Chemical and mineralogical studies of volcanic ashes

The relationships between rock types and mineralogical properties of volcanic ashes were investigated using many volcanic ash samples from different volcanoes.

The heavy mineral fraction was dominated by two pyroxenes and ferromagnetic minerals In almost all the felsic and Intermediate volcanic ashes. Therefore, It was difficult to determine the rock types of felsic and intermediate volcanic ashes according to their heavy mineral composition. It was assumed that the presence of a large quantity of hornblende was related to the origins of the volcanic ashes. On the other hand, the predominant mineral in the heavy mineral fraction of mafic volcanic ashes was volcanic glass (refractive index about 1.59), followed by olivine.

The refractive indices (γ) of plagioclases and orthopyroxenes were 1.553–1,570 and 1.703–1.715 in the felsic volcanic ashes, 1.558–1.580 and 1.705–1,715 in the intermediate ashes, and 1.565–1.576 and 1.697–1.705 in the mafic ashes, respectively. These results indicate the difficulty of determining the rock types of volcanic ashes according to the optical properties of these minerals.