NOTE THE TEXTURE OF PALYGORSKITE FROM THE RIFT VALLEY, SOUTHERN ISRAEL

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1 Clay Minerals (1981) 16, NOTE THE TEXTURE OF PALYGORSKITE FROM THE RIFT VALLEY, SOUTHERN ISRAEL Based on the conventional examination methods of transmission electron microscopy, the micromorphology of palygorskite has been accepted to consist of straight fibres or laths lying with complete random orientation. The application of carbon replica techniques, and recently that of scanning electron microscopy on fracture surfaces, has shown, however, that in its undisturbed state authigenic palygorskite appears aggregated into bundles. A closer scrutiny of the micrographs published by Watts (1976), Yaalon & Wieder (1976), Hassouba & Shaw (1980) and Schwaighofer (1980) reveals elongate, almost cylindrical particles with dimensions that suggest aggregation of individual laths into larger units, resembling rods or fibres. These structures have been variously defined as 'rods', 'fibres' and 'laths', terms similar to those commonly used for the individual laths observed in transmission electron micrographs of dispersed palygorskite samples. This usage of a similar terminology for differing structures is evidently confusing. Authigenic palygorskite identified in Neogene lake sediments from the Rift Valley, south of the Dead Sea, provided an opportunity for more detailed examination of the texture of the undisturbed mineral and a comparison between this and the structures observed in dispersed samples. Materials and methods The palygorskite is from the base of the Neogene Hazeva Formation in the Rift Valley, south of the Dead Sea. The predominantly dolomitic marls associated with the palygorskite-containing sediments belong to the limnic Mashak Member which has a total thickness of about 3 m (Sneh, 1967). Two parallel sections (A and B) of the Mashak Member were examined. In each, the bottom layer is a marly limestone containing palygorskite. The upper layers in both sections are dolomitic marls and contain only minor amounts of palygorskite. The acid-insoluble fraction of the sediments was obtained by dialysis with buffered Na-acetate at ph 5. Scanning and transmission electron microscopic investigations were carried out with the JEOL JSM 35C and JEM 100CX instruments, respectively. Results and discussion Carbonate content, clay content of the acid insoluble fraction and chemical composition ofthe clay in three layers from the two parallel sections of the Mashak Member are given in Table 1. The lowermost layers (3) are those containing palygorskite. Carbonate contents decrease upwards. Carbonates are normally calcite in palygorskite-containing layers A3 and B3 and in layer A2. In the higher-lying layers, A1, B2 and B1, carbonates are either dolomite, or dolomite together with calcite. The acid-insoluble residue in all /81/ The Mineralogical Society

2 416 Note TABLE 1. Carbonates, clay content and chemical composition of the clay fractions in the acid-insoluble residues of palygorskite-containing marly sediments from the Neogene Hazeva Formation, Rift Valley, Israel. D = dolomite, C = calcite Section A Section B Layers l Carbonates 39.8(D,C) 48.8(D) 69.9(C) 68-1(D) 17.2(C) 82.1(C) Acid insoluble residue Clay SiO A Fe MgO CaO Na K H A B 7 t9~ ~5 50~ 1 I I 1 I I L Cu Ko~ 2e Cu K(I 2e FIG. 1. XRD traces of oriented, Mg-saturated clay (< 2 #m) samples separated from the acid-insoluble residues of Neogene marly-lacustrine sediments; left--section A, layers 1, 2 and 3; right--section B, layers 1, 2 and 3.

3 Note 417 layers is composed predominantly of clay. The clay content is particularly high in palygorskite-containing layers A3 and B3. XRD patterns of oriented, Mg-saturated clay ( < 2 /~m) fractions from the acidinsoluble residues indicate that in layer A3 palygorskite and smectite dominate and are accompanied by minor amounts of kaolinite and mica (Fig. 1). In layer B3, palygorskite is accompanied by smectite only. The clay fractions in the layers higher in the sections (A2, 3 and B2, 3) are dominated by smectite, accompanied by considerable amounts of kaolinite and some mica. The silt fractions are composed almost exclusively of quartz. The high palygorskite contents in layers A3 and B3 are reflected in the high Mg contents of the clay fractions (Table 1). The relatively high Mg contents in the smectite-dominated layers A2, 3 and B2, 3 suggest the presence of high-mg smectites. Considerable mica in all layers is indicated by the relatively high potassium contents. Scanning electron micrographs of fracture surfaces reveal the in situ morphology of the palygorskite. In Fig. 2(A) the fibres tightly enmesh calcite crystals and sprout out from their surfaces. The fibres, average length 2-3/~m, frequently branch out or intertwine (Fig. 2B). The central parts of the apparently cylindrical fibers appear darker, suggesting a lower density. Closer inspection of some of the fibres at a higher magnification shows that transverse cross-sections are actually not cylindrical but rather polygonal, with a diameter of,,~0.15 um (Fig. 2C). Fig. 2(D) shows that the diameters of the fibres may vary FrG. 2. Scanning electron micrographs of fracture surfaces from layer B3, Neogene, marlylacustrine limestone;(a) and (B}--palygorskitefibres tightly enmeshingcalcite crystals and sprouting out from their surfaces;(c)--some of the fibresfrom (A) at a higher magnification; (D)--fracture surface exhibitingtransverse cross-sectionsof fibres. Bars in all pictures= 1 /~m.

4 418 Note between #m. Pei-lin-Tien (1973), in his description of palygorskite from Leicestershire, U K, mentions the six-sided transverse sections of palygorskite fibres obtained by ultramicrotome cuts. They were elongate, with the long axis ranging from #m and short axis from #m. The angles suggested a prismatic crystal habit combined with a basal. Bates (1958) found fine, dispersed Attapulgus palygorskite fibres (laths) to have a width of A. He also mentioned that they gave the impression of almost cylindrical rods, aggregated into bundles of a few fibres. Transmission electron micrographs of dispersed Rift Valley palygorskite show mainly aggregates of 2-5 laths (Fig. 3A). The laths have a length of /tm and a width varying between 200 and 350 A (Fig. 3B). More rarely, bundles can be observed composed of laths in parallel orientation (Fig. 3C). At their frayed ends, the individual laths aggregated in the bundles can be seen to protrude (Fig. 3D). These bundles, which are ~ /~m wide, are the equivalents of the 'fibres' observed in the scanning electron micrographs. Thus two distinct textural units can be distinguished: the 'laths' with a width varying between A, which constitute the primary units, and the 'fibres' (or 'rods') which are the aggregates, with a width varying between 0" pm. The interfibrous bonds do not appear to be very strong, since only few of the fibrous aggregates survive intact even mildly dispersive treatments. Compari- C FIG. 3. Transmissionelectronmicrographsof dispersed clay ( < 2 #m) samplesseparated from the acid-insolubleresidue of Neogenemarly-lacustrinesediments;(A)--laths aggregatedinto units of 2-5; (B)--laths at a higher magnificationshowingstriations with a width of ~ 50 A; (C)--bundles of laths, equivalentto the 'fibres' seenin the scanningelectronmicrographs;(d)--the frayed ends of the bundle from (C) at a higher magnification, showing the individual laths.

5 Note 419 son of the lengths estimated for the fibres with those of the laths seen in the transmission electron micrographs suggests that extensive transverse breaking of the laths also takes place during dispersion. Distinct striations can be seen in the transmission electron micrographs of individual laths (Fig. 3B). The width of the bands is approximately 50/k. This figure is close to the spacing of 45 A observed by Pei-lin-Tien (1973) on transverse sections of palygorskite laths, and has also been observed by others (Vivaldi & Robertson, 1971). Spacings of 45 A, or close to multiples of 9, within palygorskite laths can be explained by the sepiolitepalygorskite intergrowth model of Gard & Follett (1968) which contains pyroxene chains linked at regular intervals with sepiolite chains. The palygorskite textures described here also have relevance to the problem of palygorskite formation. Yaalon & Wieder (1976), Weaver & Beck (1977) and others suggested that palygorskite may have formed from the smectite with which it was associated. The Rift Valley palygorskites are also associated with large amounts of smectite. If the palygorskite had formed from smectite, it would be reasonable to expect to identify transitional textures by electron microscopy. Yet, as shown above, palygorskite appears in aggregates of a monomineralic composition. This lends support to the assumption that the mineral had formed by precipitation from solution. The Seagram Centre for Soil and Water Sciences, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot, Israel. ARIEH SINGER REFERENCES BATES T.F. (1958) Selected Electron Micrographs of Clays (nos 45, 46). Circular No. 51, Mineral Industries Experiment Station, State Univ., Pennsylvania. GARD J.A. & FOLLETT E.A.C. (1968) Clay Miner. 7, 368. HASSOUBA H. & SHAW H.F. (1980) Clay Miner. 15, 77. PEI-LIN-TmN (1973) Clay Miner. 10, 27. SCHWAIGHOrER B. (1980) Clay Miner. 15, 283. SYEH A. (1967) Progress Rep. No. 1023, Institute for Petroleum Res. & Geophys. VIVALDI M.J.L. & ROBERTSON R.H.S. (1971) The Electron-Opticallnvestigation of Clays (J. Gard, editor) p Mineralogical Society, London. WATTS N.L. (1976) Am. Miner. 61, 299. WEAVER C.E. & BECK K.C. (1977) Sediment. Geol. 17, 1. YAALON D.H. & W1EDER M. (1976) Clay Miner. 11, 73.