MINERAL AND CHEMICAL COMPOSITIONS OF THE SECONDARY SHIRASU

Abstract

The so-called “Secondary Shirasu (lacustrine or marine sediments of pyroclastics)” is distributed in separate forms and in small scale, and occurs in association with the socalled “Shirasu or Primary Shirasu (pyroclastic flow deposits)” which is widely distributed over southern Kyushu. Mineral and chemical compositions, and optics of constitutent minerals of the Secondary Shirasu, which was derived from the Aira Shirasu, have been studied, and are compared with those of the Primary Shirasu, i. e. the Aira Shirasu which came from the Aira volcanic caldera. Mineral and chemical compositions show that the Secondary Shirasu is characterized by lower content of mafic minerals and by higher silica and lower ferrous iron oxide and magnesia contents than the Primary Shirasu. The differences in chemical composition are consistent with those in mineral composition between the Secondary and Primary Shirasu. The nature of the Secondary Shirasu is reffected in these differences.