Nomenclature of the micas

End members and species defiiied with permissible ranges of composition are presented for the true micas, the brittle micas and the interlayer-cation- deficient micas. The determination of the crystallochemical formula for dif ferent available chemical data is outlined, and a system of modifiers and s uffixes is given to allow the expression of unusual chemical substitutions or polytypic stacking arrangements. Tables of mica synonyms, varieties, ill -defined materials and a list of names formerly or erroneously used for mic as are presented. The Mica Subcommittee was appointed by the Commission on New Minerals and Mineral Names ("Commission") of the International Mineralo gical Association (IMA). The definitions and recommendations presented were approved by the Commission.