7 Photos
Acanthite
Chemical Formula	Ag2S
Species	Sulfides
Crystal System	Monoclinic
Mohs Scale	2
Specific Gravity	7.20-7.22
Color	Iron-black
Streak	Black
Luster	Metallic
Diaphaneity	Opaque
Cleavage	None Observed
Fracture	Sub-Conchoidal
Crystal Habit：Primary crystals rare, prismatic to long prismatic, elongated along , may be tubular; massive. Commonly paramorphic after the cubic high-temperature phase (“argentite”), of original cubic or octahedral habit
Geological Setting：A common silver species in moderately low-temperature hydrothermal sulphide veins, and in zones of secondary enrichment. Widespread in silver deposits. Localities for fine primary and paramorphic crystals include: Jáchymov (St Joachimsthal), Czech Republic [TL]; In Germany, at Freiberg, Schneeberg, Annaberg, and Marienberg, Saxony; and from St Andreasberg, Harz. In Mexico, large paramorphs from Arizpe, Sonora; In the Rayas and other mines at Guanajuato; And from many mines in Zacatecas, Chihuahua, etc; In the USA, at Butte, Silver Bow Co., Montana; Tonopah, Nye Co. and the Comstock Lode, Virginia City, Storey Co., Nevada; From various mines at Cobalt, Ontario, Canada; At Cha?arcillo, south of Copiapó, Atacama, Chile.

Acanthite, Ag₂S, crystallizes in the monoclinic system and is the stable form of silver sulfide below 173 °C. Argentite is the stable form above that temperature. As argentite cools below that temperature its cubic form is distorted to the monoclinic form of acanthite. Below 173 °C acanthite forms directly. Acanthite is the only stable form in normal air temperature.

Occurrence

Acanthite is a common silver mineral in moderately low-temperature hydrothermal veins and in zones of supergene enrichment. It occurs in association with native silver, pyrargyrite, proustite, polybasite, stephanite, aguilarite, galena, chalcopyrite, sphalerite, calcite and quartz.

Acanthite was first described in 1855 for an occurrence in the Jáchymov (St Joachimsthal) District, Kru?né Hory Mts (Erzgebirge), Karlovy Vary Region, Bohemia, Czech Republic. The name is from the Greek "akantha" meaning thorn or arrow, in reference to its crystal shape.

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