Proustite

Proustite
Proustite on matrix, crystal size: 1 cm, Chañarcillo district, Chile
General
Category	Sulfosalt mineral
Formula (repeating unit)	Ag3AsS3
Strunz classification	2.GA.05 Neso-sulfarsenites
Dana classification	03.04.01.01 Proustite group
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M Symbol: (3 2/m)
Space group	R3c
Unit cell	a = 10.79 Å, c = 8.69 Å; Z = 6
Identification
Color	Scarlet-vermilion
Crystal habit	Crystals prismatic and scalenohedral, massive, compact
Twinning	Common
Cleavage	Distinct on {1011}
Fracture	Conchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	2 – 2.5
Luster	Adamantine
Streak	Vermilion
Diaphaneity	Translucent, darkens when exposed to light
Specific gravity	5.57 measured, 5.625 calculated
Optical properties	Uniaxial (-)
Refractive index	nω = 3.087 - 3.088 nε = 2.792
Birefringence	δ = 0.295 - 0.296
Pleochroism	Moderate; cochineal-red to blood-red
References

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 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 2.5, and the specific gravity 5.57.

Proustite occurs in hydrothermal deposits as a phase in the oxidized and supergene zone. It 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 Sainte-Marie-aux-Mines in Alsace.