MICA, a group of widely distributed rock-forming minerals, some of which have important commercial applications. The principal members of the group are muscovite, biotite, phlogopite and lepidolite (q.v.). The name is the Lat. mica, a grain, but is confused with micare, to glitter, the German word for mica, Glim mer, having also the meaning of glitter.
When a cleavage flake of mica is struck a sharp blow with a blunt needle-point a "percussion figure" or six-rayed star of cracks is developed ; the rays intersect at angles of approximately 6o°, and the pair most prominently developed are parallel to the plane of symmetry of the crystal. A similar six-rayed system of cracks, bisecting the angles between the rays of the previous set, is produced when a blunt punch is gradually pressed against a sheet of mica; this is known as the "pressure figure." These cracks coincide with planes of easy separation or of gliding in the crystal ; they are especially useful in helping to determine the crystallographic orientation of a cleavage flake when crystal faces are absent. Sheets of mica which have been subjected to earth movements are frequently cracked and ridged parallel to these directions, and are then valueless for economic purposes.
In their optical characters the micas exhibit considerable varia tions. The indices of refraction are not high, the mean index being about 1.58-1.6o, but the double refraction is very strong (0.04 0.05) and is negative in sign. The angle between the optic axes
varies from 7o–so° in muscovite and lepidolite to io–o° in biotite and phlogopite ; the latter are thus frequently practically uniaxial. The acute bisectrix of the optic axes never deviates from the normal to the basal plane by more than a degree or two, hence a cleavage flake of mica will always show an optic figure in con vergent light when placed on the stage of a polarizing microscope. The plane of the optic axes may be either perpendicular or paral lel to the plane of symmetry of the crystal, and according to its position two classes of mica are distinguished. To the first class, with the optic axial plane perpendicular to the plane of symmetry, belong muscovite, lepidolite, paragonite, and a rare variety of biotite called anomite; the second class includes zinnwaldite, phlo gopite, lepidomelane and most piotites. Dark coloured micas are strongly pleochroic.
The different kinds of mica vary from perfectly colourless and transparent—as in muscovite—through shades of yellow, green, red and brown to black and opaque—as in lepidomelane ; the former have a pearly lustre and the latter a submetallic lustre on the cleavage surfaces. Sheets very often show coloured rings and bands (Newton's rings), due to the interference of light at the surfaces of internal cleavage cracks. The sp.gr. varies between 2.7 and 3.1 in the different species. The hardness is 2-3 ; smooth cleavage surfaces can be just scratched with the finger-nail. Micas are bad conductors of heat and electricity, and it is on these properties that many of their technical applications depend. In clusions of other minerals are frequently to be observed, and flattened crystals of garnet, films of quartz, and needles of tour maline are not uncommon. Cleavage sheets are frequently dis figured and rendered of little value by brown, red or black spots and stains, often with a dendritic arrangement of iron oxides. Minute acicular inclusions, probably of rutile, arranged parallel to the rays of the percussion figure, give rise to the phenomenon of "asterism" in some micas, particularly phlogopite; a candle-flame or spot of light viewed through a cleavage sheet of such mica appears as a six-rayed star.