Chemical Formula: Ag3AsS3
Locality: Himmelsfurst mine, Erbisdor, near Freiberg, Germany
Name Origin: After the French chemist, J. L. Proust (1755-1826).
Proustite is a sulfosalt mineral consisting of; silver sulfarsenide, Ag3AsS3, known also as light red silver 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 lustre 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 2.5, and the specific gravity 5.57.
Proustite occurs in hydrothermal deposits as a phase in the oxidized and supergene zone. I is associated with other silver minerals and sulfides such as native silver, native arsenic, xanthoconite, stephanite, acanthite, tetrahedrite and chlorargyrite.
Magnificent groups of large crystals have been found at Chañarcillo in Chile; other localities which have yielded fine specimens are Freiberg and Marienberg in Saxony, Joachimsthal in Bohemia and Markirch in Alsace.
Optical and misc. Properties: Transparent – Translucinte
Reflective Power : 28,2-30,3% (580)
Refractive Index: from 2,79 to 3,08
Hardness: from 2,00 to 2,50
Density : 5,57
Color : red; cinnabar-red; reddish grey
Luster : adamantine; submetallic
Streak : brick-red; brownish; pale red
Break : conchoidal; irregular
Cleavage : yes