Isomorphous substitution micas

Ruotsala et al. (1964) reported a partial analysis of a chamosite which contained 18.7% MgO and only 9.70% FeO. This is enough Mg to fill approximately half the octahedral positions and Fe2+ to fill one-seventh. If this analysis is valid, the composition of the octahedral sheet would be similar to that of Mg-biotites (Fig.24), considerably extending the range of isomorphous substitution. If chamosite behaves as the trioctahedral micas (Foster, 1960), then as octahedral Mg increases at the expense of Fe2+, octahedral Al and tetrahedral Al both tend to decrease and the composition... 

Vermiculite, a 2 1 clay mineral, is also a hydrous mica, with isomorphous substitution in the tetrahedral sheet, resulting in a charge of between 1.2 and 1.9 mol negative charge per unit cell. In this case the... 

High fluoride content in trioctahedral micas impedes release. This is actually a special case and an exception to the first rule. Since F", which can proxy for OH" in mica structures, is not a dipole, it attracts electrostatically regardless of whether the mica is trioctahedral or dioctahedral. As a result, trioctahedral micas in which F isomorphously substitutes for much of the structural OH" release with difficulty. That is, this particular type of trioctahedral mica behaves much like a dioctahedral mica with respect to removal. 

Fluorine occurs exclusively as the fluoride anion, F , in soils, where it complexes strongly with metals such as and Fe ". It is found in structures of hydrous minerals, isomorphously substituting for structural OH . Thus, F can be found in micas, amphiboles, layer silicate clays, apatite (rock phosphate), and numerous other minerals. Because it is associated with clay structures, the natural concentrations of fluorine in fine-textured mineral soils and sedimentary rocks can be high. 

Kitaj ima K, Takusagawa N (1990) Effects of tetrahedral isomorphic substitution on the IR spectra of synthetic fluorine micas. Clay Minerals 25 235-241... 

Guidotti CV, Sassi FP (1998b) Miscellaneous isomorphous substitutions in Na-K white micas A review with special emphasis to metamorphic micas. Rend Fis Acc Lincei 9 57-78 Guidotti CV, Sassi FP, Blencoe JG (1989) Compositional controls on the a and b cell dimensions of 2M, muscovite. EurJ Mineral 1 71-84... 

Like the montmorillonites, the micas can be regarded as derived from talc or pyrophyllite by isomorphous substitution. Imagine that we start with pyrophyllite, i 2Si40io(OH)2, and replace one in four of the Si atoms by A1 this substitution would result in a charged lattice having the formula [Al2(Si8Al)Oio(OH)2] Note that the substituent A1 is written separately because it is in the silica, as distinct from the gibbsite layer. 

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