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, H2KA13(SiO4)3 . As the common white mica obtainable in thin, transparent cleavage sheets of large size it was formerly used in Russia for window panes and known as " Muscovy glass "; hence the name muscovite, proposed by J . D . Dana in r8so . It crystallizes 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 (c in the figure), on which the lustre is pearly in character . The hardness is 2—22f and the spec. gray . 2.8–2.9 . The plane of the optic axes is perpendicular to the plane of symmetry and the acute bisectrix nearly normal to the cleavage; the optic axial angle is 6o-7o°, and double refraction is strong and negative in sign . 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, cyanite, &c.; several varieties depending on differences in structure have been distinguished . Fine scaly varieties are damourite, margarodite (from Gr. napyapfrrls, a pearl), gilbertite, sericite (from vflpLKbs, silky), &c .

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 sericiteschists . Oncosine (from 6yKOVis, intumescence) is a compact variety forming rounded aggregates, which swell up when heated before the blowpipe . Closely related to oncosine are several compact minerals, included together under the name pinite, which have resulted by the alteration of iolite, spodumene and other minerals . Other varieties depend on differences in chemical composition . Fuchsite or " chrome-mica " is a bright green muscovite containing chromium; it has been used as a decorative stone . Oellacherite is a variety containing some barium . In phengite there is more silica than usual, the composition approximating to H2KAI3(Si308)a . 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, the United States and Brazil (see MICA), and to a limited extent, together with felspar, in southern Norway and in the Urals . Large sheets of muscovite were formerly obtained from Solovetsk Island, Archangel . (L .

J .