TOURMALINE, a mineral of interest from several points of view, but of no commercial value, except occasionally when used as a gem-stone. The name is from the Cingalese toramalli, brown gem-stones having come from Ceylon early in the 18th century. The mineral is of interest chemically on account of its complex composition, containing, perhaps, a greater variety of chemical elements than any other mineral, and these are present in very variable proportions. Many attempts have been made to arrive at a general formula to express the composition. The one now generally accepted is that of S. L. Penfield and H. W. Foote (1899), who regarded the several varieties of tourmaline as salts of a hypothetical alumino-borosilicic acid H9A13(BOH)2Si4019. Here hydrogen is partly, but not wholly, replaced by aluminium, ferric and ferrous iron, chromium, manganese, magnesium, cal cium, sodium, potassium, lithium, etc., and fluorine is usually also present in small amounts. Rather than a single species, tourmaline thus represents an extensive series of isomorphous min erals, but the sub-division into species is not practicable. According to the predom inance of certain elements, a chemical dis tinction has been made between iron tourmaline, magnesium-tourmaline and alkali-tourmaline.
All crystals of tourmaline belong to the ditrigonal pyramidal (hemimorphic-hemi hedral) class of the rhombohedral system, and no variation can be detected in the in terfacial angles corresponding with the wide range in chemical composition. The crystals are of interest in showing a dif ferent development of faces at the two ends, the vertical trigonal axis, or principal axis, being uniterminal or polar. At the upper end in fig. I are two trigonal pyramids r and o, and at the lower end the trigonal pyramids r and e. The crys tals are usually prismatic in habit with one hexagonal prism a and a trigonal prism m, and between the two there may be sev eral ditrigonal prisms each of six faces. All these prism faces are deeply striated or furrowed parallel to the vertical axis, so much so that the cross-section is usually triangular with curving convex sides. This characteristic feature enables the mineral to be readily distinguished from others of similar appearance.
Another interesting feature of tourmaline is shown in the pyro electrical properties, which are intimately related to the hemi morphic or polar development of the crystals mentioned above. When a crystal is slightly warmed, one end becomes charged with positive electricity and the other end acquires a negative charge. On cooling these charges are reversed. This is well shown by dust ing a cooling crystal over with a mixture of red-lead and sulphur, when the yellow sulphur is attracted to the positively charged end and the red lead to the negatively charged end. A faceted
gem-stone of tourmaline may be tested in this way. Tourmaline also shows the related phenomenon of piezoelectricity. When a plate cut perpendicular to the principal axis is subjected to varia tions of pressure it develops positive and negative charges on the two surfaces. This property can be made use of for detecting small variations in pressure, as in depth-sounding apparatus.
The optical properties are also of exceptional interest. A ray of light entering a crystal of tourmaline is split up into two rays; one, the ordinary ray, vibrating perpendicular to the principal axis, and the other, the extraordinary ray, vibrating parallel to this axis. Coloured crystals are very strongly dichroic, the ordinary ray being almost completely absorbed. Plates cut parallel to the principal axis of the crystal, therefore, allow only the extraordinary ray to pass through, and if two such plates are placed in crossed position the light is entirely cut out. A pair of such plates form a very simple polarizing apparatus known as tourmaline tongs. The crystals are optically uniaxial and negative, and the refractive indices vary with the chemical composition, for the ordinary ray 0.) =1.6315-1.6854, and for the extraordinary ray e = 1.6123— 1.6515 for sodium-light. The specific gravity shows a correspond ing range from 3.o to 3.2. The hardness is 7.5.
In general appearance, tourmaline is extremely variable. It may be quite colourless and water-clear, black and opaque, or various shades of red, yellow, brown, green or blue. Some crystals show very strikingly bands of different colours. The coloured varieties when clear and of good quality are cut as gem-stones under the names rubellite or "Siberian ruby," "Brazilian emerald," indicolite, etc. In addition to well-formed crystals, the mineral may take the form of rounded triangular rods or fine needles, which are often aggregated in bundles or radiating groups, or it may form compact granular masses. A common form, especially in the Cornish tin mines, is as bundles of black needles, this form being known as schorl (German, Schorl).
Tourmaline generally occurs in connection with granitic rocks, and it is often an indication of the presence of tin-ore. It would appear to have originated in most cases by the interaction of boron-bearing emanations from the granite magma on the sur rounding rocks and minerals. The best crystals are found in peg matite veins and in metamorphic limestones in contact with granite masses. Being resistant to weathering processes the mineral accumulates in detrital deposits and in sedimentary rocks. Gem tourmaline comes from the gem-gravels of Ceylon and is quarried in pegmatite veins in the Ural mountains, California and Mada gascar. (L. J. S.)