Mica is composed of silex, alumina, and potash. It is found in America, Switzerland, Siberia, Norway, Bohemia, and Russia. Siberia and the United States probably furnish the best and largest specimens. It occurs in granite and quartz, also in rubellite, green tourmaline, felspar, lepidolite, and several other minerals. It is one of the constituents of granite, gneiss, mica-schist, talc-slate, etc. It sometimes occurs in granular limestone, and rarely in lava, dolomite, and magnetic iron ores. According to Dana, mica is usually in thinly foliated plates or scales; colour from white, through green, yellowish, and brownish shades, to black; with a pearly lustre, transparent or translucent; before the blowpipe infusible, but becomes opaque white. There are a number of varieties. That in which the scales are arranged in plumose form is called plumose mica; that in which the leaves or scales have a transverse cleavage is called prismatic mica. The crystals are chiefly rhombic, or six-sided, though not always. The cleavage of mica is highly perfect, and, according to Prof. Henry, it can be split or divided into leaves, 250,000 to the inch. The uses of mica are various, Diamond dust, with which ladies powder their hair, is ground mica.
The costly French silver mouldings are cast from ground mica. The wonderful showers of diamonds in scenic plays are mica scales. As a lubricator, mixed with oil it wears longer than any other ingredient. For stoves, it has now become indispensable, and the demand for clear, transparent mica is rapidly increasing. Large plates, when they could be procured, were at one time used in the Russian naval vessels for deck or dead lights, because not liable to fracture from concussion. It is in common use for lanterns, and is rapidly coming into use for lamp chimneys. On account of its transparency and toughness, and the thinness of its folia, it has been used as glass in Siberia, but is now too costly for common use. Sizes as large as 14 in. have been found in North Carolina, and not unfrequently 18 to 24 in. One block or crystal weighed fully 100 lb. Crude mica (pieces too small for cutting) and the cuttings are too far from market, and the uses of waste mica are too limited, to render them valuable. Sizes less than 2 1/2 x 4 or 4 1/2 in. are hardly worth saving.
Sizes 5 x 7 and 9 in. are worth 27s. to 30s. per lb., with a rapid increase in price for larger sizes. (Cary French.)
The minerals which form the group of micas divide readily into 2 classes: those which are silicates of alumina and an alkaii, and those which are silicates of magnesia. They are all notable for their lustre and for their distinct cleavage, which permits of their being separated into thin sheets. In granite, the plates are rarely of a useful size, although in the coarser descriptions of that rock plates are occasionally found 1 ft. and more in width; but in limestone formations, mica is often found in masses of considerable size, plates having been met with in Siberia several ft. in diameter. The micas chiefly occurring in commerce are muscovite and lepidolite (or lithia-mica) of the first division, and phlogopite (rhombic mica) and biolite of the latter division, or magne-sian micas. of these, the most extensively used in the arts are muscovite and phlogopite. The former is mainly a silicate of alumina and potash, with traces of iron, fluorine,' chromium, etc, which impart colour to the otherwise grey or silver-white plates of mica. The crystals of muscovite are usually 6-sided-the colour varying from black through grey to green, chromium being invariably present in the crystal of the last-mentioned tint.
This variety of mica is proof against acids, is very refractory, the thin edge only fusing before an ordinary blowpipe, while the laminae are very tough and flexible. Phlogopite, or rhombic mica, as it is sometimes called, is mostly found in limestone, and is composed mainly of silica, alumina, and magnesia, with traces of iron, potash or soda, and fluorine. Its colour varies from brown, through brownish yellow, to grey. If it is previously reduced to fine powder, it is attacked by hot sulphuric acid, but, like muscovite, although it whitens in the blowpipe flame and fuses on the thin edges, it is virtually refractory to anything short of an intense heat. These extraordinary properties, combined with toughness and elasticity, and the peculiar facility with which it splits into thin sheets, some of which approach closely to transparency, led naturally to the use of mica for windows, and especially to its employment in lanterns. For many years it has been used in Russia for windows, and in some parts is still to be found, though it is of course rapidly giving way to the more transparent glass.
So common, however, was its employment for this purpose at one time in Russia, that it was frequently called "Muscovy glass." It is found in Siberia, Sweden, and Moravia, which also supply the lepidolite, or lithia-mica. In America it is found in various parts, as North Carolina, New Jersey, and Canada. In some coarse granitic rocks of the first-named state, the mica is found in considerable abundance, and there are unmistakable evidences that it was worked many years ago. The commercial value of mica varies through a wide scale; the large, sound, and clear sheets being naturally the highest priced, fetching as much as 40s. a lb. In the United States, where large quantities are used for what is called " stove glass," that is, for the fronts of gas and other stoves, the utilization of mica has been carried further than in this country. The small and waste stuff is there made into a coarse powder, and sprinkled over tar in roof-making; finely ground, it is used as a lubricant, and is sometimes used in packing deed-boxes and safes to render them fireproof.
The finer sheets are used for such purposes as the dials of compasses, for the letters of fancy signs, and the very finest and thinnest pieces are sometimes employed in lieu of enamel for covering photographs; but one of the principal uses to which the better qualities are put is the construction of shades for lamps, the nature of the material rendering its decoration a comparatively easy process; chromolithograph)' being extensively employed in this manufacture. The preparation of the mica is very easy. When first obtained, it is in plates and crystals of various sizes, from 1/4 in. to even occasionally 1 ft. in thickness, and from 6 in. to 1 ft. and upwards in diameter. These plates are dull and opaque, and when taken to the workshop they are split sufficiently thin to render them semi-transparent, when they are examined for flaws, and sorted into different qualities. The comparatively thick plates are then taken by the workman and split into the thinnest sheets, a stout knife and skill being all that is required for the purpose.