The constituent minerals of the pale-green vein (ca. 6 m in the vertical direction) in the granitic rock of the Ondo-cho district, Hiroshima Prefecture, as well as those in its surrounding rock, has been examined by X-ray powder diffraction, DTA, optical and electron microscopical observations and EPMA.
The clay vein consists mainly of quartz, montmorillonite and the interstratified mineral of illite/swelling-layer (illite: 0.8-0.1, swelling-layer: 0.2-0.1). The montmorillonite exists dominantly in the upper part and shows a distinct tendency to decrease towards thelower part, the other way the interstratified mineral tend to increase. The interstratified minerals are divided into two types in both chemical composition (Fe
2O
3: 16%, 2.5%) and morphology (the elongated tabular habit, the irregular plate) are comparable with those of mica group. Comparing chemical composition of the interstratified mineral with that of montmorillonite, the amount of Al
2O
3 in the latter is less than that in the former. In the granitic rock surrounding the lower level of the clay vein, plagioclase grains are almost completely replaced by illite, and biotite grains are completely replaced by chlorite, while potash feldsper grains are only scarcely replaced by secondary mineral (Fe-rich minerals) along their grain boundaries.
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