GEOLOGICAL DISTRIBUTION
It has been estimated that mica constitutes about 4 per cent. of the igneous rocks of the world. Segregation into deposits of workable size containing mica of commercial size and quality is comparatively rare. Mica mines are worked for the small percentage of sheet or block mica they contain, the large amount of waste mica being utilized only where a ready market warrants the grinding.
Individual deposits of both muscovite and phlogopite are characterized by their extreme irregularity, so that any prediction as to reserve is uncertain. This fact is responsible for the crude methods of mining which have for so long been almost universally employed. The output of a district as a whole is from many small mines rather than from any single large one.
An important consideration in the geological distribution of mica from an economic standpoint is the degree of dynamic metamorphism to which the region has been subjected during or subsequent to the formation of the mica-bearing deposits. This is due to the fact that the value of sheet mica depends, among many other factors, upon the freedom from distortion of the sheets.
Muscovite mica in commercial quantities invariably occurs in dikes and lenses of pegmatite which are considered to represent segregations of certain portions of granitic intrusions. The principal associated minerals are quartz and feldspar, both of which are usually considerably in excess of the mica. The dikes or lenses of pegmatite may be within the granite itself or in other inclosing rocks that may seem unrelated to a parent intrusive. Schists or gneisses form the inclosing rock of the pegmatite in most commercial deposits.
Phlogopite is much rarer than muscovite and occurs in deposits structurally similar to granitic pegmatites. The important associated minerals are pyroxene, apatite and calcite, although in certain deposits the mass is almost entirely of mica. The Canadian deposits, which are best known, consist of veins and pockets in pyroxenite dikes and the inclosing rock is usually limestone, a significant fact as regards the origin of the deposits.