ORIGIN OF MICA
Mica minerals are an essential constituents of various igneous rocks, such as granite, metamorphic rocks and igneous rocks. Mica content of most of these rocks is low and the crystal size is small, not economical for commercial explorations.
Economic deposits of muscovite mica containing large crystals of these minerals are only found in granitic or acidic pegmatites intruding mica schists. The best quality sound muscovite sheet mica occurs in regions of earth relatively free from earthquake and volcanic disturbances. It crystallizes in monoclinic system in a rock formation called acid pegmatites having mineral composition as ordinary granite, felspar and quartz, etc.
The occurrence of economic deposits of phlogopites confined to areas under lain by large batholithic masses of basic phrozenic rocks in asociation with calcite, apatite, limestone, dolomite, talc, marble or serpentina and phospate rocks but rarely in granite pegmatites.
The mode of formation of mica, which is found in the shape of slabs (books), cleavage into the fine laminae, is still the subject of active research. However, it has been proved by practical experience that the degree of absence of orthoclase felspar in the pegmatites to a great extent is indicative of the presence of economic deposits of mica in a vein. This establishes that mica forms at the expense of orthoclase felspar. The presence of tourmaline crystals and decomposed felspar in the pegmatite also indicates the possibilities of finding good deposit of mica as has been found true in the case of mica fields of Bihar (India).
Natural mica is not like other commercial materials which could be produced by a machine and purchased by the square yard. It is a mineral of natural occurrence which has to be dug out from below the earth's womb, some time as deep as 1000 feet vertically below the surface, at immense costs and ardous labour. The mineral has to be spotted by means of prospecting, surveying and assaying the rock. The earth nursed and the material produced with great skill by drilling and blasting mineral rocks.