Acanthite
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(ə′kan·thīt)
(mineralogy) Ag2S A blackish to lead-gray silver sulfide mineral, crystallizing in the orthorhombic system.
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The mineral name applied to the monoclinic form of silver sulfide (Ag2S) that is stable at room temperature. Two different crystalline structures are assumed in succession as temperature is increased. At a temperature of about 176–178°C (349–352°F), depending on stoichiometry, acanthite transforms rapidly to argentite, which is body-centered cubic, space group Im 3m. The cell contains two Ag2S units and has a lattice constant that ranges from 0.4860 nm at 186°C (367°F) to 0.4889 nm at 325°C (617°F). At 586–620°C (1087–1148°F), again depending on stoichiometry, argentite transforms to a face-centered cubic structure. The details of the atomic arrangement in this phase are not known.
Acanthite is an important silver ore. Notable deposits in the United States include Butte, Montana; Aspen and Leadville, Colorado; and the Comstock Lode in Nevada. The mineral is also common in Mexican mines at Guanajuato. Other major deposits occur in Bolivia, Peru, and Chile. Especially fine crystals have been found in Saxony and in the Harz Mountains in Germany. Silver sulfide is encountered in everyday life as the black tarnish that develops on silver objects. See also Crystal structure.
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