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arsenic trisulfide, AS2S3. Orpiment contains 60.91 percent arsenic and 39.09 percent sulfur. It has a monoclinic system. The crystals are prismatic; rodlike and columnar aggregates, as well as powdery, nodular, and clustered bodies, are characteristic. Orpiment is easily cut with a knife. Its color ranges from golden- and lemon-yellow to orange-yellow, sometimes brownish. Its luster is oily to diamondlike; on the cleavage planes it is like mother-of-pearl, semimetallic, and tarnishes rapidly. Its hardness on the mineralogical scale is 1.5–2. Its density is 3,490 kg per cu m. It is soluble in aqua regia and bases. It is a nonconductor and diamagnetic. During heating it becomes red. Orpiment is found in low-temperature, hydrothermic deposits in association with realgar, antimony, and other substances, as well as in deposits of hot mineral springs and in volcanic encrustations. It is the raw material from which arsenic compounds are obtained.
A. B. PAVLOVSKII