Abstract: Roxbyite (Cu_1.74–1.82S), a new mineral, was isolated from drill core from the Olympic Dam deposit, from copper flotation concentrates from the Olympic Dam pilot plant, and from reaction residues obtained by treating certain concentrates with sulphuric acid. Powder X-ray diffraction data and the composition of the mineral are similar to results reported previously for the unnamed mineral of composition Cu_1.83S (with a trace of iron) from the El Teniente deposit in Chile, and to copper sulphides of composition Cu_xS, 1.75 ⩽ x ⩽ 1.86, prepared electrochemically by several workers. A Weissenberg single-crystal study shows roxbyite to be monoclinic with a = 53.79, b = 30.90, c = 13.36Å, β = 90.0° and space group alternatives C2/m, Cm or C2. Some iron in the structure may stabilize the mineral.