Abstract
Accretionary lapilli were formed by the eruption of Sakurajima volcano on the 22nd of May, 1983. The accretionary lapilli have been studied mineralogically. Each accretionary lapillus contains commonly crystals of gypsum and spherical vacant spaces. The gypsum was formed due to the reaction of moisture and sulfide which adhered to the surface of volcanic ash, and the spherical vacant spaces were bubbles formed in the fine particles of ash. Gypsum crystals were not found in the fresh volcanic ash from Sakurajima, but gypsum crystallized after the fresh volcanic ash got wet with water. Presence of gypsum crystals and bubbles in the accretionary lapilli suggests that fine particles of volcanic ash and moist condition are the most basic conditions amongst many conditions for the formation of accretionary lapilli.
