DATOLITE FROM THE OBIRA MINE, ÔITA PREFECTURE, JAPAN

抄録

Datolite of the Ginjiki deposit of the Obira mine is white to pale greenish white in colour and massive, occurring in association with skarn minerals. Refractive indices are: α=1.6273, β=1.652 ?? 1.655, γ=1.666, and (-)2V is 73°±1.5°. The chemical composition is as follows; SiO₂ 37.64, Al₂O₃ 0.32, FeO 0.22, MnO none, CaO 34.90, MgO 0.60, B₂O₃ 21.31, H₂O(+) 5.66, total 100.65%. Trace of Ge is also contained. When weathered, it alters to a clayey mineral and decreases in the B₂O₃ content. It is paragenetic with quartz, calcite, stilpnomelane and grunerite (?), and includes or replaces skarn minerals of earlier stage. Hence, it must have been formed under hydrothermal condition in a relatively later stage of mineralization.