BORNITE, a copper-iron sulphide, of importance as an ore of copper. It crystallizes in the cubic system, the usual form being that of interpenetrating cubes twinned on an octahedral plane. The faces are usually curved and rough, and the crystals confusedly aggregated together. The colour on a freshly fractured surface is coppery, but in moist air this rapidly tarnishes with iridescent blue and red colours ; hence the names purple copper ore, variegated copper ore (Ger. Buntkupfererz), horse-flesh ore and erubescite (from the Lat. erubescere, "to grow red"). The lustre is metallic, and the streak greyish-black ; hardness 3; sp. gr. 5.0. Owing to the frequent presence of mechanically admixed chalcopyrite and chalcocite the published analyses of bornite show wide variations, the copper, for example, varying from 5o to 70%. Bornite occurs in copper-bearing veins as one of the most impor tant minerals of the zone of secondary enrichment and has been mined as an ore of copper at Redruth in Cornwall, Montecatini in Tuscany, Bristol in Connecticut, Acton in Canada, etc.
(L. J. S.)