On kaersutite from the Iki island, Nagasaki Prefecture

Abstract

Kaersutite is often found in trachybasalts, trachyandesites, trachytes and alkali rhyolites in the Iki island. Large crystals attainning a length of 3cm. are included in scoria from Takenotsuji and in lava flow from Numazu. The chemical compositions and optical properties of phenocrysts in Numazu and Takenotsuji and of crystals in cognate inclusions in trachyte at the foot of Kakujo-san are given in Table 2. Total iron of these kaersutites is almost same but a remarkable difference is seen in the Fe₂O₃/FeO. Kaersutite from Takenotsuji is poor in FeO and rich in Fe₂O₃ and shows extremely high refractive indices and very strong pleochroism. These facts are considered to be due to remarkable oxidation of kaersutite in the scoria when they were erupted. So the authors intend to propose a name“oxykaersutite”which shows high Fe₂O₃/FeO ratio and refractive indices and strong pleochroism.