Abstract
Magnesium-rich clays are distributed around the hydrothermal replacement deposit of gypsum in the Tertairy rocks of the Iwami mine, Shimane Prefecture. Generally speaking, the principal constetuent mineral is a magnesium-rich chlorite, accompanied by other accessory minerals such as montmorillonite, illite, 11 Å mineral, 30 Å mineral, gypsum, quartz, calcite, and pyrite. In the light of X-ray data, the 30 Å mineral can be identified as interstratified mineral of chlorite-montmorillonite, and 11 Å mineral is hydrous complex of mica clay mineral; intex stratified combination of illite and its more hdrated layers. Particularly some specimens have very complicated mineral compositions containing 30 Å mineral, chlorite, and montmorillonite Chemical compositions and differential thermal analysis curves are shown.
