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Dominant species
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Proustite
Proustite
Chemical
Formula
Ag3AsS3
Species
Sulfates
Crystal
System
Trigonal
Mohs
Scale
2
Specific
Gravity
5.625
Color
Scarlet-vermilion
Streak
Vermilion
Luster
Adamantine
Refractive
Index
n = 3.087 - 3.088 n = 2.792
Diaphaneity
Translucent, darkens when exposed to light
Cleavage
Distinct/GoodDistinct on
Fracture
Irregular/Uneven, Conchoidal
Crystal Habit:Crystals prismatic and scalenohedral, massive, compact
Geological Setting:A late forming mineral in hydrothermal veins, also in the supergene zone.
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 Cha?arcillo in Chile; other localities which have yielded fine specimens are Freiberg and Marienberg in Saxony, Joachimsthal in Bohemia and Markirch in Alsace.