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Originally appearing in Volume V18, Page 356 of the 1911 Encyclopedia Britannica.
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H2K Al3(SiO4)3 ^ H2Na Als(SiO4)a • KLi[Al(OH,F)2]Al(SiOs)s • (K,Li) 3[AI (OH,F)2]FeAl2SisOia ^ (H,K)2(Mg,Fe)2(A1,Fe)2(SiO4)a • [H,K, (MgF)]3MgaAl (SiO4) a The See also:

water which is See also:present in See also:muscovite to the extent of 4 to 6 %, and rather less in the other See also:species, is expelled only at a high temperature; it is therefore water of constitution, existing as basic See also:hydrogen or as hydroxyl replacing See also:fluorine. Roscoelite is a See also:mica in which the See also:aluminium is largely replaced by See also:vanadium (V2O3, 30 %) ; it occurs as brownish-See also:green scaly aggregates, intimately associated with See also:gold in See also:California, See also:Colorado and Western See also:Australia. Various attempts have been made to explain the See also:variations in See also:composition of the micas. G. Tschermak, in 1878, regarded them as isomorphous mixtures of the following fundamental molecules: H2KA13(SiO4)3, corresponding with muscovite; Mg6Si3Oi2, a hypothetical polymer of See also:olivine; and H4Si;Oi2, a hypothetical silicic See also:acid. F. W. See also:Clarke (1889–1893) supposes them to be substitution derivatives of normal aluminium orthosilicate A14(SiO4)3, in which See also:part of the aluminium is replaced by alkalis, See also:magnesium, See also:iron and the univalent See also:groups (MgF), (AIF2),(AlO), (MgOH); an excess of See also:silica is explained by the isomorphous replacement of H4SiO4 by the acid H4Si3O3. Artificially formed crystals of the various species of mica have been observed in See also:furnace-slags and in silicate fusions. Occurrence.—Mica occurs as a See also:primary and essential constituent of igneous rocks of almost all kinds; it is also a See also:common product of alteration of many See also:mineral silicates, both by weathering and by contact- and See also:dynamo-metamorphic processes. In sedimentary rocks it occurs as detrital material. Muscovite and See also:biotite are commonly found in siliceous rocks, whilst See also: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 See also:

limestone blocks ejected from See also:Monte Somma, See also:Vesuvius. Large sheets of muscovite, such as are of commercial value, are found only in the very coarsely crystallized See also:pegmatite See also:veins traversing See also:granite, See also:gneiss or mica-schist. These veins consist of See also:felspar, See also:quartz and mica, often with smaller amounts of other crystallized minerals, such as See also:tourmaline, See also:beryl and See also:garnet; they are worked for mica in See also:India, the See also:United e Muscovite Paragonite See also:Lepidolite. Zinnwaldite Biotite Phlogopite 356 States (See also:South Dakota, Colorado and See also:Alabama), and See also:Brazil (See also:Goyaz, See also:Bahia and See also:Minas Geraes). The commercially valuable micas of See also:Canada and See also:Ceylon are mainly phlogopite (q.v.), which has a rather different mode of occurrence. The mica See also:mined in India is practically all muscovite. The See also:principal See also:mining districts are those of See also:Hazaribagh in See also:Bengal and See also:Nellore in See also:Madras; in the former See also:district the mica has usually a See also:ruby tint, whilst in the latter it is more often greenish. In the Inikurti mine, Nellore, " books " of mica measuring to ft. across, and up to 15 ft. across the folia have been found, and rectangular sheets measuring 30 by 24 in. and See also:free from cracks and flaws have frequently been obtained. Uses.—On See also:account of its transparency and its resistance to See also: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 See also:gas-burners. At one See also:time it was used for window panes of houses and the See also:port-holes of See also:Russian men-of-See also:war, being commonly known as " Muscovy See also: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 specularis of See also:Pliny, scattered over the See also:Circus See also:Maximus to produce a shining whiteness, was probably mica. Large quantities of ground mica are used in the manufacture of See also:wall-See also:paper, and to produce a frosted effect on toys, See also:stage scenery, &c.

Powdered mica is also used in the manufacture of paints and paper, as a lubricant, and as an absorbent of nitro-glycerine and See also:

disinfectants. Sheets of mica are used as a See also:surface for See also:painting, especially in India; for See also:lantern slides; for carrying photographic films; as a protective covering for pictures and See also:historical documents; for mounting soft and collapsible natural See also:history specimens preserved in spirit; for the vanes of anemometers; mirrors of delicate See also:physical See also:instruments; for various See also:optical and many other purposes. Being a See also:bad conductor of See also:heat it is used for the packing and jackets of boilers and See also:steam-pipes. Other applications depend on the strength of its resistance to acids. The most extensive application of mica at the present See also:day is for See also:electrical purposes. Being a bad conductor of See also: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 See also:cloth " is much used; this consists of small sheets of mica cemented with shellac or other insulating See also: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 See also:potassium it contains renders it of value as a manure. The species lepidolite is largely used for the manufacture of See also:lithium and See also:rubidium salts.

Mining, Preparation and Value.—Mica mining is an See also:

industry of considerable importance, especially in India; but here the methods of mining are very See also: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 See also: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 See also:vertical See also:shaft, See also:cross-cuts, and levels See also: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 See also:sharp See also:knife, See also:shears or See also:guillotine, stained and damaged portions being rejected. The dressed sheets are sorted according to See also:size, transparency, See also:colour and freedom from spots or stains. Scrap mica is ground to See also:powder or used in the manufacture of micanite. The See also:price of mica varies very considerably according to the size, transparency and quality of the sheets. .An See also:average price for cut sheets of all sizes is about 4S. per lb, but for large sheets it may be as high as 54S. per lb.

End of Article: H2K

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