The composition of the several species of mica is given by the following formulae, some of which are only approximate, and many attempts have been made to explain the variations in com position. It will be seen that they may be divided into two groups —alkali-micas (potash-mica, etc.) 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 3o%) ; it occurs as brownish-green scaly aggregates, intimately associated with tellurides of gold in Cali fornia, Colorado and Western Australia.
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 lime stone 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 Ala bama), and Brazil (Goyaz, Bahia and Minas Geraes). The com mercially valuable micas of Canada and Ceylon are mainly phlogo pite (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 i of t. across, and up to r 5f t. across the folia have been found, and rectangular sheets measuring 3o by 24in. 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, etc. Powdered mica is also used in the manu facture of paints and paper, as a lubricant, and as an absorbent From figures of the United States Bureau of Mines, Department of the Interior, and other sources.
a Sales. b Sold or used by producers. e Exports. d Includes Euro Russia as well as Asiatic Russia. e Russian output for 1933. Russian output for 1934. g Russian output for 1935. Other Russian figures not available except for year 1922 when the output was 8 metric tons.
The 1938 Minerals Yearbook, Bureau of Mines, United States De partment of the Interior, reported that in 1937 the total imports of mica into the United States rose to 22,678,1471bs., valued at $2,067,599, as compared to 13,355,5871bs., valued at $1,2o5,568 in 1936. Of these 1937 imports about 38% of the unmanufactured sheet mica and 87% of splittings and a large quantity of scrap mica for grinding were from British India. In 1937 Madagascar replaced Canada as the second largest source of mica imports to the United States. London is the leading world market for mica and much of the Indian mica in export trade is reshipped. In addition to the widespread use of mica in electrical apparatus, the unique American development of mica as a coating for roofing paper and for dusting automobile tires has increased the use of the mineral enormously.