## Review of Cation Ordering in Micas

S. W. Bailey

Department of Geology and Geophysics, University of Wisconsin—Madison, Madison, Wisconsin 53706

*Note added in proof:*
Sokolova, C. V., Aleksandrova, V. A., Drits, V. A., and Vairakov, V. V. (1979) Crystal structure of two lithian brittle micas: in *Crystal Chemistry and Structures of Minerals,* Nauka, Moscow, 55–66 (in Russian).
The above reference, which has just come to the author's attention, describes two additional mica structures that are ordered in subgroup symmetry. For ephesite-1*M*: R = 11.5% in *C*2, 284 refl. Mean tet. bonds for Si_{2.2}Al_{1.8}: T(1) = 1.609 Å, T(2) = 1.764 Å, Δ = 11.0σ_{Δ}. Mean oct. bonds for Al_{1.97}Fe_{0.02}Li_{0.67}Na_{0.10}: M(1) = 2.128 Å, M(2) = M(3) = 1.927 Å. For bityite-2*M*_{1}: R = 11.5% in *Cc*, 450 refl. Mean tet. bonds for Si_{2.00}Al_{1.29}Be_{0.71}: T(1) = 1.642 Å, T(2) = 1.710 Å, T(11) = 1.717 Å, T(22) = 1.622 Å, ave. Δ = 5.7σ_{Δ}. Mean oct. bonds for Al_{2.00}Li_{0.48}Mg_{0.10}Fe_{0.03}: M(1) = 2.184 Å, M(2) ≅ M(3) = 1.898 Å.

**Abstract:** Long-range ordering of tetrahedral cations in micas is favored by phengitic compositions, by the 3*T* stacking sequence of layers, and by tetrahedral Si:Al ratios near 1:1. Phengites of the 1*M*, 2*M*_{1}, and 2*M*_{2} polytypes are said to show partial ordering of tetrahedral cations, although the amounts of tetrahedral substitutions are small and the accuracies of determination are not as large as desired. The 3*T* structures of muscovite, paragonite, lepidolite, and protolithionite show tetrahedral ordering, as do the 2*M*_{1} brittle micas margarite and an intermediate between margarite and bityite. Muscovite-3*T* and margarite-2*M*_{1} are also slightly phengitic relative to their ideal compositions. Examples of octahedral cation ordering in micas are more abundant and are to be expected when cations of different size and charge are present. Octahedron M(1) with its OH,F groups in the *trans* orientation tends to be larger than the mean of the two *cis* octahedra as a result of the ordering of cations and vacancies. In some samples ordering has reduced the true symmetry to a subgroup of that of the ideal space group. If ordering in subgroup symmetry results in ordered patterns of different geometries but similar energies in very small domains, the average over all unit cells may simulate long-range disorder.