ERUBESCITE, a native copper-iron sulphide, Cu5FeS4, 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. Compact and granular masses are of more frequent occurrence. The colour on a freshly fractured surface is bronzy or 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. Bornite (after Baron Ignaz von Born, b. 1742, d. 1791) is a name in common use for this mineral, and it predates erubescite, the name given by J. D. Dana in 1850, but afterwards rejected by him; French authors use the name phillipsite, after the English mineralogist, R. Phillips, who analysed the mineral; both these earlier names had, however, been previously used for other minerals.
Owing to the frequent presence of mechanically admixed chalcopyrite and chalcocite, the published analyses of erubescite show wide variations, the copper, for example, varying from 50 to 70%. Even the best Cornish crystals enclose a nucleus of chalcopyrite (CuFeS2), and an analysis of these made in 1839 led to the long-accepted formula Cu3FeS3. Recently, B. J. Harrington has analysed carefully selected material and obtained the formula Cu5FeS4.
Erubescite occurs in copper-bearing veins, and has been mined as an ore of copper at Redruth in Cornwall, Montecatini in the province of Pisa, Tuscany, Bristol in Connecticut, Acton in Canada, and other localities in North America. The best crystallized specimens are from the Carn Brea mine and other copper mines in the neighbourhood of Redruth, and from Bristol in Connecticut. Recently a few large isolated crystals with the form of icositetrahedra have been found with calcite and albite in a gold-vein on Frossnitz-Alpe in the Gross-Venediger, Tirol. (L. J. S.)