MUSCOVITE, a rock-forming mineral belonging to the mica group (see MICA). It is also known as potash-mica, being a potassium, hydrogen and aluminium orthosilicate, As the common white mica obtainable in thin, transparent cleav age sheets of large size it was formerly used in Russia for window panes and known as "Muscovy glass"; hence its name. It crys tallizes in the monoclinic system; distinctly developed crystals, however, are rare and have the form of rough six-sided prisms or plates; thin scales without definite crystal outlines are more common. The most prominent feature is the perfect cleavage parallel to the basal plane, on which the lustre is pearly in char acter. The hardness is 2-21, and the specific gravity 2.8-2.9.
Muscovite frequently occurs as fine scaly to almost compact aggregates, especially when, as is often the case, it has resulted by the alteration of some other mineral, such as felspar, topaz, kyanite, etc. Varieties depending both on differences in structure and in chemical composition have been distinguished. Scaly
varieties are damourite, gilbertite, sericite (Gr. anpucbs, silky), etc. In sericite the fine scales are united in fibrous aggregates giving rise to a silky lustre: this variety is a common constituent of phyllites and sericite-schists. Several compact minerals, in cluded together under the name pinite, have resulted by the alteration of cordierite, spodumene and other minerals. Fuchsite )1- "chrome-mica," is a bright green variety containing chromium.
Muscovite is of wide distribution and is the commonest of the micas. In igneous rocks it is found only in granite, never in volcanic rocks; but it is abundant in gneiss and mica-schist, and in phyllites and clay-slates, where it has been formed at the expense of alkali-felspar by dynamo-metamorphic processes. In pegmatite-veins traversing granite, gneiss or mica-schist it occurs as large sheets of commercial value, and is mined in India, East Africa, the United States and Brazil.