proustite

{{infobox mineral | name = Proustite | category = Sulfosalt minerals | boxwidth = | boxbgcolor = | image = Proustite-209739.jpg | imagesize = 260px | alt = | caption = | formula = Ag3AsS3 | IMAsymbol = Prs | molweight = | strunz = 2.GA.05 Neso-sulfarsenites | dana = 03.04.01.01 Proustite group | system = Trigonal | class = Hexagonal scalenohedral (m) H-M Symbol: ( 2/m) | symmetry = Rc | unit cell = a = 10.79 Å, c = 8.69 Å; Z = 6 | color = Scarlet-vermilion | colour = | habit = Crystals prismatic and scalenohedral, massive, compact | twinning = Common | cleavage = Distinct on {101} | fracture = Conchoidal to uneven | tenacity = Brittle | mohs = 2 – 2.5 | luster = Adamantine | streak = Vermilion | diaphaneity = Translucent, darkens when exposed to light | gravity = 5.57 measured, 5.625 calculated | density = | polish = | opticalprop = Uniaxial (−) | refractive = nω = 3.087 – 3.088 nε = 2.792 | birefringence = δ = 0.295 – 0.296 | pleochroism = Moderate; cochineal-red to blood-red | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | references = }} Proustite is a sulfosalt mineral consisting of silver sulfarsenide, Ag3AsS3, known also as ruby blende, light red silver, arsenic-silver blende or ruby silver ore, and an important source of the metal. It is closely allied to the corresponding sulfantimonide, pyrargyrite, from which it was distinguished by the chemical analyses of Joseph L. Proust (1754–1826) in 1804, after whom the mineral received its name.

The prismatic crystals are often terminated by the scalenohedron and the obtuse rhombohedron, thus resembling calcite (dog-tooth-spar) in habit. The color is scarlet-vermilion and the luster adamantine; crystals are transparent and very brilliant, but on exposure to light they soon become dull black and opaque. The streak is scarlet, the hardness 2 to 2.5, and the specific gravity 5.57. Its transparency differs from specimen to specimen, but most are opaque or translucent.