Brittle mica crystal structure

In this section, we consider and discuss the structural and chemical features of more than 200 micas. Most are true micas (146 trioctahedral and 55 dioctahedral). Brittle-mica crystal-structure refinements number about twenty, of which only three are dioctahedral (Tables 1-4, at the end of the chapter). Of the six simple polytypes first derived by Smith and Yoder (1956) and reported by Bailey (1984a, p. 7), only five (i.e., IM, 1M, 3T, IM2, and 20) have been found and studied by three-dimensional crystal-structure refinements. 

Most of the trioctahedral tme-mica stmctures are M polytypes and a few are 2Mi, 2M2, and 3T polytypes. In dioctahedral micas, the 2Mi sequence dominates, although 3T and M structures have been found. Brittle mica crystal-structure refinements indicate that the IM polytype is generally trioctahedral whereas the 2Mi polytype is dioctahedral. The 10 structure has been found for the trioctahedral brittle mica, anandite (Giuseppetti and Tadini 1972 Filut et al. 1985) and recently was reported for a phlogopite from Kola Peninsula (Ferraris et al. 2000). The greatest number of the reported structures were refined from single-crystal X-ray diffraction data, with only a few obtained from electron and neutron diffraction experiments. 

Clintonite. Clintonite is the trioctahedral brittle mica with ideal composition of Ca(Mg2Al)(SiAl3)Oio(OH)2. This structure violates the Al-avoidance principle of Loew-enstein (1954). It crystallizes in H20-saturated Ca-, Al-rich, Si-poor systems under wide P-T conditions. Clintonite, usually found in metasomatic aureoles of carbonate rocks, is rare in nature because crystallization is limited to environments characterized by both alumina-rich and silica-poor bulk-rock chemistry and very low CO2 and K activities (Bucher-Nurminen 1976 Olesch and Seifert 1976 Kato et al. 1997 Grew et al. 1999). The IM polytype and IMj sequences are the most common forms. The 2Mi form is rare (Akhundov et al. 1961) and no 3T forms have been reported. Many IM crystals are twinned by 120° rotation about the normal to the 001 cleavage. Such twinning causes extra spots on precession photographs that simulate an apparent three-layer periodicity (MacKinney et al. 1988). 

Sokolova GV, Aleksandrova VA, Drits VA, Bairakov W (1979) Crystal structures of two lithium-bearing brittle micas. In Kristallokhimaya 1 Struktumaya Mineralogiya. Krank-Kamenetskii VA (ed) Nauka, Leningrad, p 55-66... 

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... 

Filut MA, Rule AC, Bailey SW (1985) Crystal structure refinement of anandite-2C>r, a barium- and sttlfur-bearing trioctahedral mica. Am Mineral 70 1298-1308 Giuseppetti G, Tadini C (1972) The Crystal Stracture of 20 Brittle Mica Anandite. Tschermaks mineral petrogrMitt 18 169-184... 

There are two classes of micas— true and brittle. True micas contain univalent cations between each pair of layers. Cations are alkali metal ions, such as Na+ or K+. The layers separate in cleavage manner on application of a shearing stress. The layers represent basal planes in the crystal structure, and are flexible and elastic. 

Cleavage of bulk crystals to expose clean, defined lattice planes is possible for brittle substances. Some materials, like mica or highly oriented pyrolytic graphite (HOPG) that exhibit a layered structure, are readily cleaved by just peeling off some layers or using a razor blade. For other materials the so-called double-wedge technique can be used (Fig. 8.8). 

In case of brittle solid materials, especially single crystals with clearly defined layer structure (e.g. mica), it is possible to use the cleavage method developed by Obreimow [44], In this method the crystal is split along the cleavage plane (Fig. 1-23), and the force that has to be applied to cause further development of the crack, Fc, is measured. 

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