tourmaline

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tourmaline A member of the cyclosilicates and a borosilicate Na(Mg,Fe2+,Mn,Li, Al)3Al6(BO3)3[Si6O18](OH,F)4. There are three important members of this family of minerals: dravite (NaMg3Al6(BO3)3[Si6O18](OH,F)4); schorl (Na(Fe2+,Mn)3Al6(BO3)3 [Si6O18](OH,F)4); and elbaite (Na(LiAl)3Al6 (BO3)3[Si6O18] (OH,F)4); sp. gr. 2.9–3.2; hardness 7.0–7.5; trigonal; black, bluish, pink, or green, never colourless; elongate crystals common, also acicular needles and massive or radiating aggregates; cleavage good {1120} prismatic; occurs in granite pegmatites, pneumatolytic (see PNEUMATOLYSIS) veins, and granites as elbaite and schorl varieties; it may occur in the rock luxullianite formed by pneumatolytic action after boron has been introduced, where it will occur with topaz, spodumene, cassiterite, fluorite, and apatite. Dravite variety occurs in metamorphosed impure limestones and rarely in some basic igneous rocks; tourmaline is a common detrital ‘heavy’ mineral in sedimentary rocks. Good multicoloured crystals can be used as gemstones.

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tourmaline (tŏŏr´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.

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tour·ma·line / ˈtoŏrmələn; -ˌlēn/ • n. a brittle gray or black mineral that occurs as prismatic crystals in granitic and other rocks. It consists of a boron aluminosilicate and has pyroelectric and polarizing properties, and is used in electrical and optical instruments and as a gemstone.

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tourmaline Silicate mineral, sodium or calcium aluminium borosilicate, found in igneous and metamorphic rocks. Its crystals are hexagonal system and glassy, either opaque or transparent. Some are prized as gems. Hardness 7.5; r.d. 3.1.

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tourmaline brittle pyro-electric mineral orig. from Ceylon. XVIII. — F., ult. f. Sinhalese toramalli cornelian.