The Kumogi granite located in Kanagi-cho, Shimane Prefecture intruded along the Hamada cauldron wall in the late stage of Paleogene. Some parts of Kumogi granite are overlain by the Tsunodu formation. Several kinds of alteration phenomenon are recognized in the granite body, such as white clay veins, black manganese veins and pinkish and whife color altered parts. The mineralogical and chemical characterization of clay minerals formed in these veins and alteration parts of granite were analyzed by means of X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM) with EDX and Electron probe X-ray micro analyzer (EPMA). As results, tubular halloysite is mainly formed in altered granites and clay veins. In white and black veins, illite is recognized as associated mineral. Smectite is main constituent mineral with halloysite in the pinkish color altered parts. The black veins are constituted mainly of Mn (10-65wt% as oxide).
Tubular halloysite crystals are generally formed by both weathering and hydrothermal process. According to Kitagawa et al.(1984), the tube width of hydrothermal halloysite is clearly narrower than that formed by weathering. The surface microtopographies of tubular halloysite crystals are different between weathering and hydrothermal products respectivelly.
It is noted from these results that the Kumogi granite was altered by both hydrothermal and weathering.