X-ray diffraction and chemical analyses were performed on clay fractions separated from an acid brown soil (Dystrochrept) by means of size fractionations using high-gradient magnetic separation techniques. Breakdown of large phyllosilicates preexisting in the saprolite involved physical fragmentation and mineralogical transformations strongly related to chemical weathering.

Compared to the C horizon, the proportion of chlorite and vermiculite decreased strongly in the silt and coarse-clay fractions of the Al horizon, but was maintained in the finer clay fraction (< 1 μm). The distribution of mica in the different fractions was quite the opposite. Micas are the major component of the Al, 1–2 μm fractions, and their proportion progressively decreased with decreasing fraction size. Thus, it is concluded that during fragmentation and/or simple transformation of the larger phyllosilicates, clusters of chlorite, mica/vermiculite, and vermiculite layers were preferentially affected. A concentration of mica layers took place in the coarse clay fractions as chlorite and vermiculite residues were accumulated in the fine clays.

The process involved the loss of Fe and Mg, leaving, or forming, more aluminous dioctahedral minerals. As the transformation processes occurred, dissolution of preexisting minerals led to the precipitation of amorphous and/or crystalline Fe- and Al-oxides, and possibly of phyllosilicates. The new phyllosilicates appear to be montmorillonitic.

The most abundant end products of the weathering processes in either the Al or the Bw horizons appeared to be quite different. In the Al horizon they were identified mainly a hydroxyl-Al (Fe) intergrade smectite (montmorillonite), whereas in the Bw horizon the major component was an intergrade vermiculite originating, at least in part, from chlorite.

X-ray diffraction, FTIR, and chemical analyses were performed on clay fractions (1–2 µm, <0.1 µm), separated by means of size fractionations and high-gradient magnetic separation techniques, from a Cryorthod developed in a chlorite-mica schist saprolite. Weathering of large phyllosilicates pre-existing in the saprolite involves physical fragmentation and mineralogical transformations. Chloritic minerals in the coarse fractions were the most affected by physical breakdown, while micas were generally preserved. As a consequence, a concentration of mica layers occurred in the coarse clay fraction, while chloritic residues accumulated in the fine clays. These residues exhibited the typical XRD pattern of hydroxy-interlayered intergrade minerals, but the interlayered contaminants were found to be mainly hydroxy-Mg cations. Further mineralogical transformations of the intergrade minerals involved the progressive removal of the hydroxide interlayered sheet and dissolution of chloritic layers. Illite/smectite mixed-layers were formed in the surface horizon of the soil profile. These processes were associated with a strong decrease in Fe and Mg contents in the clay fractions.

Fine clays (<0.1 μm) extracted from an acid soil developed in a granite saprolite from the Massif Central, France, were characterized by X-ray diffraction (XRD) using a curve decomposition program, and high-resolution transmission electron microscopy (HRTEM) associated with a method of impregnation of moist samples. Direct measurement of d-spacings were performed on HRTEM photographs. Decomposition of XRD patterns indicated 5 to 6 different clay phases including chlorite (and/or hydroxy-interlayered vermiculite), vermiculite/smectite, illite/vermiculite and illite/smectite mixed layers. Expandable phases with decreasing layer charge (vermiculite, high- and low-charge smectite) were shown in the clay assemblage. When performed on K-saturated samples subjected to wetting and drying cycles, HRTEM observations were consistent with the XRD results. The major clay mineral phases identified by the decomposition of XRD patterns were also found by direct measurement of d-spacings on HRTEM images. Vermiculite and high-charge smectite appeared to be impregnated with preservation of their initial hydration state, whereas low-charge smectite interlayers were penetrated by the resin molecules during the impregnation procedure. It was concluded that the decomposition of XRD patterns gave a realistic analysis of the clay phases present in a complex soil clay sample, as well as the direct measurement of a limited number (50) of clay crystals on HRTEM images.

Hydroxy-interlayered minerals (HIMs) are typical of moderately acidic soils. Barnhisel and Bertsch (1989) defined the hydroxy-interlayered clay minerals as a solid-solution series between smectite, vermiculite and pedogenic or aluminous chlorite end-members. Their experimental data for the relationship between the decrease in cation exchange capacity (CEC) and the amount of Al fixed in the interlayers of smectites and vermiculites is reinterpreted using calculated structural, chemical and X-ray diffraction (XRD) evidence. The adsorbed Al ions are in a 6-fold coordination state: [Al(OH)x(H2O)y](3−x)+ with x+y = 6. The polymerization process occurs before saturation of the exchange sites by Al ions. Some of the adsorbed Al ions form polynuclear cations keeping a constant positive charge.

X-ray diffraction patterns of oriented preparations in the ethylene glycol-solvated state suggest that HIMs consist of randomly interstratified expandable and chlorite-like layers (17 and 14.2 Å). Chlorite-like layers result from the selective adsorption of Al complex ions in specific interlayer zones that behave similarly to Al-chlorite (donbassite-like) with incomplete (60%) ‘gibbsite-like’ sheets (chlorite60). Using this framework, HIM XRD patterns can be interpreted by comparison with calculated chlorite60-dismectite mixed-layer mineral patternss using the NEWMOD software.

Three soil profiles classed as brown forest soils on schistose parent material, in close proximity but under different land uses, have been studied. The profile under 50 year-old Sitka spruce trees is the most acidic, the second profile under rough grassland is less acidic, and the third profile in an arable field which has been fertilized is the least acidic. The clay fractions (ς μm) of all three profiles are very similar. However, computer simulations of the X-ray diffraction patterns of clay phases using a modified version of NEWMOD revealed two possible weathering trends: (1) an increasing proportion of vermiculite in interstratified mica-vermiculite in the upper horizons of the arable and forested soils; (2) formation of high-charge corrensite by weathering of chlorite in all three profiles but least pronounced in the arable soil. The differences in clay mineralogy amongst the profiles are minor, but these two different weathering trends may be due to the effects of different land use.