros factores. Com vista a avaliar a mobilidade dos elementos maiores, foram feitas análises químicas de amostras alteradas e frescas. Os diagramas binários e ternários, considerando o comportamento de óxidos com um único elemento ou de óxidos com um grupo de elementos durante uma meteorização progressiva, forneceram informações interessantes. Estes estudos químicos revelaram que a cristalinidade das rochas, intensidade de fracturação e a profundidade a que se expoem a vários processos de meteorização, controlaram em grande medida a composição química dos produtos meteorizados. Sem relação com o tipo de rocha, as composições dos produtos meteorizados parecem ter sido governadas pela mobilidade dos elementos durante a meteorização. As rochas durante a meteorização, apesar das suas diferenças mineralógicas e químicas, mostraram por um lado, uma deflecção generalizada de alcalis (K e Na), de alcalino-terrosos (Ca e Mg), Fe++ e Si, mas por outro lado, um enriquecimento em Al, Fe+++ e H20. Contudo, a magnitude da deflecção ou do enriquecimento nestes elementos foi diferente, principalmente, devido à composição química da rocha.

Palavras-chave: Meteorização química, mobilidade dos elementos, geoquímica, controlo litológico. Department of Geology, Poona College (affiliated to Savitribai Phule Pune University), Camp, Pune 411001, India. [email protected]

1. Introduction

Weathering is the breakdown and alteration of material near the earth’s surface to the products that are in equilibrium with the newly imposed physico-chemical conditions. Thus the weathering implies strong dependency on processes associated with hydrosphere, atmosphere and biosphere (White and Brantley, 1995) thereby changing the physico-chemical state of the parent rock which results in the higher order of heterogeneity (Duzgoren-Aydin et al., 2002). Although weathering of rocks and minerals are nearly ubiquitous on the earth’s surface the chemical, physical and mineralogical properties in the altered products are poorly understood (Dearman, 1995; Hencher and McNicoll, 1995; Price, 1995; Moon and Jayavardane, 2004) and much of the recent research is devoted on the geochemical changes during rock weathering (Aiuppa et al., 2000; Sharma and Rajamani, 2000, Malpas et al., 2001; Stefansson and Gislason, 2001; Thanchit et al., 2006). Thus the processes and rate of alteration of the rocks and minerals from their original state to the phases which are more stable at the surface conditions of the earth is one of the basic aspect of the landscape development as weathering keeps the landscape dynamic forever. Since the rock weathering has been playing important role in the formation and evolution of soils, it has its importance for evolution and existence of life on the Earth. Recently the weathering studies have been focused more at the elemental mobility and water rock interactions (Nesbitt, 1979; Gouveia, et al., 1993; Ohlender et al., 1996; Minarik et al., 1998; Tapia et al., 1998). The important issues in the field of chemical weathering studies are global and local impacts of weathering processes and chemical weathering history of a landscape (Anderson and Blum, 2003). A number of factors which control the chemical weathering have been discussed in a special issue of Chemical Geology (2002) volume 202. Furthermore the knowledge of chemical weathering is fundamental to the successful analysis of environmental issues such as influence of acid deposition on soils, biomass production in forestry and agricultural system, global warming or level of atmospheric CO2 over geological time scale (Egli et al., 2006). Caspari et al. (2006) studied the elemental mobility during the formation of soils in different lithologies in Bhutan and confirmed initial mineralogy and climate as the controlling factors.

The rock weathering has been a function of variety of processes and is shown to be controlled by climate, biological activity, topography, composition of parent rock and time. If all these factors

Artigo original Original article

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are taken together it becomes very difficult to understand their influences on weathering processes as they exhibit very complex relationships. In order to understand the influence of mineralogy and chemical composition of parent material on rock weathering the area north of Mumbai (Bombay, India) has been chosen (Fig. 1), which exposes rocks of variety of lithology. These rocks have been exposed to the weathering processes more or less at same time. While the topography of the area does not show many variations as far as altitudinal variations and/or slope conditions are concerned, it also exhibits uniform biological activity in terms of faunal and floral assemblages. The variety of rocks from this area which are included for the present study are basalt, dolerite, nepheline-syenite, andesite, rhyolite, camptonite lamprophyre, gabrro, diorite, granodiorite and associated volcanoclastic rocks like trachytic agglomerate, basaltic agglomerate and bedded tuff (Fig. 2). In the present paper the control of lithology on the mobility of elements during weathering has been discussed.

Fig.1. Location map of the Vasai-Virar area, Thane district, Maharashtra, India. Fig.1. Mapa de localização da área de Vasai-Virar, distrito de Thane, Maharashtra, India.

2. Methods of study

The samples of various geological units representing different stages of weathering were crushed and powdered to analyze their chemical composition. Major elements oxides were estimated by following the standard procedures (Shapiro and Brannock, 1962; Thompson and Wood, 1982; Bassett et al., 1985). Varian AA 1275 Atomic absorption Spectrophotometer (for total Fe, Ca, Mg, Mn), Hitachi UV 2000 Spectrophotometer (for Si, Al, P and Ti) and Corning 400 Flame Photometer (K and Na) were used to determine the major elements concentrations in terms of their oxides. The crystal lattice water (H2O+) was determined by heating the rock sample at 600 ºC in an oven for about six hours. The oxide weight percent data were later recalculated to molecular percent data as the stoichiometric proportions of various elements are more informative in weathering studies than the weight percent data (Jenny, 1941, p. 26; Reiche, 1943). The molecular percent data were further processed to depict trends of chemical weathering in rocks.

3. Geochemistry of elemental mobility during weathering

Chemical weathering of rocks is quite vital in geochemical cycling of elements. Mobilization and redistribution of certain elements during weathering is particularly complicated because these elements are affected by various processes such as dissolution of primary minerals, formation of secondary phases, transport of materials, co-precipitation and ion-exchange of various minerals (Harris and Adams, 1966; Nesbitt, 1979; Nesbitt et al., 1980; Fritz and Ragland, 1980; Chesworth, et al., 1981; Cramer and Nesbitt, 1983; Fritz and Mohr, 1984). Many workers have related the elemental mobility to the mineral breakdown with the progressive weathering (Colman, 1982; Eggleton et al., 1987; Nesbitt and Wilson, 1992; Gavshin et al., 1997; Hill et al., 2000; Patino et al., 2003) besides other factors like solubility of parent mineralogy, pressure, temperature, redox potential and type of leaching agent generally control the mobility of elements during weathering.

Fig.2. Geological map of the Vasai-Virar area, Thane district, Maharashtra, India.

Fig.2. Mapa geológico da area de Vasai-Virar, Distrito deThane, Maharashtra, India.

Lithological control on the mobility of elements during chemical weathering 65

In thermodynamic sense weathering systems are open, irreversible and incongruent (Middleburg et al., 1988). It is evident that the chemical activity of the given element determines whether it is being retained in the weathering products or removed from it. Along with the chemical composition the factors like topography, availability of organic acids and rainfall also govern the mobility. Since the rock weathering consists largely of differential losses of elements, the amounts of different elements in fresh and weathered rocks, thereby, represent expression of the type and extent of chemical weathering. There is relative increase or decrease of element oxides during progressive weathering (Birkeland, 1984) and in general during progressive weathering there is: considerable depletion of highly soluble bases; considerable incorporation of water (H2O

+); relative little loss of silica due to its immobility over wide

range of pH; increase of the concentrations of relatively immobile

alumina, ferric iron and titania; appreciable depletion of ferrous iron due to its oxidation to

ferric state. The key geochemical reactions in the weathering are the

chemical breakdown of some minerals (transformations) and neoformation of physils. In the first instance it is probably an oxidation brought about by incomplete bonding of oxygen atoms on the outside of the crystals. This results in the formation of hydroxyl group and subsequent removal of cations. The more soluble cations such as K+, Na+, Ca+ and Mg2+ may be completely removed. Natural water dissociates slightly into hydrogen and hydroxyl ions, which interact with the minerals. Hydroxyl ions become bonded to the exposed cations and hydrogen to exposed oxygen and other anions giving rise to hydration (Friedman and Sanders, 1978). The fractured surface is disordered and has higher energy than the interior of the crystal (Keller, 1978). When the surface comes in contact with the water, H+ ions displace the alkali and alkaline earth cations leading to hydrolysis. Thus the hydrolysis resulting in the physical disaggregation of rocks and removal of alkali and alkaline earth elements (Kramer, 1968) is the most significant process of weathering in normal humid environment. Weathering reactions continue as repeated flushing by percolating water removes the soluble constituents ions and atoms, which are carried away in solution and relatively insoluble constituents, remain behind as clays or hydroxides. It has been observed that the univalent and the bivalent cations readily go into solution (Mason, 1982, p. 147) but the fate of Al and Si has been less understood. Some workers believe that the hydrolysis of alumino-silicates results in the formation of colloidal silicic acid and aluminum hydroxide which later react to form clay minerals. Amongst the highly mobile elements alkali metals are most stable followed by those of alkaline earth, which are for the most part carried away in solution. Silicon, aluminum and iron on the other hand are soon redeposited as insoluble compounds and new minerals are formed from them.

Relative mobility of major elements during weathering has been established fairly well. Smyth (1913) by calculating the ratio of the percent of an element in stream or spring water to the percent in the parent rock on many rock types had found that the ease of removal of major elements follows the following sequence which agrees well with the observations made by Polynov (1937), Goldich (1938), Tiller (1958), Rode (1962) and Feth et al. (1964):

Ca > Na > Mg > K > Si > Al = Fe

However according to Colman (1982) the sequence of mobility based on the abundance of each element in the weathering rinds compared with the unaltered rock is:

Ca > Na > Mg > Si > Al > K > Fe > Ti The small amount of K originally present may be fixed in the

formation of incipient minerals like halloysite or kaolinite. When the data for the relative mobilization of element are

compared with those for their ionic potential, the ions are not ranked in the same order (Birkeland, 1984). This apparent discrepancy is due to the following factors: the minerals weather at different rates owing to their different

compositions; fixation of certain ions in the crystal lattice of the clay

minerals; ion exchange in which certain ions can replace others on clay

mineral surfaces; selective uptake by the plants.

Two competing processes (leaching and fixation) along with the three main factors viz. degree of weathering, the type and the abundance of sesquioxides and clay minerals control the behavior of chemical elements during weathering (Malpas et al., 2001). The rate of weathering is also dependent on size and amount of fracturing (Krauskopf and Bird, 1995). Eggleton (1986) considered the rate of weathering to depend upon i) The existence of cracks, fractures and defects, which increase the effective surface area for weathering agents, and ii) The rate of growth of new stable phases. The rapid growth blocks the diffusion pathways and slows down weathering while slow growth would allow time for elements to diffuse out, leaving pathways open.

From the foregoing discussions it is evident that the physico-chemical characteristics of the rocks play vital role in the rock weathering processes. Under given uniform climatic, topographic, biologic and chronologic conditions the rocks of different origin from the present study area have yielded different weathering trends.

4. Results and discussion

4.1 Weathering trends with reference to single elemental mobility

The molecular proportions of various major element oxides from the fresh rocks and their weathering products (Tab. 1) are plotted (Figs. 3, 4, 5 and 6) with respect to their depth from the surface. The mobility of different elements in the various rock types is discussed below. a) Basic igneous rocks (basalt, dolerite, camptonite, gabbro):

In all the basic igneous rocks SiO2 shows systematic depletion but the camptonite shows little lesser depletion. Basalts show stronger enrichment of Al2O3 than other rocks. Fe2O3 in all rocks shows consistent increase, only in the most weathered basalt it shows lesser concentration. H2O

+ shows continuous increase with weathering the most conspicuous being in the dolerites FeO, although show general depletion, has a trend of initial rapid depletion in basalt and gabbro while initial slow depletion and later little rapid rate in the dolerite and camptonite. K2O and Na2O show erratic behavior and in different rocks they show enrichment or depletion in different stages of weathering. The trends of CaO and MgO are also irregular; however the trend of CaO shows its relatively higher chemical activity than MgO.

b) Intermediate igneous rocks (andesite, nepheline syenite, diorite): Similar to basic igneous rocks these rocks too show systematic depletion of SiO2 but the depletion rate is low.

66 M. G. Sayyed / Comunicações Geológicas (2014) 101, 1, 63-69

Al2O3 shows general enrichment but the rate is more in nepheline syenite. Fe2O3 shows consistent enrichment. H2O

+ enrichment is more in nepheline syenite than other rocks. FeO depletion is faster initially in nepheline syenite and andesite while in diorite it shows rather linear trend of depletion. K2O and Na2O show general depletion in andesite and nepheline syenite but in diorite they show remarkable enrichment. CaO and MgO show general depletion trend in all these rocks, that only in diorite in the most advanced stage of weathering CaO has been enriched. CaO, however, does not show linear trend indicating its most reactive nature.

c) Acid igneous rocks (rhyolite, granodiorite): Although SiO2

is lost during the weathering of these rocks the loss is more rapid in rhyolite than the granodiorite. Also the incorporation of H2O

+ is more rapid initially in rhyolite than granodiorite. In general the depletion (SiO2), and enrichment (H2O

+) trend in the acid igneous rocks are symmetrical i.e. the loss of SiO2

is roughly proportional to gain in H2O+. The Al2O3 and Fe2O3

show general enrichment trends, however rhyolite shows rather rapid enrichment. FeO shows more pronounced depletion in rhyolite than in granodiorite. Na2O shows

general depletion while K2O shows initial enrichment and later depletion the magnitude, however, is different. The alkaline earths (CaO and MgO) shows similar trends as in case of alkalis, but the magnitude of changes is more pronounced.

d) Pyroclastic rocks (basaltic agglomerate, trachytic agglomerate, bedded tuff): Similar to the rocks discussed before, in these rocks also the SiO2 shows depletion but the rate is not pronounced in bedded tuff. Al2O3 and Fe2O3 show consistent linear enrichment trend in all these three rocks. H2O+ however has more pronounced enrichment in basaltic agglomerate and trachytic agglomerate than bedded tuff. This indicates that the bedded tuff is less susceptible to weathering. FeO shows general depletion, which is initially more pronounced in basaltic agglomerate, and trachytic agglomerate while in bedded tuff there is initial enrichment. Na2O shows depletion in basaltic agglomerate and trachytic agglomerate and enrichment in bedded tuff. K2O however shows general depletion. CaO here too shows higher reactivity with depleting tendency while depletion in MgO is not as pronounced as CaO.

Table 1. Chemical composition (molecular %) of fresh rocks and weathered products.

Tabela 1. Composição química molecular (%) de rochas frescas e produtos meteorizados.


Fig.3. Behaviour of Major Elements during weathering.

Fig.3. Variação dos elementos maiores durante a meteorização.







68 M. G. Sayyed / Comunicações Geológicas (2014) 101, 1, 63-69

4.2 Weathering trends with reference to group of elemental mobility

It is evident from the previous section that there is general depletion of SiO2 depletion and/or enrichment of alkalies and alkaline earth and general enrichment of Al2O3 and Fe2O3 with progressive weathering. To another observation the molecular percent data were plotted on CNKM (CaO+Na2O+ K2O+MgO) – R2O3 (Al2O3+ Fe2O3) - SiO2 triangular diagramme (Fig. 7).

In basic igneous rocks (Fig. 7A) basalts show initial continuous depletion of SiO2 and CNKM (bases) and corresponding enrichment in R2O3. However in more advanced stage there is appreciable increase in the bases indicating that alkalies and alkaline earths are fixed in the clay minerals and retained. Dolerite however shows initial enrichment of bases and SiO2 and later enrichment of R2O3 indicating fixing of bases into clays initially as against basalts. Gabbro typically shows depletion of bases and SiO2

systematically with corresponding enrichment in R2O3. Camptonite shows extraordinary initial enrichment of CNKM, which latter remains more or less constant.

In intermediate igneous rocks (Fig. 7B) nepheline syenite and andesite show strong depletions of bases and moderate to low enrichment of (R2O3) or depletion of SiO2. Diorite however shows initial depletion of bases, which later get enriched, with corresponding enrichment/depletion of SiO2 and R2O3.

The acidic rocks show initial enrichment of bases (Fig. 7C), which later get depleted, with corresponding enrichment/depletion of SiO2 and R2O3. Thus acidic igneous rocks initially weather to clay minerals and later get into lateritic material.

Pyroclastic rocks show strong depletion of bases and to some extent SiO2 with general enrichment of R2O3 (Fig. 7D).

Fig.7. Triangular plots showing behaviour of group of element oxides. Fig.7. Diagramas triangulares mostrando a variação dos grupos de óxidos.

4.3 Discussion

In general in all the rocks studied for their chemistry of weathering, the SiO2 shows general depletion. This is chiefly due to the fact that SiO2 is normally present (in excess) in the parent

rock more than it is necessary to form the clay mineral and therefore it always decrease with progressive weathering (Birkeland, 1984). Al2O3 on other hand has low solubility over wide range of pH and is required for the formation of clay minerals and hence always shows relative increase. Iron in the rock forming minerals occur in Fe2+ state and when exposed to the oxidizing environment, get converted to Fe3+ state making a part of clay or related iron bearing minerals. Thus iron normally shows gain with weathering. The bases (Ca, Mg, Na, K) are the major exchangeable cations in weathering environment and being highly soluble in water invariably show strong depletion in their concentration from the weathering horizons. However under highly semiarid environment with paucity of percolating water they may get concentrated in weathering horizons.

As different elements are affected differently by the various weathering processes (like dissolution of primary minerals, formation of secondary minerals, redox processes, transport of materials and ion exchange) the mobilization and redistribution of elements during rock weathering follow various pathways (Middleburg, 1988). The solubility of each principal ions involved in weathering of silicates may be considered to be a function of five interrelated factors namely Eh, pH, leaching potential, time and fixation. Examining the mobilities of alkalies and alkaline earths during continental weathering, Nesbitt et al. (1980) have made the following generalizations: Ca2+, Sr2+ and Na+ are most strongly removed (as dissolved species) during weathering of fresh continental rocks. Although large quantities of Mg2+ are transported to the marine environment as dissolved species appreciable amount remain (fixed in secondary clay minerals) at the weathering sites to be removed during the mass wasting of the continental weathering profiles. According to Harnois (1988) during the weathering of granite and basalt Si4+, Mg2+, Ca2+ and Na+ are leached. Al3+ and Ti4+ remained essentially in the residues, whereas Fe3+ and K+ behaved more complicatedly.

4.3 Conclusions

From the present studies it is evident that the physico-chemical characteristics of rocks play a vital role in the mobility of elements during weathering. Under given climatic, topographic, biologic and chronologic conditions the rocks of different origin from the Vasai-Virar area yielded different trends of elemental motilities during weathering. These are attributed mainly to their chemical and mineralogical compositions, intensity of fracturing and jointing in the rocks and their depth of exposure to weathering processes. The chemical composition of weathering products appears to have been governed by mobility of the elements during weathering i.e. depletion of some and enrichment of other elements. The magnitude of depletion or enrichment of these elements, however, has been different, chiefly due to their chemical compositions. Thus loss of CaO is more in basalt while granodiorite shows appreciable loss of Na2O. This is because of weathering of calcic plagioclase and augite in basalt and sodic plagioclase and alkali feldspars in granodiorite. Similarly the transformation of Fe2+ to Fe3+ is more in basic rocks as they contain more mafic minerals with higher amounts of ferrous iron than in the acid rocks. H2O

+ has shown significant rise in those rocks, which developed larger quantities of clays and hence andesite, rhyolite and basalts show higher H2O

+. Gabbro and granodiorite show lesser mobility of elements than basalt and rhyolite which could be due to their coarse grained nature. Similarly the rocks like basalt and rhyolite with the presence of numerous joints and fractures (providing diffusion pathways) show relatively more mobility than the rocks like camptonite and diorite. Thus the magnitude of the mobility of element does not depend upon its geochemical activity alone but


also upon the parent rock composition, its grain size, joints and fractures etc.

These conclusions are drawn on the basis of relative chemical changes from the fresh rocks to the highly weathered rocks. Studying the absolute chemical changes can further substantiate these observations.

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