Study on Kaolin Minerals of the Seto Group in Onada District, Tajimi City, Gifu Prefecture, Japan

The Plio-Pleistocene Seto Group is distributed in the Onada district, Tajimi City, Gifu Prefecture. The Tokiguchi Formation that is the lower part of the Seto Group is rich in clay. The constituent minerals of the sediment are quartz, kaolin mineral and smectite. Some samples contain small amount of mica mineral and lepidochrocite. Moreover, the mineralogical properties of the clay were examined by means of X-ray diffraction, electron microscopy and infrared spectroscopy. In the present investigation, the kaolin mineral in the Tokiguchi Formation is mainly composed of b-axis disordered kaolinite. Two beds with short tubular halloysite are distributed in the upper and lower parts of the Tokiguchi Formation. Relative contents of halloysite in these beds are independent of the lithofacies and sedimentary depth. The shape of the halloysite is similar to that of kaolin in pyroclastic materials in the Obora district. It was proposed that kaolin mineral in this area was derived from weathered granite. On the basis of the occurrence and distribution of kaolin minerals, we concluded that the b-axis disordered kaolinite and the short tubular halloysite were derived from weathered granite and pyroclastic materials, respectively.