Proustite

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Proustite
Proustite - Chanarcillo, Copiapo Province, Atacama Region, Chile.jpg
Proustite on matrix, crystal size: 1 cm, Chañarcillo district, Chile
General
Category Sulfosalt mineral
Formula
(repeating unit)
Ag3AsS3
Strunz classification 02.GA.05 Neso-sulfarsenites
Dana classification 03.04.01.01 Proustite group
Crystal symmetry H-M Symbol (32/m)
Unit cell a = 10.79 Å, c = 8.69 Å Z = 6
Identification
Color Scarlet-vermilion
Crystal habit Crystals prismatic and scalenohedral, massive, compact
Crystal system Trigonal - Hexagonal Scalenohedral Space Group: R 3c
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 [1][2][3]

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.[1]

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.

See also[edit]

References[edit]

Public Domain This article incorporates text from a publication now in the public domainChisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.