Hisingerite Occurring as a Weathering Product of Iron-Rich Saponite

Hisingerite occurs as a weathering product of iron-rich saponite in rhyolitic tuff of Oya, Tochigi Prefecture. The chemical composition: SiO 2 , 40·68%; Al 2 O 3 , 4·44%; Fe 2 O 3 , 28·54%; MgO, 1·85%; CaO, 1·42%; Na 2 O, 1·50%; K 2 O, 0·33%; H 2 O + , 6·43%; H 2 O − , 13·70%; total 98·89%. Very broad and weak X-ray powder reflections occur at 16, 4·5, 2·5, 1·7 and 1·5 Å, which correspond to the main peaks of nontronite. It is considered that trioctahedral iron-rich saponite was altered to hisingerite, a very poorly crystalline form of nontronite, by oxidation of iron and advanced leaching of octahedral Mg ions.