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micas 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.

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lepidolite Lithium-bearing mica with composition K2(Li,Al)5–6 [Si6–7A12–1 O20] (OH,F)4; Sp. gr. 2.8–2.9; hardness 2.5–4.0; monoclinic; crystals as small flakes; perfect basal hardness cleavage; lilac to greyish; occurs in late-stage pegmatites associated with other lithium minerals, such as spodumene, and minerals of a pneumatolytic origin, such as tourmaline and topaz. Lepidolite is an important ore mineral of lithium.