Copper Sulfides

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.