Abstract
On the occasion of the Field Trip of the 20th Meeting of the Clay Science Society. of Japan, we visited Itabashi, Tochigi Prefecture, where a pile of weathered pyroclastics was exposed. It is composed of Shichihonzakura Pumice, Imaichi Pumice, Tawara Volcanic Ash, Takaragi Volcanic Ash and Kanuma Pumice in descending order. The Takaragi Volcanic Ash is composed of halloysite. Although the Kanuma Pumice is composed of allophane, halloysite was formed in its uppermost part along the boundary with the Takaragi Volcanic Ash. This fact seems to support the Kanno's opinion that halloysite is formed by resilication to which silica carried by percolating water is responsible. Allophane formation in the upper part of pyroclastic deposits and halloysite formation in the lower part of them may be coupled chemical reactions; silica released by allophane formation migrates downwards and is used for halloysite formation.
