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The Kurodake Mine embedded in the Pleistocene biotite andesite and its tuff breccia is situated on the southern slope of Mt. Kurodake (891.1 m) in the central part of Oita Prefecture. Brown and white ...clays of this mine consist mainly of halloysite with small amounts of relic minerals such as biotite, hornblende and magnetite. The halloysite deposit is composed of a broad flat body, 8 to 13 m in thickness, and is covered by tuff breccia. The boundary between the ore body and the tuff breccia is distinct, although some weakly crystallized halloysite also occurs in the tuff breccia. In the thick portion of the ore body we find dark brown clay veins composed of halloysite and biotite, up to 15 cm in width. They are considered as passage-ways of the hydrothermal solution. Halloysite in brown and white clays has 10 A diffraction peak which is expanded to 11 A by ethylene-glycol treatment. The d.t.a. curve of this mineral has two endothermic peaks at about 130°C and 570°C, and one exothermic peak at about 960°C. The chemical analyses of two kinds of ores show that the brown clay has more Fe2 03 than the white clay.show more
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