Muscovite polytypes

Amouric M, Baronnet A (1983) Effect of early nucleation conditions on synthetic muscovite polytypism as seen by high resolution transmission electron microcopy. Phys Chem Minerals 9 146-159... 

Liang, J.-J, Hawthorne, EC, Novdk, M., Cern), F, Ottolini, L. (1995) Crystal-structure refinement of boro-muscovite polytypes using a coupled Rietveld-static-structure energy-minimization method. Can. Mineral.,33, 859-865. 

Polytypism in layered silicates has been well characterized. There are as many as 19 polytypes known among micas. The repeat unit in muscovite mica, [KAl2(OH)2(Si3Al)0 o], for instance, consists of a sheet of octahedrally coordinated aluminium ions sandwiched between two identical sheets of (Si, AOO tetrahedra, the large ions being located in interlayer positions. Surface oxygens of the tetrahedra in... 

Yoder and Eugster (1955) and Velde (1965) have established by hydrothermal experiments that the 2M polytype is the stable form of muscovite and that the lMd polytype is metastable. Velde (1965) found that the 2M polytype is stable at temperatures as low as 125°C and suggested that the sequence of polytype transformation is lMd- TM->-2M with an increase of either time, temperature, or pressure. He concluded ... 

Takeda H, Haga N, Sadanaga R (1971) Stractmal investigation of a polymorphic transition between 2M2-, lA/-lepidohte and 2Mi-muscovite. Mineral J 6 203-215 Takeda H, Ross M (1975) Mica polytypism Dissimilarities in the crystal structures of coexisting IM and 2M biotite. Am Mineral 60 1030-1040 Takeuchi Y (1965) Stractrrres of brittle micas. Clays Clay Minerals 13 1-25... 

The partial occupancy of the Ml site by divalent cations, and a greater Si content in the tetrahedral sheet, should decrease the compressibility. Therefore the presence of a phengitic component probably increases the mica stiffness, as shown by the increased bulk modulus of phengite-SZ in comparison with that of end-member muscovite (Pavese et al. 1999b). The difference found, however, could be partially ascribed to the different mica polytype. A statistical study on the occurrence of metamorphic phengites by Sassi et al. (1994) seems to show that the crystallization of the 3T polytype is mainly favored by high PIT ratio conditions. 

Mauguin MCh (1927) Etude du mica muscovite au moyen des rayons X. CR Aead Sci Paris 185 288-291 MauguinMCh (1928) Etude de Micas au moyen du rayons X. Bull Soc frang Miner Crist 51 285-332 McLaman TJ (1981) The number of polytypes in sheet silicates. Z Kristallogr 155 247-268 Merlino S (1990) OD stractmes in mineralogy. Per Mineral 59 69-92... 

Thirty years passed before another structural determination of a mica was made by Radoslovich (I960), a refinement of the structure of muscovite, in particular 2Mj polytype. The purpose was to clarify some points left open by Jackson and West (1933) a - the presence of reflections forbidden the space group C2/c, b - a monoclinic angle different from the ideal of 13 = cos i(-a/3c), c - the non-correspondence between the dimensions of the tetrahedral sheet and the octahedral sheet, d - the too-large K-0 distance, e - the order or disorder in the tetrahedral sites, f - an explanation of the frequency of the polymorphs of the micas. 

Accordingly, the disordered one-layer structure (IMd) is generally assumed to indicate authigenic growth, whereas the 2M polytype can be taken as evidence for the detrital origin of muscovite. Consequently, the lMd/2M ratio in a sediment has been proposed as a criterion for the degree of metamorphosis (Reynolds [1963]). 

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