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topaz(tō`păz), aluminum silicate mineral with either hydroxyl radicals or fluorine, Al2SiO4(F,OH)2, used as a gem. It is commonly colorless or some shade of pale yellow to wine-yellow; pale blue and pale green also occur, but natural red stones are uncommon. Some natural yellow stones lose their yellow coloring when heated and become permanently pink ("pinked" topaz). The stone is transparent with a vitreous luster. It has perfect cleavage on the basal pinacoid, but it is nevertheless hard and durable. The brilliant cut is commonly used. Topaz crystals, which are of the orthorhombic system, occur in highly acid igneous rocks, e.g., granites and rhyolites, and in metamorphic rocks, e.g., gneisses and schists. Important sources of topaz are in Russia, Siberia, Brazil, Australia, and Mexico and in New Hampshire, Colorado, and Utah in the United States. The name topaz is commonly but incorrectly used for various other yellow stones, e.g., for citrine quartzquartz,
one of the commonest of all rock-forming minerals and one of the most important constituents of the earth's crust. Chemically, it is silicon dioxide, SiO2.
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a silicate mineral of the nesosilicate class with the chemical composition Al2[SiO4l (OH, F)2. The contents of OH– and F– vary, and the ions Fe3+, Cr3+, Ti3+, and V4+ may replace aluminum.
Topaz crystallizes in the orthorhombic system, usually forming prismatic crystals with perfect basal cleavage. The mineral’s structure is based on a four-layered arrangement of densely packed oxygen and fluorine atoms and OH– groups; Al atoms are located in the octahedral interstices, and Si atoms in the tetrahedral interstices. The distribution of fluorine and of OH– groups in the structure may or may not be ordered, a situation affecting the mineral’s optical constants. Topaz may be yellow, green, pale blue, or pink; colorless varieties are also encountered. Under the action of ionizing radiation (gamma radiation, X-radiation), the mineral acquires a smoky, dark yellow, or red color owing to the formation of structural defects. Topaz has a hardness of 8 on Mohs’ scale and a density of 3,400–3,640 kg/m3.
Topaz is found in granitic pegmatites and pneumatolytic deposits in association with feldspar, quartz, tourmaline, beryl, and micas. Transparent colorless and beautifully colored crystals of topaz are gems. Topaz rocks (greisens) are used as a refractory material. Deposits of topaz, which can be faceted, exist in the Ukraine (Volyn’), the Ural Mountains, and Transbaikalia and, outside the Soviet Union, in Brazil, Japan, and Madagascar.
L. V. BERSHOV