Tourmaline

Tourmaline, a mineral of much interest to the physicist on account of its optical and electrical properties; it is also of some geological importance as a rock-constituent (see Schorl), whilst certain transparent varieties have economic value as gem-stones. The name is probably a corruption of turmali, or toramalli, the native name applied to tourmaline and zircon in Ceylon, whence specimens of the former mineral were brought to Europe by the Dutch in 1703. The green tourmaline of Brazil had, however, been known here much earlier; and coarse varieties of the mineral had passed for centuries under the German name of Schorl, an old mining word of uncertain origin, possibly connected with the old German Schor (refuse), in allusion to the occurrence of the mineral with the waste of the tin-mines. The German village of Schorlau may have taken its name from the mineral. It has been suggested that the Swedish form skorl has possible connexion with the word ski*, 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. 1 are terminated at one end by rhombohedral faces, o, P, and at the other by the basal plane k'. Doublyterminated 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 fragments having been cemented together FIG' I.

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 Ixpoos (uncoloured). Red tourmaline, which when of fine colour is the most valued of all varieties, is known as rubellite. 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. Holly 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 H 20 B 2 Gi 4 O 21. 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 Hls(B OH)2S14019. 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 (I) 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 in granitic rocks, such as those of San Piero in Elba, where some of the pale pink and green prisms are tipped with black, and have consequently been called "nigger-heads." Lepidolite is a common associate of tourmaline, as at Rozena in Moravia. Tourmaline occurs, with corundum, in the dolomite of Campolongo, in canton Ticino, Switzerland. Fine black crystals, associated with apatite and quartz, were formerly found in granite at Chudleigh, near Bovey Tracey in Devonshire. The Russian localities for tourmaline are mentioned under Rubellite. Most of the tourmaline cut for jewelry comes from the gem-gravels of Ceylon. The green tourmaline has generally a yellowish or olive-green colour, and is known as "Ceylon chrysolite." Fine green crystals are found in Brazil, notably in the topaz-locality of Minas Novas; and when of vivid colour they have been called "Brazilian emeralds." Green tourmaline is a favourite ecclesiastical stone in South America Blue tourmaline occurs with the green; this variety is found also at Uto in Sweden (its original locality) and notably near Hazaribagh in Bengal. Certain kinds of mica occasionally contai.I flat crystals of tourmaline between the cleavage-planes.

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 1820 by two students, E. L. Hamlin and E. Holmes. It occurs in granite, with lepidolite, smoky quartz, spodumene, &c.; and some of the prismatic crystals are notable for being red at one end and green at the other. Mt Rubellite at Hebron, and Mt Apatite at Auburn, are other localities in Maine which have yielded fine tourmaline. At Chesterfield, Massachusetts, remarkable crystals occur, some of which show on transverse section a triangular nucleus of red tourmaline surrounded by a shell of green. Red and green tourmalines, with lepidolite and kunzite, are found in San Diego county, California. Fine coloured tourmalines occur at Haddam Neck, Connecticut; and excellent crystals of black tourmaline are well known from Pierrepont, New York, whilst remarkable brown crystals occur in limestone at Gouverneur in the same state. Canada is rich in tourmaline, notably at Burgess in Lanark county, Ontario, and at Grand Calumet Island in the Ottawa river. Heemskirk Mountain, Tasmania, and Kangaroo Island, South Australia, have yielded fine coloured tourmaline fit for jewelry. Madagascar is a well-known locality for black tourmaline in large crystals.

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, occurring near Unter Drauburg in the Drave district in Carinthia. Taltalite was a name given by I. Dorneyko 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. *)