True mica

Bailey S. W. (1984b). Crystal chemistry of true micas. In Reviews in Mineralogy, vol. 13, P. H. 

Fig. 2.12E. From Bailey, S. W. (1984). Crystal Chemistry of the True Micas. Figs. 3 and 4, p. 15. In Micas, S. W. Bailey, ed. Reviews in Mineralogy 13, Min. Soc. America, Washington, D.C. With permission of Min. Soc. America and the author.

There is, therefore, a difference between the true mica minerals... 

The symbol signifies a vacancy A indicates an unoccupied A-site. The thicknesses given for the case of the occupied A-site are for the true micas. 

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. 

In true micas, the tetrahedral mean bond distance varies from 1.57(1) A in boromuscovite-2Mi (Liang et al. 1995 Table 4) to 1.750(2) A in an ordered (A1 vs. Si) ephesite-2Mi (Slade et al. 1987 Table Id) in brittle micas, the (T-0> mean bond distance varies from 1.620(2) to 1.799(2) A, both values are from anandite-20 (Filut et al. 1985 Table 3a). 

Preiswerkite. Preiswerkite, a trioctahedral true mica with the ideal formula Na(Mg2Al)(Si2Al2)Oio(OH)2 was described by Keusen and Peters (1980). The sample occurs in a metarodingite from the Geisspfald ultramafic complex, Swiss Penninic Alps. Other occurrences include (i) Allalin gabbro, Zermatt-Saas zone, Switzerland (Meyer... 

In some naturally occurring true micas. Si nearly fills all the tetrahedral sites (e.g., polylithionite, tainiolite, norrishite, and celadonite), whereas in the most common mica species (i.e., muscovite and phlogopite) Al substitutes for Si in a ratio near 1 3. In some true micas and brittle micas, the Al for Si substitution corresponds to a ratio of Al Si = 1 1 (e.g., ephesite, preiswerkite, siderophyllite, margarite, and kinoshitalite), whereas the... 

The displacement obtained was used to isolate the x value from the influence of the divalent interlayer cation. The x values of tetrahedrally disordered brittle micas which was thus isolated (i.e., x ) follow the same trend defined for true micas, confirming the influence of interlayer cations on x (Fig. 4). 

In addition, i reflects an adjustment for the misfit between the tetrahedral sheet and the octahedral sheet (the regression coefficient, r, of x vs. the difference between mean basal tetrahedral edges and mean octahedral triads is r = 0.92). Furthermore, as the mean <0-0> basal distance decreases, the tetrahedral cation moves away from the basal oxygen-atom plane. Thus, x increases in value (Fig. 7). The deviation of the parameters for clintonite and kinoshitalite from the trend for true micas further suggests that there is a significant influence of the interlayer cation on the value of x. 

In conclusion (i) x increases as the distance between the tetrahedral cation and the basal oxygen-atom plane increases from its ideal value (ii) x increases as <0-0>basai decreases, thus reflecting a dimensional a ustment between the tetrahedral sheet and octahedral sheet and (iii) x increases with Si content. Differences between x values of brittle micas from the true micas are related in part to electrostatic features. It is useful to understand why the tetrahedral cation moves from its ideal position. Drits (1969) stated that the position of the tetrahedral cation depends not only on the degree of substitution of Si by A1 in the tetrahedra (Brown and Bailey 1963), but also in the position and distribution in compensating positive charges. This assumption is related to electrostatic forces in the following way (see Appendix I for derivation) ... 

In K-rich trioctahedral micas, both a and interlayer separation increase from norrishite to tetra-ferriphlogopite (and aluminian phlogopite) toward values for Fe-rich polylithionite, Fe-rich phlogopite, Mg-rich annite, and phlogopite. Annite deviates from the trend of trioctahedral true micas owing to a larger interlayer separation. In the Ba-rich brittle mica, ferrokinoshitalite (M sites mainly occupied by Fe ), a- and interlayer-separation values are smaller with respect to those of kinoshitalite (M sites mostly occupied by Mg). With respect to trioctahedral micas, the interlayer separation in both muscovite and celadonitic muscovite is smaller, but a values are similar. To explain this behavior, the octahedral, tetrahedral, and 0(4) site chemistry must be considered. 

TABLE 1. STRUCTURAL DETAILS OF TRIOCTAHEDRAL TRUE MICAS... 

Table la. Structural details of trioctahedral true Micas-IM, space group dim... 

Bailey SW (1975) Cation ordering and psendosymmetiy in layer sihcates. Am Mineral 60 175-187 Bailey SW (1984a) Classification and structures of the micas. Rev Mineral 13 1-12 Bailey SW (1984b) Crystal chemistry of the true micas. Rev Mineral 13 13-60 Bailey SW (1984c) Review of cation ordering in micas. Clays Clay Minerals 32 81-92 Bailey SW (1986) Report of the AlPEA Nomenclature Committee (llhte, Glauconite and Volkonskoite). AIPEA Newsl 22 1-3... 

Bailey SW (1987) Crystal chemistry of the true micas. Rev Mineral 13 13-60... 

TRUE MICAS BRITTLE MICAS INTERLAYER-DEFICIENT MICAS ... 

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