Permalink : http://hdl.handle.net/2115/34826

Compositional variation of biotites from the Takakumayama granite, located in the central part of the Ōsumi Peninsula, Kagoshima Prefecture, was observed. The biotites show compositionally a substitution of 3Mg → 2(Al^VI+Fe^+3)+ Vacancy during the formation of the granite. Hydrothermal experiments on two analyzed biotites were carried out with the HM buffer at temperatures up to 750℃ and 1 kb. An alkali feldspar phase was not formed by partial decomposition, but one or two of iron oxides. Comparison of physical and chemical properties between natural and experimentally treated biotites was examined. By the partial decomposition, Al^+3 is hardly depleted from biotite, and Fe^+3 and octahedral vacancies in biotite increase. The results indicate that the substitution will take place in an oxidizing condition during crystallization of a granitic magma.