This mineral is found in an ahnost pure state, forming a layer 1–2 m. thick
at an average depth of about 50 m. beneath ground level, on the northern
slope of the Monteti near to Capelbio (Orbetello, Tuscany). Above the layer
of micaceous mineral there is developed a series of clays containing
halloysite, metahalloysite, montmorillonite and kaolinite, locally rich in
iron and manganese hydroxides. The series is quaternary, and lies on top of
phyllites which have been attributed to the Permian. The whole series of
clayey and ochreous minerals is of low-temperature hydrothermal origin, as I
have shown in a recent communication (1948).