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MUSCOVITE , a See also:rock-forming See also:mineral belonging to the See also:mica See also:group (see Mica). It is also known as potash-mica, being a See also:potassium, See also:hydrogen and See also:aluminium orthosilicate, H2KA13(SiO4)3.
As the See also:common See also:- WHITE
- WHITE, ANDREW DICKSON (1832– )
- WHITE, GILBERT (1720–1793)
- WHITE, HENRY KIRKE (1785-1806)
- WHITE, HUGH LAWSON (1773-1840)
- WHITE, JOSEPH BLANCO (1775-1841)
- WHITE, RICHARD GRANT (1822-1885)
- WHITE, ROBERT (1645-1704)
- WHITE, SIR GEORGE STUART (1835– )
- WHITE, SIR THOMAS (1492-1567)
- WHITE, SIR WILLIAM ARTHUR (1824--1891)
- WHITE, SIR WILLIAM HENRY (1845– )
- WHITE, THOMAS (1628-1698)
- WHITE, THOMAS (c. 1550-1624)
white mica obtainable in thin, transparent cleavage sheets of large See also:size it was formerly used in See also:Russia for window panes and known as " Muscovy See also:glass "; hence the name muscovite, proposed by J. D. See also:Dana in r8so. It crystallizes in the See also:monoclinic See also:system; distinctly See also:developed crystals, however, are rare and have the See also: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 See also:plane (c in the figure), on which the lustre is pearly in See also:character. The hardness is 2—22f and the spec. See also:- GRAY
- GRAY (or GREY), WALTER DE (d. 1255)
- GRAY, ASA (1810-1888)
- GRAY, DAVID (1838-1861)
- GRAY, ELISHA (1835-1901)
- GRAY, HENRY PETERS (1819-18/7)
- GRAY, HORACE (1828–1902)
- GRAY, JOHN DE (d. 1214)
- GRAY, JOHN EDWARD (1800–1875)
- GRAY, PATRICK GRAY, 6TH BARON (d. 1612)
- GRAY, ROBERT (1809-1872)
- GRAY, SIR THOMAS (d. c. 1369)
- GRAY, THOMAS (1716-1771)
gray. 2.8–2.9. The plane of the optic axes is perpendicular to the plane of symmetry and the acute See also:bisectrix nearly normal to the cleavage; the optic axial See also:angle is 6o-7o°, and See also:double See also:refraction is strong and negative in sign.
Muscovite frequently occurs as See also:fine scaly to almost compact aggregates, especially when, as is often the See also:case, it has resulted by the alteration of some other mineral, such as See also:felspar, See also:topaz, See also:cyanite, &c.; several varieties depending on See also:differences in structure have been distinguished. Fine scaly varieties are damourite, margarodite (from Gr. napyapfrrls, a See also:pearl), gilbertite, sericite (from vflpLKbs, silky), &c. In sericite the fine scales are See also: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 See also:blowpipe. Closely related to oncosine are several compact minerals, included together under the name pinite, which have resulted by the alteration of See also:iolite, See also:spodumene and other minerals. Other varieties depend on differences in chemical See also:composition. Fuchsite or " chrome-mica " is a See also:bright See also:green muscovite containing See also:chromium; it has been used as a decorative See also:- STONE
- STONE (0. Eng. shin; the word is common to Teutonic languages, cf. Ger. Stein, Du. steen, Dan. and Swed. sten; the root is also seen in Gr. aria, pebble)
- STONE, CHARLES POMEROY (1824-1887)
- STONE, EDWARD JAMES (1831-1897)
- STONE, FRANK (1800-1859)
- STONE, GEORGE (1708—1764)
- STONE, LUCY [BLACKWELL] (1818-1893)
- STONE, MARCUS (184o— )
- STONE, NICHOLAS (1586-1647)
stone. Oellacherite is a variety containing some See also:barium. In phengite there is more See also:silica than usual, the composition approximating to H2KAI3(Si308)a.
Muscovite is of wide See also:distribution and is the commonest of the micas. In igneous rocks it is found only in See also:granite, never in volcanic rocks; but it is abundant in See also:gneiss and mica-schist, and in phyllites and See also:clay-slates, where it has been formed at the expense of See also:alkali-felspar by See also:dynamo-metamorphic processes. In See also:pegmatite-See also:veins traversing granite, gneiss or mica-schist it occurs as large sheets of commercial value, and is See also:mined in See also:India, the United States and See also:Brazil (see MICA), and to a limited extent, together with felspar, in See also:southern See also:Norway and in the Urals. Large sheets of muscovite were formerly obtained from Solovetsk See also:Island, See also:Archangel. (L. J.
End of Article: MUSCOVITE
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