BIOTITE, an important rock-forming mineral belonging to the group of micas (q.v.). The name was given in honour of the French physicist, J. B. Biot, who in 1816 found the magnesia micas to be optically uniaxial or nearly so. The magnesia-micas are now referred to the species biotite and phlogopite, which differ in that the former contains a considerable but widely vary ing amount of iron. Biotite is an orthosilicate of aluminium, magnesium, ferrous and ferric iron, potassium and basic hydro gen, containing small amounts of calcium, sodium, lithium, fluorine, titanium, etc., and ranges in composition between and Like the other micas, it is monoclinic with pseudo-hexagonal symmetry, and possesses a perfect cleavage in one direction. Biotite is, however, readily distinguished by its darker colour, strong pleochroism, and small optic axial angle. The colour is usually brown or dark-green; thick crystals are often deep black and opaque. The absorption of light-rays vibrating parallel to the cleavage is much greater than of rays vibrating in a di rection perpendicular thereto, and in dark-coloured crystals the former are almost completely absorbed. The specific gravity of biotite is, as a rule, higher than that of other micas, varying from 2.7 to 3.1 according to the amount of iron present. The hardness is 21 to 3.
Several varieties of biotite are distinguished. By G. Tschermak it is divided into two classes, meroxene and anomite; in the former the plane of the optic axes coincides with the plane of symmetry, whilst in the latter it is perpendicular thereto. Haughtonite and siderophyllite are black varieties rich in ferrous iron, and lepidomelane is a variety rich in ferric iron. Rubellane, hydrobiotite, pseudobiotite and others are altered forms of biotite.
Biotite is a common constituent of igneous and crystalline rocks : in granite, gneiss, and mica-schist it is often associated with muscovite (white mica), the two kinds having sometimes grown in parallel position. In volcanic rocks, and in nearly all other kinds of igneous rocks with the exception of granite, biotite occurs to the exclusion of the muscovite. In the dyke-rocks known as mica-traps or mica-lamprophyres biotite is especially abundant. It is also one of the most characteristic products of contact-metamorphism, being developed in sedimentary and other rocks at their contact with granite masses.
Although biotite (black mica) is much more common and widely distributed than white mica, yet it is of far less economic importance. The small size of the sheets, their brittleness and want of transparency render the material of little value. Large, cleavable masses yielding fine smoky-black and green sheets, sufficiently elastic for industrial purposes, are, however, found in Renfrew county, Ontario.
(L. J. S.)