acanthite

{{infobox mineral | name = Acanthite | category = Sulfide mineral | boxwidth = | boxbgcolor = | image = Acanthite - Imiter mine, Jbel Saghro, Tinghir, Drâa-Tafilalet, Morocco.jpg | imagesize = 280px | alt = | caption = Crystallized acanthite (4.0 × 2.5 × 1.5 cm) from Imiter mine, Jbel Saghro mountain range, Morocco | formula = Ag2S |IMAsymbol=Aca | molweight = | strunz = 2.BA.30a | dana = | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P21/n | unit cell = a = 4.229 Å, b = 6.931 Å c = 7.862 Å; β = 99.61°; Z = 4 | color = Iron-black | colour = | habit = Primary crystals rare, prismatic to long prismatic, elongated along [001], may be tubular; massive. Commonly paramorphic after the cubic high-temperature phase ("argentite"), of original cubic or octahedral habit | twinning = Polysynthetic on {111}, may be very complex due to inversion; contact on {101} | cleavage = Indistinct | fracture = Uneven | tenacity = Sectile | mohs = 2.0–2.5 | luster = Metallic | streak = Black | diaphaneity = Opaque | gravity = 7.20–7.22 | density = | polish = | opticalprop = | refractive = | birefringence = | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | references = }} Acanthite is a form of silver sulfide with the chemical formula Ag2S. It crystallizes in the monoclinic system and is the stable form of silver sulfide below . 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.