CHARACTERIZATION OF SOIL CLAY-MINERALS - DECOMPOSITION OF X-RAY-DIFFRACTION DIAGRAMS AND HIGH-RESOLUTION ELECTRON-MICROSCOPY

Fine clays (<0.1 mu m) extracted from an acid soil developed in a gran ite saprolite from the Massif Central, France, were characterized by X -ray diffraction (XRD) using a curve decomposition program, and high-r esolution transmission electron microscopy (HRTEM) associated with a m ethod of impregnation of moist samples. Direct measurement of d-spacin gs were performed on HRTEM photographs. Decomposition of XRD patterns indicated 5 to 6 different clay phases including chlorite (and/or hydr oxy-interlayered vermiculite), vermiculite/smectite, illite/vermiculit e and illite/smectite mixed layers. Expandable phases with decreasing layer charge (vermiculite, high- and low-charge smectite) were shown i n 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 deco mposition of XRD patterns were also found by direct measurement of d-s pacings 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 p hases present in a complex soil clay sample, as well as the direct mea surement of a limited number (50) of clay crystals on HRTEM images.