CHALCOCITE, a mineral consisting of cuprous sulphide (Cu2S), crystallizing in the orthorhombic system. It is known also as copper-glance, redruthite and vitreous copper. The crystals have the form of six-sided tables or prisms which are usually twinned, with the result that they simulate hexagonal symmetry. The mineral more often occurs as compact masses, which are sometimes of considerable extent. The colour is dark lead-grey with a metallic lustre, but this is never very bright, since the material is readily altered, becoming black and dull on exposure to light. The mineral is soft (H. = 23) and sectile and can be readily cut with a knife; specific gravity 5.7.
Next to chalcopyrite, chalcocite is the most important ore of copper, and is perhaps the most characteristic mineral of the zone of secondary enrichment. It seems probable however that at some localities, e.g., Butte, Mont., it is of primary origin. The best crys tals are from St. Just, St. Ives and Redruth in Cornwall, and from Bristol in Connecticut.