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Tourmaline

crystals, tourmalines, black, red and brown

TOURMALINE, a common and wisely dis tributed mineral, so called from an East Indian name, and known to earlier writers as schod. It is a very complex aluminum boro-silicate, with several marked varieties depending on the presence and proportions of other metallic oxides. The precise constitution of tourmaline has been recently studied elaborately by eminent mineralogical chemists both in Europe and America, without exact agreement, save in its general features, as derived from a complicated boro-silicic acid. According to the oxides pres ent, three types are clearly determined,— iron tourmalines, mostly black; magnesia tourma lines, usually brown; and alkali tourmalines, in which some lithia is present, of red, green and other rich colors. These last, when transparent, yield beautiful gems, of a hardness of seven to seven and five-tenths and specific gravity three to three and one-tenth. The black variety is quite common in schists, gneisses and granites; the brown is usually in crystalline limestones; the brightly colored varieties occur in dikes of albitic granite, often associated with lepidolite. The gem tourmalines have received a number of special names; the pink or red is called rubellite or Siberian ruby; the green, Brazilian emerald; the deep blue, indicolite, or Brazilian sapphire; the colorless, achroite. The crystals are rhombohedral, hemimorphic and of pris matic habit, either short and stout or long and slender, with three, six, nine or 12 sides, and with rhombohedral, or more rarely, simple basal terminations. The prisms are often so deeply

striated vertically as to completely obliterate the faces. The physical properties of tourmaline are very interesting; it is rendered highly elec tric, both by heating and by friction and it has remarkable polarizing action on light; so that plates cut from transparent crystals, parallel to their length, are much used in experiments in optics, mounted in the so-called tourmaline pincers or tongs. With this is connected a very high dichroism, such that the color is frequently quite different according as light traverses a crystal lengthwise or across. Entirely distinct from this is another peculiar feature, namely, the intermixture of two or more colors in the same crystals, either transversely (concentrically) or lengthwise, sometimes gradually and some times sharply; so much so that elegant gems have lately been cut from some of the crystals from Southern California which are half red and half green, with perfectly sharp demarca tion between the two brilliant tints. The most noted localities for bright-colored tourmalines are in the Ural Mountains; the island of Elba; Brazil; Paris, Me.; Haddam Neck, Conn., and above all, several mines recently opened in San Diego and Riverside counties, Cal. (See GEms). Superb black tourmalines occur at Pierrepont, N. Y.; fine brown crystals at Gou verneur, N. Y., and Hamburg, N. J.