ABSTRACT

Metallic element contents of biotite collected along the Nishiki river, Southwest Japan, were investigated by means of atomic absorption spectrophotometry and the results obtained were examined from mineralogical and statistical view points. Biotite in the district is commonly altered to vermiculite and to lesser extent to vermiculite-chlorite interstratified mineral and kaolinite. The main variations of the chemical composition are mostly caused by decomposition and the vermiculitization of biotite. The content of the representative interlayer cation, K, depends on that of biotite in the parent granite. Rubidium behaves together with K, so that the Rb fraction is almost constant in the interlayer cations (K, Na, Ca and Rb), despite its low content (below 550ppm) of biotite. Among the possible octahedral cations, substitution of Fe by Mg is remarkable. Contents of minor elements, such as Cu, Zn and Pb, vary locally but no geological evidences for explaining the local enrichments of these elements can be found. Correlation coefficients of Fe and Zn against K suggest that a part of the two elements are contained in the interlayer position.