STANNITE  , a rare

mineral consisting of tin, copper and iron sulphide (a sulpho-stannate, Cu2FeSnS4), containing, when pure, tin 27.5, copper 29.5% . It has a metallic lustre, and, when pure, is iron-black in colour: more often, however, it is bronze-yellow, owing to tarnish or to the presence of intimately ad-mixed chalcopyrite: for this reason it is known to miners as " bell-metal-ore " or as " tin pyrites." The hardness is 32 and the specific gravity 4.45 . It usually occurs as granular to compact masses, rarely as crystals . Minute crystals from Bolivia have been shown to be tetragonal and hemihedral, like chalcopyrite; and to be invariably twinned, giving rise to pseudocubic forms . The mineral has been found in a number of Cornish tin mines, and was formerly worked to a limited extent as an ore . At Zinnlvald in Bohemia it occurs with blende and galena, and in Bolivia with silver ores . (L . J .