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Originally appearing in Volume V18, Page 355 of the 1911 Encyclopedia Britannica.
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MICA, a group of widely distributed rock-forming minerals, some of which have important commercial applications. The principal members of the group are muscovite, biotite, phlogopite and lepidolite (q.v.). The name mica is probably derived from the Latin micare, to shine, to glitter; the German word glimmer has the same meaning. The mineral was probably included with selenite under Pliny's 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, elasticity and toughness of the extremely thin cleavage flakes. They all crystallize in the monoclinic system, often, however, in forms closely resembling those of the rhombohedral or orthorhombic systems. Crystals have usually the form of hexagonal or rhomb-shaped scales, plates or prisms, with 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 prism (Ito) at 90° from the basal plane is not developed as a crystal face, but is a plane of twinning, the two individuals of the twin being united parallel to the basal plane (fig. 2). The different 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 sharp blow with a blunt needle-point a six-rayed 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 punch is gradually pressed against a 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 orientation of a cleavage flake of mica when crystal faces are absent. Sheets of mica which have been subjected to earth-movements are frequently cracked and ridged parallel to these directions, and are then valueless for economic purposes. In their optical characters the micas exhibit considerable variations. The indices of refraction are not high, the mean index being about 1.58–i•6o, but the double refraction is very strong (0.04–0.05) and is negative in sign. The 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 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 light when placed on the stage of a polarizing 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-curate determinations of the optical orientation, as well as the symmetry of the etching figures on the cleavage planes, seem to suggest that the micas, except muscovite, may be anorthic rather than monoclinic in crystallization. The different kinds of mica vary from perfectly colourless and transparent—as in muscovite—through shades of yellow, green, red and brown to 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 (Newton's rings), due to the interference of light at the surfaces of internal cleavage cracks. The spec. 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 finger-nail. The micas are bad conductors of heat and 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 garnet, films of quartz, and needles of 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 iron oxides. Minute acicular inclusions, probably of rutile, arranged parallel to the rays of the percussion figure, give rise to the phenomenon of " asterism " in some micas, particularly phlogopite: a candle-flame or spot of light viewed through a cleavage sheet of such mica appears as a six-rayed star. Chemical Composition.—The micas are extremely complex and variable in composition. They are silicates, usually orthosilicates, of aluminium together with alkalis (potassium, sodium, lithium, rarely rubidium and caesium), basic hydrogen, and, in some species magnesium, ferrous and ferric iron, rarely chromium, manganese and barium. Fluorine is also often an essential constituent, and titanium is sometimes 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 groups—alkali-micas (potash-mica, &c.) and ferromagnesian micas—which correspond roughly with the division into light and dark micas.
End of Article: MICA

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