十和田火山灰層におけるハロイサイトの産状について

抄録

The occurrence of halloysite in Towada Holocene and Pleistocene Tephras was investigated. Formation of halloysite took place in the buried tephras occurring in "Si accumulating zone" where thickness of overburden tephra. deposits were mostly 2 m or greater, and the following forms of halloysite were observed ; spheroidal, elongated spheroidal, twin of spheroidal, chestnut-sheell-like and tubular. Tubular halloysite was dominated in buried A horizon or crack zone of outcrop, whereas spheroidal halloysite was dominated in B, C horizons or noncrack zone of outcrop. The absence of chestnut-shell-like halloysite in Holocene tephras indicate that spheroidal and tubular forms of halloysite were formed concurrently in these tephras. On the other hand, in Pleistocene tephras, the following transformation of halloysite was also suggested ; spheroidal → chestnut-shell-like → tubular. The mean size of spheroidal halloysite in B or C horizon or noncrack zone of outcrop and that of tubular halloysite in all tephras increased with age of tephra. Beside, the mean diameter of spheroidal halloysite in buried A horizon or crack zone of outcrop increased till Takadate tephra, but decreased in Tengutai tephra indicating the depletion of Al supply in this oldest tephra. Results obtained in this study thus indicate that surface weathering of tephra influenced the form and amounts of halloysite formed by geochemical weathering.