Abstract
Phenocrysts of pigeonite were found in a pigeonite hypersthene andesite of a somma lava of Usu volcano in the roof mountain part of Showa-Shinzan. The phenocrystic minerals of this lava are bytownite, hypersthene, pigeonite and a little augite surrounded by pigeonite, while groundmass consists of andesine_??_labradorite, pigeonite, magnetite, brown glass and apatite. The occurrence of phenocrystic pigeonite can be divided into the following three types: (1) single crystal, (2) intergrowth with augite (thick rims around augite), (3) inversion from hypersthene. The chemical composition of those pigeonite, inferred from the optical properties, are plotted in Fig. 6. The mode of occurrence and Fig. 6 show that pigeonite phenocrysts are crystallized only in the latest intratelluric stage.
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