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(also copper glance), a sulfide mineral, Cu2S. Chalcocite usually contains admixtures of Ag; sometimes it contains Fe, Co, Ni, and As. A considerable copper deficit is occasionally observed in its composition.
Three polymorphic modifications of chalcocite are known. The most common is the orthorhombic modification, which at a temperature of 103°C alters to the hexagonal modification. Above 425°C, hexagonal chalcocite alters to the isometric modification. The structure of orthorhombic and hexagonal chalcocite is based on an extremely dense hexagonal packing of the sulfur atoms. The copper atoms occupy triangular vacancies in the plane of the densely packed “sulfur” layers.
Chalcocite is found in the form of fine-grained aggregates or phenocrysts in ores and rock. Crystals are rare, but hexagonal twins are common. Chalcocite is lead gray in color and has a metallic luster. An opaque and brittle mineral, it has a hardness of 2.5–3 on Mohs’ scale and a density of 5,780 kg/m3. It is a good conductor of electricity and an ore of copper.
The most valuable commercial accumulations of chalcocite are associated with the zones of oxidation of copper deposits, where chalcocite is found in association with bornite, chalcopyrite, and other sulfides (deposits at Bisbee, Ariz., and Butte, Mont., in the United States, at Braden and Chuquicamata in Chile, at Tsumeb in southwestern Africa, and at Dzhezkazgan in the USSR). Commercial concentrations of chalcocite occur in copper sandstones and shales. When it oxidizes at the earth’s surface, chalcocite forms cuprite, malachite, azurite, native copper, and other copper minerals.
REFERENCESMineraly: Spravochnik, vol. 1. Moscow, 1960.
Ramdohr, P. Rudnye mineraly i ikh srastaniia. Moscow, 1962. (Translated from German.)
Yund, R. A., and G. Kullerud. “Thermal Stability of Assemblages in the Cu—Fe—S System.” Journal of Petrology, 1966, vol. 7, no. 3.
IU. K. VOROB’EV