Accretionary lapilli formed by the eruption of Sakurajima volcano.

Abstract

A mineralogical study was carried out on the accretionary lapilli formed by the eruption of Sakurajima volcano, Kyushu, Japan. Each accretionary lapillus commonly contain crystals of gypsum and empty spheres. The gypsum was formed by reaction between moisture and sulphide adhering to the surface of volcanic ash, and the spheres were bubbles formed in the fine particles of ash. Gypsum crystals were not found in fresh volcanic ash from the volcano, but crystallized after that ash became wet with water. The presence of gypsum crystals and bubbles in the accretionary lapilli suggests that fine particles of volcanic ash and a moist condition are most important for the formation of accretionary lapilli. -K.O.