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thenardite:see sodium sulfatesodium sulfate,
chemical compound, Na2SO4. It is a white, orthorhombic crystalline compound at ordinary temperatures; above 100°C; it assumes a monoclinic structure, and above about 250°C; it assumes a hexagonal structure.
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(named for the French chemist L. J. Thénard, 1777–1857), a mineral of the sulfate class, Na2SO4. Thenardite contains small amounts of K, Mg, Cl, Br, H2O, and, as a component of a mechanical mixture, CaSO4. It crystallizes in the ortho-rhombic system, whereas its high-temperature polymorph, me-tathenardite, crystallizes in the hexagonal system. The structure consists of a framework of Na polyhedrons surrounded and interconnected by SO4 tetrahedrons. The colorless transparent crystals have a dipyramidal or tabular habit; cruciform twinning and perfect cleavage are characteristic features. Thenardite occurs most commonly in the form of milky white granular aggregates. It is freely soluble in water and has a bitter, salty taste. Hardness on Mohs’ scale is 2–3, and the density is 2,680–2,690 kg/m3.
Thenardite is a chemogenic mineral formed, together with mi-rabilite, epsomite, and gypsum, in evaporating salt lakes; it is also formed upon dehydration of mirabilite. It precipitates from supersaturated solutions at temperatures above 32.4°C; in the presence of NaCl, however, it may crystallize at temperatures as low as 13.5°C. The mineral is also known as a product of fumar-ole activity.
There are thenardite deposits in, among other places, the USSR (Kara-Bogaz-Gol in the Turkmen SSR, the Mormyshan-skie lakes in the Kulunda Steppe in northeastern Kazakhstan), the United States (borax and alkali lakes in California and Nevada), and Canada. Thenardite is primarily used as raw material in the sodium-carbonate industry and in glassmaking.