|
BIOTITE , an important See also:rock-forming See also:mineral belonging to the See also:group of micas (q.v.). The name was given by J. F. L. See also:Hausmann in 1847 in See also:honour of the See also:French physicist, J. B. See also:Biot, who in 1816 found the See also:magnesia-micas to be optically uniaxial or nearly so. The magnesia-micas are now referred to the See also:species biotite and See also:phlogopite, which differ in that the former contains a considerable but widely varying amount of hon. Biotite is an orthosilicate of See also:aluminium, See also:magnesium, ferrous and ferric See also:iron, See also:potassium and basic See also:hydrogen, with small amounts of See also:calcium, See also:sodium, See also:lithium, See also:fluorine, See also:titanium, &c., and ranges in See also:composition between (H,K)z(Mg,Fe)4(Al,Fe)z(SiO4)4 and (H,K) z(Mg,Fe) zAlz(SiO4) 3.
Like the other micas, it is See also:monoclinic with pseudo-hexagonal symmetry (See also:figs. 1, 2) and possesses a perfect cleavage in one direction (c). Biotite is, however, readily distinguished by its darker See also:colour, strong pleochroism, and small optic axial See also:angle.
The colour is usually dark-See also:green or See also:- BROWN
- BROWN, CHARLES BROCKDEN (1771-181o)
- BROWN, FORD MADOX (1821-1893)
- BROWN, FRANCIS (1849- )
- BROWN, GEORGE (1818-188o)
- BROWN, HENRY KIRKE (1814-1886)
- BROWN, JACOB (1775–1828)
- BROWN, JOHN (1715–1766)
- BROWN, JOHN (1722-1787)
- BROWN, JOHN (1735–1788)
- BROWN, JOHN (1784–1858)
- BROWN, JOHN (1800-1859)
- BROWN, JOHN (1810—1882)
- BROWN, JOHN GEORGE (1831— )
- BROWN, ROBERT (1773-1858)
- BROWN, SAMUEL MORISON (1817—1856)
- BROWN, SIR GEORGE (1790-1865)
- BROWN, SIR JOHN (1816-1896)
- BROWN, SIR WILLIAM, BART
- BROWN, THOMAS (1663-1704)
- BROWN, THOMAS (1778-1820)
- BROWN, THOMAS EDWARD (1830-1897)
- BROWN, WILLIAM LAURENCE (1755–1830)
brown; thick crystals are often deep-See also:black and opaque. The absorption of See also:light-rays vibrating parallel to the cleavage is much greater than of rays vibrating in a direction perpendicular thereto, and in dark-coloured crystals the former are almost completely absorbed. The angle between the optic axes is usually very small, the crystals being often practically uniaxial; an axial angle of 500 has, however, been recorded in a dark-coloured biotite. The specific gravity of biotite is, as a See also:rule, higher than that of other micas, varying from 2.7 to 3•1 according to the amount of iron See also:present. The hardness is 22 to 3.
Several varieties of biotite are distinguished. By G. Tschermak it is divided into two classes, meroxene and anomite; in the former the See also:plane of the optic See also:axis coincides with the plane of symmetry, whilst in the latter it is perpendicular thereto. Meroxene includes nearly all See also:ordinary biotite, and is the name given by A. Breithaupt in 1841 to the Vesuvian crystals; on the other See also:hand, anomite (named from bop-or, "contrary to See also:law ") is of rare occurrence. Haughtonite and siderophyllite are black varieties See also:rich in ferrous iron, and lepidomelane (from ?ten-is, a See also:scale, and pEXas, black) is a variety rich in ferric iron. In barytobiotite and manganophyllite the magnesia is partly re-placed by baryta and manganous See also:oxide respectively. Rubellane, hydrobiotite, pseudobiotite, and others are altered forms of biotite, which is a mineral particularly liable to decomposition with the See also:production of chlorites and vermiculites.
Biotite is a See also:common constituent of igneous and crystalline rocks: in See also:granite, See also:gneiss and See also:mica-schist it is often associated with See also:muscovite (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), the two kinds having sometimes grown in parallel position. In volcanic rocks, and in nearly allother kinds of igneous rocks with the exception of granite, biotite occurs to the exclusion of the muscovite. In the dyke-rocks known as mica-traps or mica-See also:lamprophyres biotite is especially abundant. It is also one of the most characteristic products of contact-See also:metamorphism, being See also:developed in sedimentary and other rocks at their contact with granite masses. In the ejected blocks of crystalline See also:limestone of See also:Monte Somma, See also:Vesuvius, the most perfectly developed crystals of biotite (figs. I, 2), or indeed of any of the micas, are found in abundance, associated with brilliant crystals of See also:augite, See also:olivine, See also:humite, &c.
Although biotite (black mica) is much more common and widely distributed than white mica, yet it is of far less economic importance. The small See also:size of the sheets, their dark colour and want of transparency render the material of little value. Large, cleavable masses yielding See also:fine smoky-black and green sheets, sufficiently elastic for See also:industrial purposes, are, however, found in See also:Renfrew See also:county, See also:Ontario. (L. J.
End of Article: BIOTITE
Additional information and Comments
There are no comments yet for this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML.
Site content, images, and layout Copyright © 2006 - Net Industries, worldwide.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.
Links to articles and home page are always encouraged.
|
del.icio.us it!