Realgar

realgar [rē′al‚gär]

(mineralogy)

AsS A red to orange mineral crystallizing in the monoclinic system, having a resinous luster and found in short, vertical striated crystals; specific gravity is 3.48, and hardness is 1.5-2 on Mohs scale. Also known as red arsenic; red orpiment; sandarac.

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Realgar 

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.