In a study of “rana” soils of central Spain, hydroxy-interlayered 2:1 clay minerals were identified in the upper horizons and high-charge smectite in the lower horizons, with kaolinite the most abundant phyllosilicate. The high-charge smectite showed a basal spacing of 18 Å on Mg-saturation and glycerol solvation and 10 Å on K-saturation and air-drying. It is concentrated in the coarse fractions and appears to be basically a beidellite in the fine-silt fractions and a montmorillonite in the clay fractions. Pre-existing illite was probably transformed into hydroxy-interlayered 2:1 clays in the surface horizons, whereas high-charge smectite formed in the deeper horizons. These minerals, which were apparently not stable in the existing soil conditions, were the first stage in the degradation process of illite. The stable mineral in these soil profiles was kaolinite.

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.

Study of the transformation of smectite to illite, chlorite or vermiculite via interstratified clay minerals needs precise qualitative and quantitative determinations of the different layers in the mixed-layer clays and is generally based on X-ray diffraction (XRD) patterns after specific treatments of the clay samples. Saturation with K or Mg followed by ethylene glycol (EG) solvation are classical methods used to identify high-charge smectite and vermiculite. These procedures have been applied to two experimental clays, one composed of smectite layers and the second, a mixture of vermiculite and smectite layers. Different methods of glycolation (EG vapor or liquid EG) produce significant differences in the XRD patterns. Comparison with literature data indicates that K-saturated, high-charge smectite (≈0.8 < total charge <1/unit-cell) and Mg-vermiculite (whatever its charge) do not expand in ethylene glycol vapor (d values ≈14–15 Å). Expansion to 17 Å in liquid ethylene glycol occurs for Mg-vermiculite with a total charge of <~1.2/unit-cell and for K-saturated, high-charge smectite, when the tetrahedral charge is <≈0.7/unit-cell. This study shows that: (1) glycolation procedures need to be standardized; (2) the use of saturation protocols using both liquid ethylene glycol and ethylene glycol vapor yields useful additional information about the distribution of charges in clay minerals.

Mineralogical and chemical variations were studied in the upper half of a 1 m thick discontinuous bentonite bed interlaminated in the Lower Cenomanian sedimentary formations of the northern Aquitaine basin (France). X-ray diffraction patterns obtained from the <2 mm fraction in the Ca and K-saturated states were decomposed and compared to those calculated from decomposition parameters. They revealed the presence of two highly expandable illite-expandable (I-Exp) mixedlayer minerals (MLMs). The relative proportions of the two MLMs evolve steadily with depth leading to the decrease of the cation exchange capacity and of the (Na + Ca) content towards the centre of the bentonite bed. However, the system is essentially isochemical and Mg, Al, Si, K and Fe are roughly constant in the bulk samples. It is thought that the mineralogical zonation results from the initial stages of the smectite formation in an ash layer.

In the Ca-saturated state, the expandable component of these MLMs was for the most part homogeneous with the presence of 2 sheets of ethylene glycol molecules in the interlayer. However, the heterogeneous hydration behaviour of these expandable layers was enhanced by the K-saturation test. From this test, the presence of three layer types with contrasting layer charge was evidenced from their contrasting swelling abilities. The C12-alkylammonium saturation test applied to samples in which the octahedral charge had previously been neutralized (Hofmann-Klemen treatment) showed that the tetrahedral charge is located on specific layers. These layers are responsible for the heterogeneous hydration behaviour. Low-charge smectite layers are mostly octahedrally substituted, whereas for intermediate- and high-charge layers this montmorillonitic charge is complemented by additional tetrahedral substitutions (0.30 and 0.35–0.40 charge per O10(OH)2, respectively).