土佐硯（源谷坑）源岩「三原石」の地質学的特徴

抄録

Tosa-suzuri inkstone of Gendani, a traditional craft product of Kochi Prefecture, Japan, has been produced in Mihara Village, Hata District, using the Miharaishi slate that occurs in the Shimanto Belt. The slate-bearing sequence was thought to be Cretaceous in age. The slate is known for its fine texture that is ideal for grinding inksticks, and contains a copper-colored powder called “golden stars” that enhances the beauty of the inkstone. However, the geological characteristics of the slate have received little scientific attention. Based on a literature survey, the slate-bearing sequence is estimated to be middle Eocene-early Miocene in age. X-ray diffraction analyses show that the main minerals in the slate are muscovite and quartz, and X-ray spectroscopy identified pyrite as the main mineral in the golden stars. Microtomographic and electron microscopic observations show step-like microstructures on the surface of the inkstone, particularly at the edges of clay minerals, that may be favorable structures for grinding inksticks. The geological characteristics of the Miharaishi slate are similar to those of the source rocks of high-quality inkstones in other areas of Japan and China.