Nomenclature of the micas

The Mica Subcommittee was appointed by the Commission on New Minerals and M ineral Names of the International Mineralogical Association. The definition s and recommendations presented were approved by the Commission. The report discusses mica definition, subdivisions, principles of classific ation, end-member formulae, modifiers and suffixes, series names for incomp letely investigated materials, and invalid names. The determination of the crystallochemical formula for different available chemical data is outlined , and a system of modifiers and suffixes is given to allow the expression o f unusual chemical substitutions or polytypic stacking arrangements. Formulas and permissible ranges of composition are given for the following valid true, brittle, and interlayer-cation-deficient micas: [GRAPHICS] Tables of mica synonyms and varieties, ill-defined materials and mixtures, and a list of names formerly or erroneously used for micas are presented.