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Encyclopedia Britannica - Main :: TOO-TUM |
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TOURMALINE , a mineral of much interest
village
with the word skor, brittle. Tourmaline crystallizes in the rhombohedral division of the hexagonal system. The crystals have generally a prismatic habit, the prisms being longitudinally striated or even channelled. Trigonal prisms are characteristic, so that a transverse section becomes triangular or often nine-sided. By combination of several prisms the crystals may become sub-cylindrical. The crystals when doubly terminated are often hemimorphic or present dissimilar forms at the opposite ends; thus the hexagonal prisms in fig. I are terminated at one end by rhombohedral faces, o, P, and at the other by the basal plane k'. Doubly-terminated crystals, however, are comparatively rare ; the crystals being usually attached at one end to the matrix. It is notable that prismatic crystals of tourmaline have in some cases been curved and fractured transversely; the displaced FIG. I. fragments having been cemented together by deposition of fresh mineral matter. Tourmaline is not infrequently columnar, acicular or fibrous; and the fibres may radiate from a centre so as to form the so-called " tourmaline suns." Crystals of tourmaline present no distinct cleavage, but break with a sub-conchoidal fracture; and whilst the general lustre of the mineral is vitreous, that of the fractured surface is rather pitchy. The hardness is slightly above that of quartz (7). The specific gravity varies according to chemical composition, that of the colourless varieties being about 3, whilst in schorl it may rise to 3.2. Tourmaline has a great range of colour, and in many cases the crystals are curiously parti-coloured. Occasionally, though rarely, the mineral is colourless, and is then known as achroite, a name proposed by R. Hermann in 1845, and derived from the Greek axpoos (uncoloured). Red tourmaline, which when of fine colour is the most valued of all varieties, is known as rubellite (q.v.). Green tourmaline is by no means uncommon, but the blue is rather rare and is distinguished by the name indigolite, generally written indicolite. Brown is a common colour, and black still more common, this being the usual colour of schorl, or common coarse tourmaline. Thin splinters of schorl may, however, be blue or brown by transmitted light. The double refraction of tourmaline is strong. The mineral is optically negative, the ordinary index being about 1.64, and the extraordinary 1.62. Coloured tourmalines are intensely pleochroic, the ordinary ray, which vibrates perpendicular to the principal axis, being much more strongly absorbed than the extraordinary; hence a slice cut in the direction of the principal or optic axis transmits sensibly only the extraordinary ray, and may consequently be used as a polarizing medium. The brown tourmaline of Ceylon and Brazil is best adapted for this purpose, but the green is also used. Two plates properly mounted form the instrument used by opticians for testing spectacle-lenses, and are known as the " tourmaline tongs." In order to secure the best colour-effect when used as a gem-stone, the tourmaline should be cut with the table parallel to the optic axis. It was in tourmaline that the phenomenon of pyroelectricity was first observed. On being heated in peat ashes its attractive power was observed by the Dutch, in the early part of the 18th century; and this curious character obtained for it the name of aschtrekker, or ash-drawer. J. R. Hauy first pointed out the relation of pyroelectricity with hemimorphism. Tourmaline is also piezoelectric, that is, it becomes electric by pressure. If a crystal be subjected to pressure along the optic axis, it behaves as though it were contracting by reduction of temperature. The mineral may also be rendered electric by friction, and retains the charge for a long time. Tourmaline is a boro-silicate of singularly complex composition. Indeed the word tourmaline is sometimes regarded as the name of a group of isomorphous minerals rather than that of a definite species. Numerous analyses have been made, and the results discussed by a large number of authorities. In the view of S. L. Penfield and H. W. Foote all tourmaline may be derived from a boro-silicic acid of the formula H2nB25i4O21. It is believed that the hydrogen is present as hydroxyl, and that this may be partially replaced by fluorine. The tourmaline acid has probably the constitution H1s(BOH)2Si4O19. Nine atoms of hydrogen are replaced by three of aluminium, and the remaining nine in part by other metals. Lithium is present in red tourmaline; magnesium dominates in brown; iron, manganese and sometimes chromium are found in green; and much iron occurs in the black varieties. Four groups are sometimes recognized, characterized by the presence of (r) lithium, (2) ferrous iron, (3) ferric iron and (4) magnesium. Tourmaline occurs commonly in granite, greisen, gneiss and crystalline schists. In many cases it appears to have been formed by pneumatolysis, or the action on the rocks of heated vapours containing boron and fluorine, as in many tin-bearing districts, where tourmaline is a characteristic mineral. Near the margin of a mass of granite the rock often becomes schorlaceous or tourmaliniferous, and may pass into " tourmaline-rock," which is usually an aggregate of tourmaline and quartz. Tourmaline is an essential constituent of the west of England rocks called luxullianite (luxulyanite) and trowlesworthite. It occurs embedded in certain metamorphic limestones, where it is possibly due to fumarolic action. Microscopic crystals are common in clay-slate. By resistance to decomposition, tourmaline often survives the disintegration of the matrix, and thus passes into sands, clays, marls and other sedimentary deposits. Many of the finest crystals of tourmaline occur in druses
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Lepidolite
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Many localities in the United States are famous for tourmaline. Magnificent specimens have been obtained from Mt Mica, near Paris, Maine, where the mineral was accidentally discovered in 182o by two students, E. L. Hamlin and E. Holmes. It occurs in granite, with lepidolite
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Many varieties of tourmaline have received distinctive names, some of which are noticed above. Dravite is G. Tschermak's name for a brown tourmaline, rich in magnesia but with little iron, occur-ring near Unter Drauburg in the Drave district in Carinthia. Taltalite was a name given by I. Domeyko to a mixture of tourmaline and copper ore from Taltal in Chile. The colourless Elba tourmaline was called apyrite by J. F. L. Hausmann, in allusion to its refractory behaviour before the blow-pipe; whilst a black iron-tourmaline from Norway was termed aphrazite by J. B. d'Andrada, in consequence of its intumescence when heated. (F. W. R. *) End of Article: TOURMALINE If you wish, you can link directly to this article.
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