Formation of gibbsite in the presence of 2:1 minerals: an example from Ultisols of northeast India

Transcription

1 Clay Minerals (2000) 35, Formation of gibbsite in the presence of 2:1 minerals: an example from Ultisols of northeast India T. BHATTACHARYYA*, D. K. PAL AND P. SRIVASTAVA Division of Soil Resource Studies, National Bureau of Soil Survey & Land Use Planning, Amravati Road, Nagpur , India (Received 23 December 1998; revised 21 February 2000) ABSTRACT: There are two different views regarding the genesis of gibbsite in tropical acid soils: (1) direct weathering of primary Al-silicate minerals; and (2) transformation through clay mineral intermediates. We investigated the genesis of gibbsite in two representative Ultisols from northeastern India. Gibbsite in these Ultisols appears to be the remnant of earlier weathering products of aluminosilicate minerals formed in a neutral to alkaline pedochemical environment. The mere presence of gibbsite in these soils, therefore, does not indicate their advanced stage of weathering. The formation of typically rod-shaped and well-crystallized gibbsite in both the coarse and fine soil fractions in the presence of large amounts of 2:1 minerals indicates that the anti-gibbsite hypothesis may not be tenable in these tropical acid soils. A schematic model for the formation of gibbsite and kaolin in Ultisols is proposed. KEYWORDS: gibbsite, Ultisol, acid soil, weathering, India. The mineralogy of Ultisols in the state of Meghalaya of the northeastern region of India was dominated by gibbsite in their sand, silt and clay fractions along with kaolin (Kl) and hydroxy interlayered vermiculite (HIV). The study describes a pedochemical environment that might have favoured the direct transformation of Al silicates to gibbsites. Morphological, chemical and mineralogical studies of two representative Ultisols of Meghalaya state are described and discussed in the light of existing concepts described by Hsu (1989) for the formation of gibbsite in humid tropical, subtropical and temperate regions. * tapas@nbsslup.mah.nic.in MATERIALS AND METHODS Soils Two representative Ultisols developed on Archaean Gneiss (biotite-sillimanite-cordierite) were selected from the central gneissic highland of West Khasi hills of the Meghalaya plateau in northeastern India at an elevation of >1200 m above mean sea level (Fig. 1). Pedon 1 represents a profile on a hill top at lower elevation whereas pedon 2 represents a profile on the slopes of hills. Analytical techniques The general properties of the soils were determined following standard procedures (Jackson, 1973; Soil Survey Staff, 1975). Sand ( mm), silt (50 2 mm), total clay (<2 mm) and fine clay (<0.2 mm) fractions were separated according to the procedures of Jackson (1979). The # 2000 The Mineralogical Society

2 828 T. Bhattacharyya et al. FIG. 1. Details of the study area (Munsell notations indicate wet soil colours. In both pedons the horizon boundaries are clear smooth or clear wavy.) powder mount of the sand fractions was X-rayed using a Philips diffractometer with Fe-filtered Co-Ka radiation at a scanning speed of 282y/min. Oriented clay fractions were subjected to X-ray diffraction (XRD) analysis. Samples were saturated with: (1) Mg and solvated with ethylene glycol and: (2) K (258C), and heated to 110, 300 and 5508C. Identification of the clay minerals in the different fractions was done following the criteria laid down by Jackson (1979). For identification of

3 Formation of gibbsite 829 halloysites in silt and clay fractions, the formamide test was carried out following the method of Churchman et al. (1984). The formamide-treated samples were scanned after 1 h, after 4 h and after 10 days. Since the mineralogy of both pedons was identical, only XRD patterns of pedon 2 have been reported. A bauxite ore developed on khondalite (Nandi & Slukin, 1983) was used as a reference in the present study. The clay fraction (<2 mm) of this reference bauxite comprised gibbsite (80%) and kaolin (20%). Sand-sized gibbsite and biotite particles from the soils and bauxite ore were picked out after identification under a petrographic microscope. These particles were fixed on an Al stub with LEIT-C conductive carbon cement, coated with gold and examined in a Philips scanning electron microscope (SEM). RESULTS AND DISCUSSION Chemical characteristics The soils are clay loam in texture, rich in organic matter, poor in Ca, Mg, Na and K, and are acidic. The KCl phs of the soils were >phs in water for the lower horizons (Table 1), which might indicate the presence of gibbsite in these soils (Smith, 1986). The cation exchange capacity (CEC) and effective cation exchange capacity (ECEC) values range from 2.9 to 4.4 and 1.0 to 1.1 cmol(+) kg 1 respectively, in the soil control section (SCS) (Table 1). Due to clay illuviation, clear horizon boundaries, and the presence of appreciable amounts of weatherable minerals, these low-activity clay soils are grouped as Ultisols (Soil Survey Staff, 1994), and these soils are common in northeast India (Bhattacharyya et al., 1994). Mineralogy of the soils Sand ( mm). This fraction produced XRD peaks at ~2.4, 1.2 and 0.8 nm confirming the presence of interstratified mica-vermiculite. A small peak at ~0.7 nm was also present. The presence of gibbsite was indicated by a strong peak at nm along with its higher-order reflections. Small amounts of quartz, sillimanite and feldspars are also present (Fig. 2). The sharpness and intensity of the XRD peaks of the gibbsites indicate their high degree of crystallization. The SEM photographs of gibbsites (Fig. 7a c) also indicate that they are very well developed. The dominance of gibbsites over layer silicates in the sand fractions suggests that the formation of gibbsites entirely at the expense of 1.0 and 0.72 nm minerals is unlikely. Had gibbsite formed from layer silicates, it would have concentrated in the finer clay fractions as weathering products. Silt (50 2 mm). This fraction contains hydroxy interlayered vermiculite (HIV), interstratified minerals of the mica-hydroxy-interlayered vermiculite (M/HIV) type, kaolin, mica, quartz, feldspars and gibbsite. The presence of kaolin was ascertained by the appearance of a 0.72 nm peak after digestion with 6 N HCl for 30 min and disappearance of the 0.72 nm peak following K saturation and heating to 5508C (K5508C). The 0.72 nm peak is characterized by a relatively broad base (Fig. 3). The Mg-saturated silt samples treated with formamide failed to produce any shift of the 0.72 nm peak after 1 h, indicating that the 0.72 nm peak is not due to the presence of halloysite. The XRD patterns of Mg-saturated samples show a sharp peak at 1.40 nm along with peaks at 0.72, 0.48 and 0.35 nm without any change in position and/or intensity on glycolation (Fig. 3). Gradual heating of the K-saturated samples from 110 to 3008C led to a partial collapse of the 1.40 nm peak. On further heating to 5508C, this peak collapsed to 1.00 nm indicating the presence of HIV. The peak at ~1.20 nm did not shift with solvation, nor on K saturation and heating at 1108C (Fig. 3). This indicates the presence of a mixed-layer mineral with random interlayering of 1.00 and 1.40 nm minerals (Barnhisel & Bertsch, 1989). The peak at ~1.20 nm, however, shifted to ~1.10 nm and gradually merged with the 1.00 nm component on heating from 300 to 5508C. The 1.40 nm component of this interstratified mineral was thus of a non-swelling type and represents a HIV. The behaviour of the 1.20 nm peak indicates that it is an interstratification of mica and HIV (M/ HIV). The presence of gibbsite is confirmed by a 0.48 nm peak which subsequently disappeared due to thermal treatment at K 3008C (Fig. 3). These observations show that gibbsite can occur even in the presence of substantial amounts of 2:1 minerals (Wada & Kakuto, 1983; de Brito Galvao & Schulze, 1996). The presence of both of these minerals does not support the generally observed fact that gibbsite cannot form until all the expansible 2:1 clay minerals are decomposed (Hsu, 1989).

4 830 T. Bhattacharyya et al. TABLE 1. Selected physical and chemical properties of Ultisols. Horizon Depth Analysis (%) ph OC Extractable cations CEC ECEC (cm) Sand Silt Clay Water KCl (g kg 1 ) Ca Mg Na K Total NH4OAc SCS NH4OAc SCS ( mm) (2 50 mm) <2 mm (1:2) (1:2) cmol(+)kg 1 Pedon 1 Ap B Bt Bt Pedon 2 A B Tr Bt Bt Tr C SCS: Soil Control Section is defined by a depth of 25 cm to 1 m if the regolith is >1 m thick (Soil Survey Staff, 1975) Tr: Traces OC: organic carbon

5 Formation of gibbsite 831 FIG. 2. Representative XRD patterns of the sand fractions (Bt2 horizon of pedon 2); ML = mica-vermiculite, Kl = kaolin, G = gibbsite, Q = quartz, S = sillimanite, F = feldspars. Clay (<2 mm). The XRD patterns of the clay fractions of both the pedons exhibit well-defined diffraction maxima indicating the presence of HIV, M/HIV, mica, Kl, gibbsite, feldspars and quartz and possibly also an interstratified kaolin-hiv mineral (Kl-HIV) (Fig. 5). A formamide test of the clay samples did not produce any shift of the 0.72 nm peak indicating that the 0.7 nm peaks are not attributable to halloysite (Fig. 4). Fine clay (<0.2 mm). The peak at 1.40 nm appears clearly in both the pedons and its behaviour towards solvation, K saturation and subsequent thermal treatments confirms the presence of HIV (Fig. 6). The Kl, HIV and gibbsite are present in major proportions. The 0.72 nm peak tailed off somewhat in the low-angle regions. However, on K-saturation at 258C, this peak shifted a little towards the low-angle region and reached 0.72 nm during heating from 110 to 3008C with subsequent reinforcement of the 1.0 nm peak. This indicates the presence of Kl and HIV although the possible presence of Kl-HIV cannot be ruled out. The

6 832 T. Bhattacharyya et al. FIG. 3. Representative XRD patterns of the silt fractions (Bt2 horizon of Pedon 2); Mg = Mg-saturated; Mg-EG = Mg-saturated plus glycol vapour; K 25/110/300/5508C = K-saturated and heated to 258, 1108, 3308 and 5508C, HIV = hydroxy interlayered vermiculite, M/HIV = mica-hydroxy interlayered vermiculite, M = mica, Kl = kaolin, G = gibbsite, S = sillimanite, Q = quartz.

7 Formation of gibbsite 833 FIG. 4. Representative XRD patterns of Bt2 horizon of pedon 2: (a) silt fractions, (b) total clay fractions, and (c) fine clay fractions; fo = Mg-saturated samples, f1 = Mg saturated, treated with formamide and kept for 1 h; f4 = Mg saturated, treated with formamide and kept for 4 h 10 days.

8 834 T. Bhattacharyya et al. FIG. 5. Representative XRD patterns of the total clay (i.e. <2 mm) fraction (Bt2 horizon of Pedon 2). Abbreviations as in Fig. 3.

9 Formation of gibbsite 835 FIG. 6. Representative XRD patterns of the fine clay (i.e. <0.2 mm) fraction (Bt2 horizon of Pedon 2). Abbreviations as in Fig. 3. formamide test of the fine clay samples showed the possible presence of halloysite. The M-HIV and mica were not identified in the fine clay fractions. Genesis of hydroxy-interlayered minerals and gibbsite. The presence of mica-vermiculite in the sand fractions and mica-hydroxy-interlayered vermiculite and 1.40 nm minerals in the silt fractions suggests that mica has been transformed into vermiculite and that Al ions released in neutral to acidic soil solution were adsorbed in the interlayers of vermiculite to form HIV and then probably to Kl-HIV. Under an acid weathering environment in a humid tropical climate with plenty of available Al 3+, hydroxy-al-interlayering in the expansible 2:1 layer silicates is an essential step towards the interstratification of 2:1 and 1:1 layers (Bhattacharyya et al., 1993). Since the expanding lattice minerals act as a template for formation of HIV in such a weathering environment, formation of gibbsite has been stated as unlikely due to the anti-gibbsite effect (Jackson, 1963, 1964). It therefore rules out the presence of gibbsite and HIV in

10 836 T. Bhattacharyya et al.

11 Formation of gibbsite 837 the same weathering environment. Interestingly, the HIV and gibbsite occur simultaneously in these Ultisols. Morphological features of gibbsite of the soils show well-developed rod-shaped particles in the form of hexagonal prisms (Fig. 7a c). Tait et al. (1983) demonstrated the formation of such crystals in an alkaline environment which finds support from observations made in natural bauxite profiles of Western India (Balasubramaniam & Sabale, 1984). Well-crystallized gibbsite particles show their appreciable stability in the present-day acidic environment of the Ultisols under study. However, gibbsites undergo dissolution when the ph drops below 5 (Mulyanto et al., 1999). This suggests that the formation of gibbsite and HIV might have occurred in two distinctly different pedochemical environments. Transformation of kaolin to gibbsite appears improbable. It involves a desilication process above a ph of ~9 (Millot, 1970). Experimental studies by electrodialysis (Majumdar, 1979) and also by repeated passing through H-resin (Majumdar & Mukherjee, 1979) indicate a general resistance of the kaolin structure to degradation in mineral acids as compared to other layer silicates. Even in extremely acidic conditions (ph 2.2) in the laboratory, structural degradation of kaolinite is observed only at crystal edges by the liberation of Al 3+ ions (Majumdar & Mukherjee, 1979). Such extreme acidic conditions do not normally exist other than in acid sulphate soils. It is interesting to note that even in acid sulphate soils (CaCl 2, ph 2.4), kaolinite persists in considerable amounts (Ghosh & Tomar, 1973). The routine method of confirming the presence of kaolinite in a mixture of kaolinite and chlorite through selective dissolution of chlorite by 6 N HCl at 908C for 30 min also supports the fact that kaolinite can persist in acidic environments (Wilson, 1987). It has been observed that Al ions liberated from layer silicate structures remain as Al(OH) 3, which readily becomes crystallized to form gibbsite at phs ranging from 6.5 to 8.5, but remains as Al 3+ ions at ph <4.7 (Panda, 1987). This suggests that the gibbsite in the Ultisols studied was probably formed in the neutral to alkaline ph range and the kaolin at an acidic ph. Balasubramaniam & Sabale (1984) also observed that kaolinization was not an intermediate stage in the formation of bauxite which was transformed directly from plagioclase feldspars in an alkaline environment. Petrographic observation of minerals in the sand fraction of these soils indicated the presence of gibbsite particles (1 2 mm in size) in dominant proportions (>80%), with few particles of biotite, sillimanite or quartz. Very few highly-altered plagioclase feldspars were observed. Biotite particles were identified in the reference bauxite ore (Nandi & Slukin, 1983) studied. This suggests that micas may survive not only in bauxitic laterite (Anand & Gilkes, 1987) but also in kaolinitic ferruginous soils (Pal et al., 1989). Gibbsite crystals were not observed at the fringes of biotite particles. This is quite evident from the morphology of biotite under SEM, which indicates initiation of layer separation to the formation of vermiculite around biotite particles both in the soils (Fig. 7d,e) and in the reference ore (Fig. 7f). Although biotite did not suffer total dissolution, it does not preclude the formation of gibbsite from biotite, especially in the finer fractions. In an alkaline environment, interlayer K release from clay-sized biotite occurs mainly through dissolution of its structure (Pal, 1985). In coarser fractions, K release occurs primarily through exchange reactions, conforming to the typical vermiculitization process in trioctahedral mica at higher ph conditions (Pal, 1985). During the hydrolysis reaction at an initial stage of weathering it is most likely that sillimanite, feldspars and biotites were dissolved to give soluble forms of Al in solution which then crystallized as gibbsite on a solid mineral surface. In order to identify the core mineral of the large gibbsite crystals, these were broken to expose their inner surface. Petrographic examination of the exposed surfaces indicated partially altered sillimanite grains and their morphology under SEM showed dissolution and etch pits with profuse gibbsite crystals FIG. 7. Representative SEM photographs of sand sized fractions showing: (a) a cluster of rod-shaped gibbsite developed on sillimanite (C horizon of Pedon 2); (b) hexagonal prisms of individual gibbsite (C horizon of Pedon 2); (c) single crystal of gibbsite showing clear hexagonal planes (C horizon of Pedon 2); (d) initiation of layer separation in biotite particles of soils;(e) formation of vermiculite around biotite particles of soils (C horizon of pedon 2); (f) biotite particles with layer separation in reference ore; (g) highly altered sillimanite grain showing dissolution and etch pits with gibbsite crystals; (h) same as (g) with a part showing higher magnification.

12 838 T. Bhattacharyya et al. FIG. 8. Schematic models of the formation of gibbsite in Ultisols of northeastern India. (Fig. 7g,h). This observation confirms that the formation of gibbsite is primarily from sillimanite, although the formation of gibbsite from feldspars cannot be ruled out. The formation of gibbsite at the expense of primary aluminosilicate minerals appears to be common in soils (Lowe, 1986; Hsu, 1989) and bauxite ores (Balasubramaniam & Sabale, 1984; Hsu, 1989). With the advancement of weathering as the ph of the soils became acidic, the liberated Al ions in the form of (Al(OH) + 2) were adsorbed in the interlayer spaces to neutralize the negative charge of the vermiculite layers (Barnhisel & Bertsch, 1989). The presence of HIV in these Ultisols confirms that acidic weathering has occurred. environment gradually became acidic favouring the formation of HIV. The presence of gibbsite in these soils should, therefore, be considered as a signature of earlier pedochemical weathering and should not be considered as conclusive proof of extreme weathering conditions of soils (Macias Vasquez, 1981; Jenkins, 1985; Lowe 1986). The present acidic soil environment (ph &5) is not conducive for the desilication and precipitation of the Al and Fe that lead to the formation of tropical ferruginous soils. Thus the reconstruction of an earlier alkaline pedochemical environment can explain the presence of gibbsite in the Ultisols studied. A schematic model for the formation of gibbsite in these Ultisols has been proposed (Fig. 8). A model proposed The Ultisols studied were formed on a sillimanite-rich gneissic rock in the tropical humid climate from at least the Tertiary period (Idnurm & Schmidt, 1986) on the Meghalaya plateau which is a horst structure uplifted during the Mio-Pliocene transition (Krishnan, 1982). Since then, the pedochemical environment has changed considerably. During the initial stage of weathering it was neutral to alkaline, which helped the crystallization of gibbsite. With progressive leaching due to high rainfall during the advancement of weathering, the CONCLUSIONS It appears that gibbsite in the tropical acid soils (Ultisols) of northeast India was formed from primary Al silicates. It has occurred as a remnant of an earlier weathering cycle characterized by a neutral to alkaline pedochemical environment. It cannot be considered as an indicator of an advanced stage of weathering. The formation of gibbsite even in the presence of a considerable amount of 2:1 minerals indicates that the hypothesis of an antigibbsite effect does not hold in these tropical acid soils.

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