Hectorite collected from colemanite deposits in north-west Turkey was examined by using x-ray diffractometer and electron microscopy techniques. Structural formula of the mineral was derived from its chemical analysis results and cation exchange capacity was determined.

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Natural growth faces and cleavages of kaolinite crystals were characterized by electron microscopy using a gold decoration method. The cleaved surfaces of kaolinites treated with urea followed by wet-grinding showed slip steps but not growth spirals, whereas the surfaces of kaolinites etched with NH₄F showed growth spirals. Therefore, the surface microtopographs of kaolinites with different crystallinities from deposits and soils were compared before and after etching with NH₄F. Well-crystallized kaolinites from hydrothermal deposits exhibited growth spirals with wide spacings, irrespective of whether they were etched with NH₄F or not, and were characterized by spiral growth from solution phase of low supersaturation. Poorly-crystallized kaolinites from sedimentary deposits showed growth spirals on the surface etched with NH₄F but not on the original surface. The concentration of NH₄F solution suitable for removal of the surface layers of kaolinite crystals was 5 to 15 %. The relationships between the crystallinity and the type of growth spirals observed before and after etching with NH₄F were similar for kaolinites from residual soils other than one transported soil and kaolinites from deposits. The differences in environmental conditions of their formation were discussed.

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Thermogravimetric studies of an expansive clay soil from Adelaide (South Australia) indicate one major dehydroxylation event. Comparing the weight loss at this event with standard (API) montmorillonites allows a relatively accurate determination of the geotechnically-undesirable montmorillonite type minerals to be made.

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Homoionic (MB) and MB-Na-montmorillonites were prepared and examined by IR, ESR and X-ray methods. The IR spectrum of MB on montmorillonite was very similar to that of MB in methanol and was considered to be a typical spectrum for MB in monomer form. Protonation of MB⁺ to MB⁺⁺ on the clay surface was observed in homoionic MBmontmorillonite, which showed a deformation band of hydrogen on the group=N-at 1520 cm^-1. Aqueous and solid MB (polymer forms) showed ESR signal at g=2.0036. This signal can be ascribed to free radicals occurring within the structure of the associated molecules. MB in methanol or on the exchange sites did not show any ESR signal, confirming the IR finding that MB in methanol or adsorbed on the clay surface is mainly in the pure monomer form. The ESR signal of the clay at g-2 was not affected either by methanol wash or MB⁺ on the exchange sites showing that the presence of MB⁺ had no effect on the paramagnetic species on the clay. X-ray spacing of MB-complexes showed both single and two-layer structures.

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IR Spectral changes occurred during heating of MB-and MB-Na-montmorillonites (25-500°C) in N₂ flow have been considered in this work. Within a temperature range of 25-150°C, the H-O-H deformation band at 1635 cm^-1 diminishes progressively and MB bands shift towards low frequencies. Therefore, the thermal changes at this stage are limited to increases in the attraction between the exchangeable MB⁺ and the clay surfaces.
Within a temperature range of 200-500°C the exchangeable MB⁺ is completely decomposed. The decomposition starts near 200°C and is almost completed at 500°C. The decomposition of exchangeable MB⁺ leaves a residue consisting of a mixture of aromatic species, methyl groups, NH₃ and NH₄⁺ ions on the clay surface. Pyrolysis of these species takes place at the end of this stage.
Structural water of the clay participates in decomposition of MB⁺ on the clay surfaces. This was observed in IR spectra and confirmed by differential thermal microanalysis (DTMA).
Thermal decomposition of molecular MB adsorbed on dehydroxylated montmorillonite seems to be very similar to that of exchangeable MB⁺on normal clay surfaces.

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