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MICA

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Originally appearing in Volume V18, Page 355 of the 1911 Encyclopedia Britannica.
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MICA , a See also:

group of widely distributed See also:rock-forming minerals, some of which have important commercial applications. The See also:principal members of the group are See also:muscovite, See also:biotite, See also:phlogopite and See also:lepidolite (q.v.). The name mica is probably derived from the Latin micare, to shine, to glitter; the See also:German word glimmer has the same meaning. The See also:mineral was probably included with selenite under See also:Pliny's See also:term lapis specularis. Mineralogical Characters.—The micas are characterized by a very easy cleavage in a single direction and by the high degree of flexibility, See also:elasticity and toughness of the extremely thin cleavage flakes. They all crystallize in the See also:monoclinic See also:system, often, however, in forms closely resembling those of the See also:rhombohedral or orthorhombic systems. Crystals have usually the See also:form of hexagonal or rhomb-shaped scales, plates or prisms, with See also:plane angles of 60° and 120°, and, with the exception of the basal planes, are only rarely bounded by smooth and well-defined faces. The crystal represented in fig. r is bounded by the basal pinacoid c (oot) parallel to which is the perfect cleavage, the clinopinacoid b ((a10) parallel to the plane of symmetry, and the pyramids m (221) and o (112). The angles between these pyramids and the basal plane are 851° and 73° respectively. The See also:prism (See also:Ito) at 90° from the basal plane is not See also:developed as a crystal See also:face, but is a plane of twinning, the two individuals of the twin being See also:united parallel to the basal plane (fig. 2). The different See also:species of mica have very nearly the same forms and interfacial angles, and they not infrequently occur intergrown together in parallel position.

The best developed crystals are those of Vesuvian biotite. When a cleavage flake of mica is struck a See also:

sharp See also:blow with a See also:blunt See also:needle-point a six-rayed See also:star of cracks or " percussion figure " is developed: the rays intersect at angles of approximately 60°, and the pair most prominently developed are parallel to the plane of symmetry of the crystal. A similar six-rayed system of cracks, bisecting the angles between the rays of the previous set, is produced when a blunt See also:punch is gradually pressed against a See also:sheet of mica ; this is known as the " pressure figure." These cracks coincide with planes of easy separation or of gliding in the crystal; they are especially useful in helping to determine the crystallographic See also:orientation of a cleavage flake of mica when crystal faces are absent. Sheets of mica which have been subjected to See also:earth-movements are frequently cracked and ridged parallel to these directions, and are then valueless for economic purposes. In their See also:optical characters the micas exhibit considerable See also:variations. The indices of See also:refraction are not high, the mean See also:index being about 1.58–i•6o, but the See also:double refraction is very strong (0.04–0.05) and is negative in sign. The See also:angle between the optic axes varies from 70°–50° in muscovite and lepidolite to to–o° in biotite and phlogopite; the latter are thus frequently practically uniaxial. The acute See also:bisectrix of the optic axes never deviates from the normal to the basal plane by more than a degree or two, hence a cleavage flake of mica will always show an optic figure in convergent See also:light when placed on the See also:stage of a polarizing See also:microscope. The plane of the optic axes may be either perpendicular or parallel to the plane of symmetry of the crystal, and according to its position two classes of mica are distinguished. To the first class, with the optic axial plane perpendicular to the plane of symmetry, belong muscovite, lepidolite, paragonite, and a rare variety of biotite called anomite; the second class includes zinnwaldite, phlogopite, lepidomelane andmost biotites. Dark coloured micas are strongly pleochroic. Ac-See also:curate determinations of the optical orientation, as well as the symmetry of the See also:etching figures on the cleavage planes, seem to suggest that the micas, except muscovite, may be anorthic rather than monoclinic in See also:crystallization.

The different kinds of mica vary from perfectly colourless and transparent—as in muscovite—through shades of yellow, See also:

green, red and See also:brown to See also:black and opaque—as in lepidomelane; the former have a pearly lustre and the latter a submetallic lustre on the cleavage surfaces. Sheets of mica very often show coloured rings and bands (See also:Newton's rings), due to the interference of light at the surfaces of See also:internal cleavage cracks. The spec. See also:gray. varies between 2.7 and 3.1 in the different species. The hardness is 2–3; smooth cleavage surfaces can be just scratched with the See also:finger-See also:nail. The micas are See also:bad conductors of See also:heat and See also:electricity, and it is on these properties that many of their technical appiications depend. Inclusions of other minerals are frequently to be observed in mica. Flattened crystals of See also:garnet, films of See also:quartz, and needles of See also:tourmaline are not uncommon. Cleavage sheets are frequently disfigured and rendered of little value by brown, red or black spots and stains, often with a dendritic arrangement of See also:iron oxides. See also:Minute acicular inclusions, probably of See also:rutile, arranged parallel to the rays of the percussion figure, give rise to the phenomenon of " asterism " in some micas, particularly phlogopite: a See also:candle-See also:flame or spot of light viewed through a cleavage sheet of such mica appears as a six-rayed star. Chemical See also:Composition.—The micas are extremely complex and variable in composition. They are silicates, usually orthosilicates, of See also:aluminium together with alkalis (See also:potassium, See also:sodium, See also:lithium, rarely See also:rubidium and See also:caesium), basic See also:hydrogen, and, in some species See also:magnesium, ferrous and ferric iron, rarely See also:chromium, See also:manganese and See also:barium. See also:Fluorine is also often an essential constituent, and See also:titanium is sometimes See also:present.

The composition of the several species of mica is given by the following formulae, some of which are only approximate. It will be seen that they may be divided into two See also:

groups—See also:alkali-micas (potash-mica, &c.) and ferromagnesian micas—which correspond roughly with the See also:division into light and dark micas.

End of Article: MICA

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