PROUSTITE  , a

mineral consisting of silver sulpharsenite, AgaAsS3, known also as light red silver ore, and an important source of the metal . It is closely allied to the corresponding sulphantimonite, pyrargyrite, from which it was distinguished by the chemical analyses of J . L . Proust in 1804, after whom the mineral received its name . Many of the characters being so similar to those of pyrargyrite (q.v.) they are mentioned under that species . The prismatic crystals are often terminated by the scalenohedron 12011 and the obtuse rhombohedron { to}, thus resembling calcite (dog-tooth-spar) in habit . The colour is scarlet-vermilion and the lustre adamantine; crystals are trans-parent and very brilliant, hut on exposure to light they soon become dull black and opaque . The streak is scarlet, the hardness 22, and the specific gravity 5'57 . The mode of occurrence is the same as that of pyrargyrite, and the two minerals are sometimes found together . Magnificent groups of large crystals have been found at Chafiarcillo in Chile; other localities which have yielded fine specimens are Freiberg and Marienberg in Saxony, Joachimsthal in Bohemia and Markirch in Alsace . (L . J .