muscovite An important rock-forming mineral and member of the mica group with composition K₂Al₄[Si₃AlO₁₀]₂(OH,F)₄ (see MICAS); when K is replaced by Na the mineral is called paragonite and is related to the phyllosilicates (sheet silicates) lepidolite and zinnwaldite (both lithium-bearing micas); sp. gr. 2.8–2.9; hardness 2.5–3.0; monoclinic; normally colourless to very pale grey to green or light brown; vitreous to pearly lustre; crystals tabular, hexagonal, and also occur in foliated masses and disseminated flakes; cleavage basal perfect MICAS; widely distributed in igneous alkali granites, in pegmatites, as a secondary mineral resulting from the decomposition of feldspars, and can be a major constituent of clastic sediments (e.g. sandstone and silts).

Mus·co·vite / ˈməskəˌvīt/ • n. a native or citizen of Moscow. ∎ archaic a Russian.• adj. of or relating to Moscow. ∎ archaic of or relating to Russia.

mus·co·vite / ˈməskəˌvīt/ • n. a silver-gray form of mica occurring in many igneous and metamorphic rocks.