MICA is a finely foliated mineral, of a pearly metallic lustre. It is harder than gypsum, but not so hard as cale-spar ; flexible and elastic ; spec. gray. 2.65. It is an ingredient of granite and gneiss ; in this country commonly called isinglass ; in Europe it is occasionally called Mus covy-talc. The largest sheets are found in Siberia, Canada, and the New England States. Its primitive form is an oblique rhombic prism ; ordinarily it is a six-sid ed table ; its cleavage is perfect. There is great diversity in the composition of mica coming from different localities ,• generally it is a silicate of alumina united with silicates of iron and potash. Some times manganese replaces the alumina, as in the mica of Mt. St. Gothard ; some times magnesia replaces the potash, and titanium the iron. That from Siberia yielded to Klaproth, in 100 parts : Alumina 3425 Silica 48'00 Oxide of iron 450 Oxide of Manganese trace Magnesia 050 Potash 815 The mica of Fahlun, analyzed by Rose, afforded, silica, 46.22; alumina, 34.52;
peroxide (I) of iron, 6.04 ; potash, 8,22 ; magnesia, with oxide of manganese, 2.11 ; fluoric acid, 1.09 ; water, 0.98. Very beautiful specimens of mica abound in the United States. At Ac worth, N. H., they lie imbedded in fel spar; at Monroe, N. Y., a large vein of a green-colored variety exists. The crys tals at Goshen, Mass., are rose-red, and rhomboidal, and that in Brunswick Me., is in emerald green scales. Lepielolite is a variety of mica containinglithia and fluoric acid ; it occurs at Paris, Maine. Brown and gray mica are used in lan terns, in stove-doors, and in the windows of ships of war, and in all cases where glass is liable to be broken.
It is much used in optical experiments, and in the manufacture of artificial ultra marine.