Some notes on pisolites from Sanbe Volcano, Shimane Prefecture

Abstract

Pisolites, the products of volcanic activities of Sanbe Volcano, are collected from small exposure near Kasubuchi town, Shimane Prefecture. They are composed chiefly of the fragments of quartz, biotite, feldspar, glassy materials and a few hornblende. Quartz and feldspar, occur as in angular granular form, but biotite as in prismatic one. They are found usually as in well-spherulitic form ranging from 1 to 10mm (the maximum attains 13mm) in diameter, but the diameter of 2.5 to 6.5mm is the most predominant, and the form appears as in more or less oblate spheroidal form. Under the microscope, concentric texture is easy to be found in the specimens, in decreasing the size of grains from the core to the margin; i. e. the core composed of coarse-grained fragments is surrounded densely by the margin layer that composed of fine powdered materials.
Genetically, it is assumed that the pisolites have been formed by coagulation of volcanic ashes around a nucleus or centre of raindrop when it was falling in the air, and the concentric texture due to the change of grain size of ashes is attributed to the change of water content in a growing pisolite.