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

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: H2KA13(SiO4)3, corresponding with muscovite; Mg6Si3Oi2, a hypothetical polymer of olivine; and H4Si;Oi2, a hypothetical silicic acid . F . W . Clarke (1889–1893) supposes them to be substitution derivatives of normal aluminium orthosilicate A14(SiO4)3, in which part of the aluminium is replaced by alkalis, magnesium, iron and the univalent groups (MgF), (AIF2),(AlO), (MgOH); an excess of 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 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 e Muscovite Paragonite Lepidolite . Zinnwaldite Biotite Phlogopite 356 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 to 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 specularis 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-glycerine 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 mica cemented 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 lb, but for large sheets it may be as high as 54S. per lb .