Review of Cation Ordering in Micas

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.