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a mineral, arsenic monosulfide (AsS); contains 70.1 percent arsenic.
Realgar has a complex crystal structure. The lattice is constructed of individual As4S4 molecules, with the sulfur ions forming a square, and the arsenic ions a tetrahedron; the centers of the square and tetrahedron coincide. Realgar crystallizes in the monoclinic system, forming bright orange-red prismatic crystals. It occurs most commonly in the form of compact granular masses or powdery, earthy aggregates. The mineral has a density of 3,560–3,590 kg/m3 and is sectile; its hardness is 1.5–2 on Mohs’ scale.
Realgar is usually found together with orpiment, stibnite, and lead, silver, and gold ores of hydrothermal origin but may also occur as a sublimation product of volcanic eruptions or as a deposit from hot springs. When exposed to sunlight, it decomposes into orange to lemon-yellow orpiment, As2S3. Realgar is an important ore for the extraction of arsenic. In the USSR, deposits of realgar are found in Transcaucasia; there are also major deposits in such countries as Czechoslovakia, Rumania, Greece, the USA, and Japan.