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 bronzeyellow, owing to tarnish or to the presence of intimately admixed 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 Zinnwald in Bohemia it occurs with blende and galena, and in Bolivia with silver ores. (L. J. S.)