抄録
The kaolin deposits of the Seta mine, Hokkaido were formed by hydrothermal kaolinization of tuffaceous rocks. The kaolin of the Taisei-1 deposit replacing fine-grained rhyolitic tuff is of the highest quality, composed of about 60 percent kaolinite, 30-40 percent quartz and about 5 percent pyrite. Kaolinite may be classified into two types, A and B, by slight differences in crystallinity ; A is a well crystallized type with a triclinic cell, while B is a more disordered type, probably a “b-axis disordered kaolinite”. Kaolinite in the peripheral and basal parts of the deposit is of type A usualy accompanied by a minor amount of alunite, whereas kaolinite in the central part is mainly of type B. Micas are absent in this deposit, except for a small amount of a mixedlayer mineral of mica-montmorillonite. Mineral assemblages, represented by quartz-kaolinite-alunite in the fine-grained tuff and kaolinite-quartzmontmorillonite in propylite, show that the alteration took place at high activity of H+ relative to activities of metal cations.
