proustite

[After the French chemist, J. L. Proust; suff. -ite ( Min.).] Min .: An important silver ore occurring also in distinct crystals. Crystallization, rhombohedral. Hardne , 2-2-5; specific gravity, 5.42-5-56; luster, adamantine; color and streak, cochineal red; transparent to sub-translucent; fracture, uneven, conchoidal; brittle. Composition: Sulphur, 194; arsenic, 15-2; silver, 654=100; yielding the formula, 3AgS+AS2S3; isomorphous with pyrargyrite (q. v.). Found in many silver mines. A group of crystals from the mines of Chañarcillo, Copiapo, Chili, is stated to be unique for size and perfection of form.