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. 1 is bounded by the basal pinacoid c (001) parallel to which is the perfect cleavage, the clinopinacoid b (010) parallel to the plane of symmetry, and the pyramids m (221) and o (112). The angles between these pyramids and the basal plane are 85½° and 73° respectively. The prism (110) 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.
|Fig. 1.||Fig. 2.|
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 1.58-1.60, 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 10-0° 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 and most biotites. Dark coloured micas are strongly pleochroic. Accurate 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. grav. 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 applications 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.
The water which is present in muscovite to the extent of 4 to 6%, and rather less in the other species, is expelled only at a high temperature; it is therefore water of constitution, existing as basic hydrogen or as hydroxyl replacing fluorine.
Roscoelite is a mica in which the aluminium is largely replaced by vanadium (V2O3, 30%); it occurs as brownish-green scaly aggregates, intimately associated with gold in California, Colorado and Western Australia.
Various attempts have been made to explain the variations in composition of the micas. G. Tschermak, in 1878, regarded them as isomorphous mixtures of the following fundamental molecules: H2KAl3(SiO4)3, corresponding with muscovite; Mg6Si3O12, a hypothetical polymer of olivine; and H4Si5O12, a hypothetical silicic acid. F. W. Clarke (1889-1893) supposes them to be substitution derivatives of normal aluminium orthosilicate Al4(SiO4)3, in which part of the aluminium is re laced by alkalis, magnesium, iron and the univalent groups (MgF), (AlF2), (AlO), (MgOH); an excess of silica is explained by the isomorphous replacement of H4SiO4 by the acid H4Si3O8.
Artificially formed crystals of the various species of mica have been observed in furnace-slags and in silicate fusions.
Occurrence.—Mica occurs as a primary and essential constituent of igneous rocks of almost all kinds; it is also a common product of alteration of many mineral silicates, both by weathering and by contact- and dynamo-metamorphic processes. In sedimentary rocks it occurs as detrital material.
Muscovite and biotite are commonly found in siliceous rocks, whilst phlogopite is characteristic of calcareous rocks. The best crystallized specimens of any mica are afforded by the small brilliant crystals of biotite, which encrust cavities in the limestone blocks ejected from Monte Somma, Vesuvius. Large sheets of muscovite, such as are of commercial value, are found only in the very coarsely crystallized pegmatite veins traversing granite, gneiss or mica-schist. These veins consist of felspar, quartz and mica, often with smaller amounts of other crystallized minerals, such as tourmaline, beryl and garnet; they are worked for mica in India, the United States (South Dakota, Colorado and Alabama), and Brazil (Goyaz, Bahia and Minas Geraes). The commercially valuable micas of Canada and Ceylon are mainly phlogopite (q.v.), which has a rather different mode of occurrence. The mica mined in India is practically all Muscovite. The principal mining districts are those of Hazaribagh in Bengal and Nellore in Madras; in the former district the mica has usually a ruby tint, whilst in the latter it is more often greenish. In the Inikurti mine, Nellore, “books” of mica measuring 10 ft. across, and up to 15 ft. across the folia have been found, and rectangular sheets measuring 30 by 24 in. and free from cracks and flaws have frequently been obtained.
Uses.—On account of its transparency and its resistance to fire and sudden changes of temperature, mica has been much used for the windows of stoves and lanterns, for the peep-holes of furnaces, and the chimneys of lamps and gas-burners. At one time it was used for window panes of houses and the port-holes of Russian men-of-war, being commonly known as “Muscovy glass.” Spangles of mica are much used for decorative purposes of various kinds, and the mineral was formerly known as glacies Mariae (Ger., Frauenglas) because of its use for decorating statues of the Virgin. The lapis specular is of Pliny, scattered over the Circus Maximus to produce a shining whiteness, was probably mica. Large quantities of ground mica are used in the manufacture of wall-paper, and to produce a frosted effect on toys, stage scenery, &c. Powdered mica is also used in the manufacture of paints and paper, as a lubricant, and as an absorbent of nitro-glycerin and disinfectants. Sheets of mica are used as a surface for painting, especially in India; for lantern slides; for carrying photographic films; as a protective covering for pictures and historical documents; for mounting soft and collapsible natural history specimens preserved in spirit; for the vanes of anemometers; mirrors of delicate physical instruments; for various optical and many other purposes. Being a bad conductor of heat it is used for the packing and jackets of boilers and steam-pipes. Other applications depend on the strength of its resistance to acids.
The most extensive application of mica at the present day is for electrical purposes. Being a bad conductor of electricity it is of value as an insulator, and the smooth flexible sheets are much used in the construction of armatures of dynamos and in other electrical machinery. For various purposes a manufactured material known as “micanite” or “micanite cloth” is much used; this consists of small sheets of micazcemented with shellac or other insulating cement on cloth or paper. Muscovite and phlogopite are practically the only species used commercially, the former being the more common. Phlogopite is rarely found as colourless transparent sheets and is therefore almost exclusively used for electrical purposes. Many other uses of mica might be mentioned. The potassium it contains renders it of value as a manure. The species lepidolite is largely used for the manufacture of lithium and rubidium salts.
Mining, Preparation and Value.—Mica mining is an industry of considerable importance, especially in India; but here the methods of mining are very primitive and wasteful. In working downwards in open quarries and in tortuous shafts and passages much of the mica is damaged, and a large amount of labour is expended in hauling waste material to the surface. Since the mineral occurs in definite veins, a more satisfactory and economical method of working would be that adopted in metalliferous mines, with a vertical shaft, cross-cuts, and levels running along the strike of the vein: the mica could then be extracted by overhead stopping, and the waste material used for filling up the worked-out excavations.
In dressing mica the “books” are split along the cleavage into sheets of the required thickness, and the sheets trimmed into rectangles with a sharp knife, shears or guillotine, stained and damaged portions being rejected. The dressed sheets are sorted according to size, transparency, colour and freedom from spots or stains. Scrap mica is ground to powder or used in the manufacture of micanite.
The price of mica varies very considerably according to the size, transparency and quality of the sheets. An average price for cut sheets of all sizes is about 4s. per ℔, but for large sheets it may be as high as 54s. per ℔.
References.—For the mineralogical characters see the textbooks of J. D. Dana and C. Hintze; for economic questions, the following: T. H. Holland, “The Mica Deposits of India,” Memoirs of the Geological Survey of India (1902), xxxiv. 11–121; G. P. Merrill, The Non-Metallic Minerals (New York, 1904), pp. 163–180; “The Mining and Preparation of Mica for Commercial Purposes,” Bulletin of the Imperial Institute (London, 1904), ii. 278–291; F. Cirkel, “Mica: its Occurrence, Exploitation and Uses” (Canadian Dept. of the Interior, Mines Branch, Ottawa 1905, 148 pp.). (L. J. S.)