Mineralogical studies of a species of stevensite from the Obori mine, Yamagata Prefecture, Japan

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Abstract
A variety of stevensite was collected at the Obori mine, Yamagata Prefecture, Japan. Analysis shows close agreement with published data for stevensite. Chemical composition: SiO2 52·76%; A1203 trace; Fe203 0·38%; MgO 23·67%; MnO 1·57%; CaO 2·19%; Na2O 0·37%; K2O 0·00%; H2O+ 6·52%; H2O− 12·38%; Total 99·84%. The structural formula is (Fe0·04 Mg5·31 Mn0·02) Si80·1 020·00 (OH)4·00·Ca0·36, Na0·10.This specimen shows an interstratified structure of dehydrated and hydrated layers, the former caused by dehydration of the interlayer water, although stevensite essentially contains a non-swelling layer and shows interstratification. The intensities and spacings of the 00l reflections change successively with changes in humidity. When treated with water and glycerol, the 00l reflections are similar to those of montmorillonite. The DTA and TG curves are similar to those of stevensite and an interstratified mineral of talc and saponite.

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