Proustite: Information & Facts

Proustite, a sulfosalt mineral, silver arsenic sulfide (Ag₃AsS₃), that is an important source of silver.

Proustite is one of only an few sulfides that are not metallic or opaque. In fact it is at times cut for gemstones, although it is far too soft to wear in everyday jewelry. Its color is a bright scarlet red to reddish orange and is distinctive. The nickname "Ruby Silver" has been applied to proustite as well as the closely related mineral pyrargyrite.

Pyrargyrite is a silver antimony sulfide and is isostructural with proustite. Isostructural means that the two minerals have the same structure but a different chemistry. There is some substitution of the antimony and arsenic. But in both minerals, the substitution is minor and no solid solution is believed to exist between the two minerals.

Proustite: Information & Facts. Beautiful Proustite Crystals From: Imiter Mine, Imiter, Djebel Saghro, Ouarzazate, Souss-Massa-Draâ, Morocco. Photo Credit: Christian Rewitzer

Proustite is the rarer of the two minerals and is usually found in the same ore veins with pyrargyrite, silver and other silver sulfides. Fine crystals, with their transparency, luster and color, are very attractive mineral specimens. However, as with other silver minerals, it is reactive to light and can form a white coating upon exposure. This coating can be wiped off, but fine specimens should be stored in closed containers with exposure to light limited.

Proustite from O'Brien mine, Cobalt, Ontario, Canada Photo: Haubi

Properties of  Proustite

Class: Sulfides
Subclass: Sulfosalts
Formula: Ag₃AsS₃
System: Trigonal
Colour: Scarlet, Vermilion or ...
Lustre: Adamantine
Hardness: 2 - 2½
Specific Gravity is approximately 5.6 (very heavy for translucent minerals)
Streak is red.
Associated Minerals include pyrargyrite, silver, tetrahedrite, calcite, quartz, galena, sphalerite and other silver vein minerals.
Other Characteristics: A white film forms on the surface of crystals upon exposure to light.
Notable Occurrences include Atacama, Chile; silver mines in Saxony Germany; Poorman Mine, Idaho and Chihuahua, Mexico.
Best Field Indicators are crystal habit, density, association with pyrargyrite and color.

Name: Named in honor of Joseph Louis Proust (1754–1826), a celebrated French chemist who was able to prove that substances combine in constant and definite proportions.

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.