Clay Minerals (1976) 11, 73.

N O T E

P E D O G E N I C P A L Y G O R S K I T E IN SOME A R I D BROWN

( C A L C I O R T H I D ) SOILS OF I S R A E L

I N T R O D U C T I O N

In the frequently reported occurrences of palygorskite and/or sepiolite in calcareous soils of arid regions it is believed that these relatively unstable minerals have been inherited from calcareous sediments rich in palygorskite and sepiolite (Millot, 1970; Wiersma, 1970). This concurs with our viewpoint concerning the palygorskite in the limestone and marl derived soils of Israel (Yaalon, 1955; Yaalon et al., 1966) in which palygorskite is found in the parent rocks but disappears upon decalcification.

Pedogenic palygorskite has only occasionally been reported. However, Vanden Heuvel (1966) in New Mexico and Millot, Paquet & Ruellan (1969) in Morocco found it strongly associated with calcic soil horizons and argued therefore for a pedogenic origin. The origin of the New Mexico fibrous clay minerals is disputed. McLean, Allen & Craig (1972) considered pedogenic origin unlikely because of the close association with lacustrine materials, but Frye et al. (1974) spoke of diagenetic development and alteration, below the caliche, in a high magnesium environment produced during soil desiccation.

In Australia, Beattie & Haldane (1958) and Singer & Norrish (1974) examined coatings of almost pure palygorskite fibres. The latter established a palygorskite stability field, supporting the field evidence for its pedogenic origin under temporarily waterlogged conditions. Most recently Eswaran & Barzanji (1974) have concluded from micromorphological and SEM studies that palygorskite fibres in some alluvial and colluvial soils of Iraq could not have been transported but were formed after the crystallization of gypsum.

Pedogenic palygorskite in some calcareous Arid Brown soils of Israel is described below and its mode of origin is discussed.

Mater ia l s

Two soil catenas (eight profiles), were sampled in the northern Negev near Beer Sheva in an arid climate. A detailed examination has been made by Wieder (in prep- aration).

One catena facing north comprises loessial Arid Brown soils overlying buried quartzic Arid Brown soils at about 2.5 na below the surface (profiles EH-1 ; EH-6; EH-7). The other catena facing south is subjected to strong wind erosion (Yaalon & Dan, 1974) and as a result quartzic Arid Brown soils are exposed (profile EH-2

74 N o t e

and EH-4). The loessial Arid Brown soils were formed solely by aeolian dust accumu- lation, whereas the quartzic Arid Brown soils have developed from two parent materials: calcareous sand or sandstone (the non-carbonate sand is 98 ~ quartz and 2~o feldspars), with aeolian dust which has gradually infiltrated the sandy material. The best developed profile, EH-8, situated on the upland, contains gypsic accumulations in its lower part.

Experimental methods and results

Scanning electron micrographs. The first indication of a pedogenic clay mineral, confirmed later as palygorskite, was provided by the clay morphology in SEM micrographs of acid etched (1N HC1 or more diluted) samples from carbonate nodules and the surrounding soil (Fig. 15 in Wieder & Yaalon, 1974). Whereas in transmission electron microscopy the palygorskite, due to the necessary pre-treatment, appears as straight, flat and randomly oriented fibres or clusters of fibres, the stereoscan pictures of slightly acid etched samples indicate rather concentrated fibres, twisted and intertwined aggregates (Fig. 1). In the soil matrix, after stronger etching the twisting is less noticeable (Figs 2 and 3).

We conclude--like Eswaran & Barzanji (1974)--that aggregates of this kind are unlikely to survive transportation and redeposition within the soil, but that this structure, together with the high concentration of almost pure palygorskite indicates crystallization in situ. This contention is supported by the detailed clay mineral distribution of several profiles determined by X-ray diffraction.

X-ray diffraction

For X-ray analysis carbonates were removed with buffered Na-acetate at pH 5 (Jackson, 1956) and oriented slides were examined with Cu-radiation. Palygorskite was identified by the 10.45 A (110) and 6.45 A (20~)) reflections. Other clay minerals present in larger quantities are montmorillonite (including some mixed layers) and kaolinite. As in other Israeli soil clays the amount of illite is very small (Yaalon et al., 1966). The most significant relations are shown in Figs 4, 5 and 6.

In the loessial arid brown soils montmorillonite and kaolinite and some paly- gorskite are present (A in Fig. 4) and the diffraction pattern is similar to those of the parent loess and desert dust samples examined by us and by Ganor (1976) which occasionally contain palygorskite but only in small amounts.

Palygorskite is found concentrated in the lower B horizons of the quartzic arid brown soils, though the parent material was originally free or poor in palygorskite (B in Fig. 5). In soil profiles where the sandy horizon is overlain by the finer textured loessial material, the palygorskite accumulation increases noticeably with depth (B and C in Fig. 4 and C and D in Fig. 6, clay loam and sandy clay loam respectively), and it is always highest in the horizons adjacent to the textural transition. The highest content of palygorskite is present in the IIB 3 horizon of profile EH-8 (D in Fig. 6), which also contains pedogenic gypsum.

Note 75

FIG. 1. Acid etched carbonate nodule from E H 1 (234-265 cm) showing almost pure aggregates of palygorskite; SEM x 1410.

F~G. 2. Acid etched soil matrix from EH-1 (234-265 cm) showing palygorskite laths in the soil matrix; SEM x 7200.

F~G. 3. Detail from Fig. 2; SEM • 14,400.

76 Note

Insoluble residue nodule

K

i J i I i i i i i

.30 26 2_2 18 14

Cu K oK 2e-*

M

M

P

M

I0 6

FIG. 4. X-ray diffraction patterns of samples from loessial Serozem soil overlying quartzic Arid Brown profile (EH-1). Depth indicated in cm, followed by horizon designation; ca- calcic horizon, b- buried horizon, A, B and C- yellowish clay loam, D- reddish sandy clay loam, E- insoluble residue from carbonate nodule. E Prof. EH-1 234-265 IIB b ca; D Prof. EH-1 234-265 II B b ca; C Prof. EH-1 193534 B32 ca; B Prof.

EH 1 158-193 B31 ca; A Prof. EH-1 25-55 B1 ca.

Discussion

The main evidence for the pedogenic origin of the pa lygorski te is its compl ica ted morpho logy as exhibi ted on the SEM micrographs and its concent ra t ion in the lower hor izons of the soils. Ne i the r the calcareous quartzic sand o f the under lying bur ied horizons, nor the covering loessial l oam conta in significant amoun t s of paly- gorskite, and it seems unl ikely tha t it was concent ra ted in the B 3 hor izons by preferent ia l movement within the soils.

The concent ra t ion of the pa lygorsk i te ad jacent to the textural t rans i t ion appears significant. Mois ture moving in a soil, when passing f rom finer to coarser textured hor izons accumulates at the b o u n d a r y in o rder to a t ta in the higher pressure needed to fill the larger pores in the hor izon be low (Rode, 1969) and dur ing infi l t rat ion this

N o t e 77

boundary layer remains nearly saturated and uniformly wetted. The notion of Singer & Norrish (1974) that temporarily waterlogged conditions were needed for paly- gorskite formation seems to support this interpretation. Though infrequently wetted, the transitional layers in these soils are likely to have been wet for longer than the other soil horizons and were a suitable environment for the palygorskite neoformation, especially in the larger pores of the sandy quartzic B-horizons.

Such textural transitional horizons are also frequently a favoured environment for the development of calcic horizons (Stuart & Dixon, 1973), but in these profiles a direct relation to palygorskite with calcic horizons, (see Millot et al., 1969) is rather tenuous. The amount of palygorskite in the calcic horizon of EH-1 (B and C in Fig. 4) is similar to that in the non-calcic B 3 horizon in EH-7 (B in Fig. 3).

P

K K

C P

K M

i K t

;o ' ' ' . . . . . . . . . . . . 26 18 14 I0 6

Cu K ~ 2 e ~

FIo. 5. X-ray diffraction patterns from quartzic Arid Brown soils (EH-2 and EH~). Depth indicated in cm, followed by horizon designation; ca- calcic horizon. A - reddish- yellow sandy loam, B - yellowish-brown loamy sand, C - reddish-yellow sand. C Prof.

EH-2 172-205 C; B Prof. EH-4 100-150 II C; A Prof. EH-4 55-100 B3 ca.

78 Note

M

c U A

--3() 28 2'6 24 2~2 20 18 16 14 Cu K ~ 2 0 ~

P

,2 ,'o 8 ~ . . . .

FIG. 6. X-ray diffraction patterns from loessial Arid Brown profiles overlying sandy to sandy clay loams (EH-6-8). Depth indicated in cm, followed by horizon designation. A and B - reddish-yellow loam, C - reddish-yellow clay loam, D - gypseous reddish- yellow sandy clay loam, E - gypseous reddish-yellow sandy loam. E Prof. EH-8 175-230 II C; D Prof. EH-8 115-173 II B3; C Prof. EH-8 85-115 B23; B Prof. EH-7

248-285 B3 ; A Prof. EH-6 240-260 B3.

Though the calcareous soils provide a buffered alkaline environment which is apparently needed for palygorskite formation, the direct proximity of calcite is probably not necessary (see Singer & Norrish, 1974). A possible connection between secondary calcites and palygorskites is the action of the former maintaining a high Mg-level. Firstly by precipitating CaCO3 and some Mg in high Mg-calcites (St. Arnaud & Herbillon, 1973) and secondly by releasing the Mg subsequently, as low Mg-calcite in the most stable calcite form. When this process is repeated, magnesium accumulates and reaches the subsoil to below the calcic horizon.

The highest content of palygorskite dominating other clays is found below the calcic horizon in the B3 horizon of profile EH-8 (D in Fig. 6) which also contains

N o t e 79

pedogenic gypsum. Eswaran & Barzanji (1974) also found the highest amount of palygorskite in gypsic horizons. There is apparently a relationship between the crystallization of gypsum and palygorskite but this relationship cannot be specified at present.

Finally, the relationship of palygorskite to montmorillonite must be considered, since both require a suitable supply of silica and magnesium, for their formation. Millot (1970, p. 271) suggested that the neogenetic sequence montmorillonite-~ palygorskite--~sepiolite exists in marine and lacustrine sediments due to a gradually increasing Mg/A1 ratio in the precipitating solution. Kaolinite is incompatible with palygorskite and is considered to be detrital when present in such an assemblage (Nathan, 1969). Several investigators have suggested the transformation palygorskite --~montmorillonite during weathering and the reverse reaction, i.e. montmorillonite--~ palygorskite, is equally possible when Mg/A1 increases. This is quite likely in the slightly alkaline calcareous soil which, as outlined above, tends to maintain a very low Al concentration but a relatively high Mg level.

In this study due to the variable degree of crystallization of the montmorillonite we have estimated palygorskite semi-quantitatively by relating it to the occurrence of detrital kaolinite. But support for the montmorillonite--~palygorskite transforma- tion is found in the diffractogram of the non-carbonate residue of the calcareous nodules (E in Fig. 4). Whereas the surrounding matrix contains considerable amounts of both montmorillonite and palygorskite (D in Fig. 4), the montmorillonite in the nodules seems to have been destroyed leaving palygorskite as the dominant mineral. Similarly, the sandy C horizon (B and C in Fig. 5) shows a dominant content of montmorillonite, which seems to have decreased in the B horizons (e.g., D in Fig. 4) while palygorskite increased. The montmorillonite-~palygorskite transformation, provided additional Mg is available, seems to be a distinct possibility, but requires further study and evidence.

Frye et al. (1974) presented data of the clay mineral zonation in the caliche capped Ogallala Formation, New Mexico. A clear increase of diagenetic palygorskite as detrital montmorillonite content decreases is shown and suggests some alteration of detrital montmorillonite to palygorskite. This transformation is supported by the local presence of opal, probably derived from the excess of silica. A high- magnesium environment produced during evapotranspiration has been postulated by Frye et aL for the origin of palygorskite, an interpretation akin to ours, i.e. palygorskite formed at the expense of montmorillonite, though we favour downward infiltration and diagenitic inversion of high Mg-calcite to low Mg-calcite at the magnesium source.

Acknowledgment

Thanks are due to the technicians of the SEM and X-ray laboratories, for their assistance in the sample preparation and examination.

D. H. YAALON Department of Geology, The Hebrew University, M. WIEDER Jerusalem, Israel.

80 Note

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