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.

lepidolite Lithium-bearing mica with composition K₂(Li,Al)_5–6 [Si_6–7A1_2–1 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.