Acanthite, Silver Locality: Colquechaca (Aullagas), Chayanta Province, Potosí Department, Bolivia (Locality at mindat.org) Size: small cabinet, 7.6 x 6.1 x 5.1 — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Acanthite
Also known as: Silver glance, Argentite (above 177 °C).
The common low-temperature form of silver sulfide, stable below 173 °C; above that temperature the same compound becomes argentite.
Intro adapted from Wikipedia (CC BY-SA).
Acanthite is an IMA-recognized mineral species in the sulfide class, with the chemical formula Ag₂S.
It crystallizes in the monoclinic crystal system (space group P2_1/n). It is a member of the Acanthite structural group.
Key properties: Mohs hardness 2–2.5; specific gravity 7.2–7.22; typically gray or black; Black streak; Metallic. Anisotropism: Weak. R: (400) 32.8, (420) 32.9, (440) 33.0, (460) 33.1, (480) 33.0, (500) 32.7, (520) 32.0 luster.
IMA symbol Aca, grandfathered status — known before the IMA’s founding, named in 1855.
A common silver species in moderately low-temperature hydrothermal sulfide veins, and in zones of secondary enrichment.
It is found associated with Silver, pyrargyrite, proustite, polybasite, stephanite, aguilarite, galena, chalcopyrite, sphalerite, calcite, quartz.
Its name comes from the Greek for thorn, in allusion to the shape of the crystals.
Collections: Metals & Native Elements, Collector Favorites.