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tourmaline(to͝or`məlĭn, –lēn), complex borosilicate mineral with varying amounts of aluminum, iron, magnesium, sodium, lithium, potassium, and sometimes other elements, used as a gem. It occurs in prismatic crystals, commonly three-sided, six-sided, or nine-sided, and striated vertically. Different crystal forms are usually present at opposite ends of the vertical axis. The luster is vitreous. Colors are red and pink (rubellite), blue (indicolite, or Brazilian sapphire), green (Brazilian emerald), yellow, violet-red, and black (schorl). Colorless varieties are called achroite. Two or more colors may occur in the same stone, the colors being arranged in zones or bands with sharp boundaries between them. Some Brazilian stones have a red core with a green exterior, separated by a colorless band; some stones from California are green within and red outside. The variations in color are, of course, dependent on the variations in chemical composition. Tourmalines are found in pegmatite veins in granites, gneisses, schists, and crystalline limestone. Sources of the gem include Elba, Madagascar, Myanmar, Sri Lanka, the Urals, Siberia, Brazil, and Maine, Connecticut, and California in the United States.
a boron-containing aluminosilicate mineral of complex, variable composition; a member of the cyclosilicate class of silicates with the general formula
AB3C6(Si6O18)(BO3)3(OH, F, O)4
where A = Na, Ca; B = Mg, Fe2+, Fe3+, Li, Mn, Al; and C = Al, Fe3+. The highly complex tourmaline structure is based on the six-member rings of silicon-oxygen tetrahedrons (Si6O18), which are joined together by BO3 triangles.
Variations in the composition of the B and C cations give rise to different varieties of tourmaline. Examples include the iron-rich varieties schorl (black), verdelite (green), and indicolite (dark blue), the magnesium-rich variety dravite (brownish yellow), the lithium varieties elbaite (lilac pink) and rubellite (dark pink), and the manganese variety tsilaisite. Bright green chromium-bearing varieties of tourmaline are also known. Some tourmaline crystals exhibit pleochroism. Tourmaline crystallizes in the trigonal system to form prismatic crystals having a triangular cross section and characteristic vertical striations. Columnar masses (tourmaline “suns”) are also common.
Tourmaline has a hardness of 7–7.5 on Mohs’ scale and a density of 3,000–3,200 kg/m3. Because the mineral is piezoelectric, large iron-free crystals are used as hydrostatic pressure gauges; the crystals also find use in acoustics, optics, and radio engineering. The double refraction exhibited by tourmaline is used in tourmaline wedges, devices for demonstrating the polarization of light.
Tourmaline is a widespread endogenic high-temperature mineral; it is encountered in metasomatically transformed granites, mica schists, phyllites, and gneisses. The mineral is common in granitic pegmatites, hydrothermal deposits (copper-tourmaline), and greisens. It resists weathering and occurs in placer deposits.
Beautifully colored, transparent varieties of tourmaline (rubellite, bluish green Brazilian sapphire) are precious stones of order II. Deposits of precious varieties are found in the USSR (Urals, Transbaikalia), Brazil (Minas Gerais), Madagascar, and Sri Lanka. Tourmaline is obtained in the laboratory through hydrothermal synthesis.
L. V. BERSHOV