Chemical formula of biotite calculated based on EPMA-WDS: Crystal chemistry and genetic significance

Abstract

A formula of biotite has been commonly constructed by the procedure described by Foster(1960) and Rimsaite(1970). However, these procedures are not approved considered for H₂O content and iron redox state which have never been measured by EMPA. The present work focuses on the means to determine chemical formulas of biotite in detail by using only EMPA data, with reasonable regard to H₂O contents and iron redox state, and discusses the crystal chemistry and the genetic significance. The six samples of biotite are used. The present procedure reveals that all biotite formulae have more vacancies at I site and M site than those usual procedure. It is proved by their derived formulae that there is a coupled substitution including vacancies in the examined biotites. Moreover, it is obvious from the formulae that biotites from the Yamanoo contain more Fe³⁺ contents than those from the Tanakami. Since Fe²⁺/Fe³⁺ ratios of biotites increase in granites of magnetite-series, it is suggested that the granitic rock from Yamanoo may belong to a magnetite-series.