Abstract
The occurrence and chemical composition of smectite in hyaloclastite from the Toyohama rock fall site of Feb.10, 1996 and the Oshoro Bay area were examined by using optical microscopy, X-ray diffractometry, electron microprobe analysis and analytical transmission electron microscopy. Usually, smectite occurs as fillings and linings of pores in various aspects, and occasionally in replacement of volcanic glass in hyaloclastite. The analytical results show that the smectite is dioctahedral and Fe-rich type. Its composition varies extensively between montmorillonite and nontronite. Moreover, interlayer cations of smectite vary extensively in relation with the type of weathering in hyaloclastite. Na-dominant smectite occurs characteristically in discolored hyaloclastite affected by surficial weathering, whereas Ca-and Ksmectite occurs dominantly in hyaloclastite collected from the inner part of rockmass. These features of the smectite in hyaloclastite can be attributed to its formation and weathering environments.
