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.