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An important group of phyllosilicates (sheet silicates) with a 2:1 atomic structure (see clay minerals) and including the important minerals muscovite, biotite, and phlogopite; the group is characterized by the silicon oxygen tetrahedral layers of composition [Si4O10]n and a general composition may be written (K,Na)2Y6[Z4O10]2(OH,F)4, where Y = Mg, Fe, Fe3+, or Al, and Z = Si or Al; the micas include the common micas already mentioned, plus glauconite, lepidolite, and zinnwaldite, the brittle micas (see margarite), and also the related minerals talc, stilpnomelane, and pyrophyllite.

Subjects: Earth Sciences and Geography.

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