micas

views updated May 08 2018

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

views updated May 08 2018

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.