BIOTITE  , an important

rock-forming mineral belonging to the group of micas (q.v.) . The name was given by J . F . L . Hausmann in 1847 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 varying amount of hon . Biotite is an orthosilicate of aluminium, magnesium, ferrous and ferric iron, potassium and basic hydrogen, with small amounts of calcium, sodium, lithium, fluorine, titanium, &c., and ranges in composition between (H,K)z(Mg,Fe)4(Al,Fe)z(SiO4)4 and (H,K) z(Mg,Fe) zAlz(SiO4) 3 . Like the other micas, it is monoclinic with pseudo-hexagonal symmetry (figs . 1, 2) and possesses a perfect cleavage in one direction (c) . Biotite is, however, readily distinguished by its darker colour, strong pleochroism, and small optic axial angle .

The colour is usually dark-

green or brown; 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 direction perpendicular thereto, and in dark-coloured crystals the former are almost completely absorbed . The angle between the optic axes is usually very small, the crystals being often practically uniaxial; an axial angle of 500 has, however, been recorded in a dark-coloured biotite . 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 22 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 axis coincides with the plane of symmetry, whilst in the latter it is perpendicular thereto . Meroxene includes nearly all ordinary biotite, and is the name given by A . Breithaupt in 1841 to the Vesuvian crystals; on the other hand, anomite (named from bop-or, "contrary to law ") is of rare occurrence . Haughtonite and siderophyllite are black varieties rich in ferrous iron, and lepidomelane (from ?ten-is, a scale, and pEXas, black) is a variety rich in ferric iron . In barytobiotite and manganophyllite the magnesia is partly re-placed by baryta and manganous oxide respectively .

Rubellane, hydrobiotite, pseudobiotite, and others are altered forms of biotite, which is a mineral particularly liable to decomposition with the

production of chlorites and vermiculites . 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 allother 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 . In the ejected blocks of crystalline limestone of Monte Somma, Vesuvius, the most perfectly developed crystals of biotite (figs . I, 2), or indeed of any of the micas, are found in abundance, associated with brilliant crystals of augite, olivine, humite, &c . 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 dark colour 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 .