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End-members and species defined with permissible ranges of composition are presented for the true micas, the brittle micas, and the interlayer-deficient micas. 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 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 micas are presented. The Mica Subcommittee was appointed by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. The definitions and recommendations presented were approved by the Commission.

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The 13 adiabatic elastic stiffness moduli of natural muscovite have been measured using Brillouin scattering. Following the standard Institute of Radio Engineers convention, these constants are (in gigapascals) C11 = 181.0 ± 1.2, C22 = 178.4 ± 1.3, C33 = 58.6 ± 0.6, C44 = 16.5 ± 0.6, C55 = 19.5 ± 0.5, C66 = 72.0 ± 0.7, C23 = 21.2 ± 1.8, C13 = 25.6 ± 1.5, C12 ± 48.8 ± 2.5, C15 = −14.2 ± 0.8, C25 = 1.1 ± 3.7, C35 = 1.0 ± 0.6, and C46 = −5.2 ± 0.9. The main observed features are the very great acoustic anisotropy due to the weak interlayer bonding and the small, but not insignificant, deviation from hexagonal symmetry (monoclinicity) in the velocity patterns.