compounds of copper and sulfur, Cu2S and CuS. Cu2S (cuprous sulfide) occurs as the grayish black mineral chalcocite, or copper glance, which has a density of 5.5-5.8 g/cm3; three modifications of this mineral are known. Upon heating, Cu2S oxidizes to form CuO and SO2 (or CuSO4). It is practically insoluble in water, dilute acids, and ammonia solutions but soluble in aqueous solutions of FeSO4, cyanide, and CuCl2. It dissolves in hot HNO3 with the liberation of elemental sulfur.
CuS (cupric sulfide) forms the bluish black mineral covellite, or indigo copper, which has a density of 4.68 g/cm3. When heated to temperatures above 450°C, it decomposes into Cu2S and sulfur. CuS readily undergoes oxidation. Black CuS is precipitated from slightly acidic copper salt solutions by H2S.
The pyrometallurgical methods of copper extraction are based on the marked affinity of Cu for S.