Mica group

The minerals of the pyrophyllite [(Al2Si40io(OH)2)]-ferripyrophyllite [(Fe 2Si40io(OH)2)] series are related to the mica group. The structure of these minerals closely approximates that of muscovite with the two tetrahedral sheets and octahedrally coordinated intercalated ions, a 2 1 layering (Fig. 2.13) but there is little substitution of Si " by Al Only two-thirds... 

Examples are the mica group (e.g., KMg3(OF[)2Si3A10io) of which biotite, (K(Mg,Fe)3(OH)2Sl3A10io) (Figure 1.55) and talc (Mg3(OFl)2Si40io) are members, and contains a sandwich of two layers with octahedrally coordinated cations between the layers and clay minerals such as kaolin, Al4(OH)8Sl4C3io. 

The mica group has about 30 members but only a few are common. Muscovite, phlogopite, and biotite are important representatives of this group. Muscovite is one of the most common of the micas and occurs in a wide variety of geological environments because of its stability. Crystals measuring 2-3 m across are mined in some locations. Muscovite can vary in chemical composition as a result of atomic substitution (Na for K Mg and F for Al). 

Illite and pyrite were specifically cited in the above discussion to make a point relative to the precision and accuracy with which coal minerals can be quantified by XRD. First, illite is NOT a mineral. Illite is "...a general term for the clay mineral constituents of argillaceous sediments belonging to the mica group"... 

Mica group is a large one, with more than 30 minerals (muscovite, phlogopite, biotite are the most common among them). They are often found in limestones, dolomites, and other magnesium-rich rocks. 

Mica-group minerals 2549 Micro-Tech Lasso 66... 

Whipple ER (1968) Quantitative Mossbauer Spectra and Chemistry of Iron Earth and Atmospheric Science. PhD dissertation, Massachusetts Irrstitute of Technology, Cambridge, 187 p White AF, Yee A (1985) Aqueous oxidation-reduction kinetics associated with coupled electron-cation transfer from iron-containing silicates at 25°C. Geochim Cosmochim Acta 49 1263-1275 Winchell AN (1925) Studies in the mica group. Am J Sci 5lh series 9 309-327... 

Foord EE, Martin RF, Fitzpatrick JJ, Taggart JE Jr, Crock JG (1991) Boromuscovite, a new member of the mica group, from the Little Three mine pegmatite, Ramorra district, San Diego County, California. Am Mineral 76 1998-2002... 

Within the context of a general proposal for all the silicates, Berman (1937) suggested a general formula for the mica group which is still valid today. It was Berman who introduced the crystal-chemical symbols W, X, Y, Z for the different lattice sites, thus changing symbols previously suggested by Bragg (1930). 

Biotite a mineral, member of the mica group, of dark color. A common rock-forming mineral. 

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