Proustite, tricky reading of such a beautiful mineral that consists of sulfur, arsenic and silver (Ag3AsS3). However, its color as red as Hugo Chavez' flag is as magnificent as a rubi, or as a red garnet (pirope), or as a cuprite, a beauty just out of this world. Just see the picture on your right, which by the way, is a specimen from Chañarcillo, in Atacama, Chile. Among other features, the rarity of proustite is that being a sulfide, the crystals are not metallic, but semi-metallic, and glassy and weak as a wulfenite, or a rodochrosite.
Fine proustite crystals, with their transparency, luster and color, are very attractive mineral specimens. However, just as with other silver minerals, it is sensitive to light, and it can form a white coating upon light exposure. This coating can be cleaned off, but fine specimens should be stored in closed containers with limited or no light exposure.
Proustite is usually found in the same ore veins with pyrargyrite (consisting of sulfur, antimony and silver), and other silver sulfides. Proustite and Pyrargyrite are iso-structural, in other words, both minerals have the same structure but with a different chemistry (proustite has antimony, and pyrargyrite has arsenic).
Chemical Formula: Ag3AsS3
Molecular Weight = 494.72 gm
Composition: Silver 65.41%; Arsenic 15.14%; Sulfur 19.44%
Environment: Late forming mineral in hydrothermal deposits, in the oxidized and enriched zone, associated with other silver minerals and sulfides.
Locality: Himmelsfurst mine, Erbisdor, near Freiberg, Germany; Chañarcillo mine in Atacama, Chile; Chihuahua, Mexico.
Name Origin: After the French chemist, J. L. Proust (1755-1826).
Crystal System: Trigonal - Hexagonal Scalenohedral
Color: Vermilion, Reddish gray.
Density: 5.5 - 5.6, Average = 5.55
Diaphaniety: Transparent to translucent
Fracture: Brittle - Sectile - Brittle fracture with slightly sectile shavings possible.
Habit: Blocky - Crystal shape tends to be equant (e.g. feldspars).
Habit: Crystalline - Poor - Occurs primarily as crudely formed crystals.
Habit: Massive - Uniformly indistinguishable crystals forming large masses.
Hardness: 2-2.5 - Gypsum-Finger Nail
Luster: Sub Metallic
Streak: Vermilion redHugo Chavez