Composition of an unusual zinnwaldite from southeastern Missouri

Abstract

Major and minor element composition is reported for a Li–Fe mica (1M) occurring in topaz greisen of Precambrian age in southeastern Missouri, U.S.A. The Missouri zinnwaldite is high in MnO (2.44 wt%) and MgO (0.85 wt%) and is characterized by a 0.94 ion octahedral vacancy which places it nearly midway between the trioctahedral and dioctahedral states. The vacancies occur in the large-ion sites of the octahedral sheet and are regarded as the result of both octahedral and tetrahedral substitutions. Plotting of refractive indices against compositional variables confirms the unusual compositional nature of the Missouri zinnwaldite which resembles members of the zinnwaldite–masutomilite series described in Japan.