Cu₂Fe₂S₄

Cuprous ferric sulphide, Cu₂S.Fe₂S₃, or Cu₂Fe₂S₄, occurs in nature as copper pyrites, chalcopyrite, or towanite, and is one of the commonest ores of copper. It is tetragonal, possessed of a brass-yellow colour, and exhibits a conchoidal fracture. It is decomposed by nitric acid, and tarnishes upon exposure to air, frequently yielding beautiful iridescent surfaces, a blue colour predominating. Masses of such tarnished ore are found in Cornwall, and are known as peacock ore. The blue colour is probably due to the formation of a surface layer of cupric sulphide or covellite, CuS.

Chalcopyrite may be distinguished from iron pyrites by its relative softness, and by the fact that with nitric acid it yields a green solution which becomes blue on addition of excess of ammonia - one of the characteristic reactions for copper. Blistered copper ore is a botryoidal or reniform variety, with a smooth brassy appearance.

Small crystals of artificial copper pyrites are obtained by the action of hydrogen sulphide upon a mixture of copper oxide and ferric oxide gently warmed in a glass tube.

Copper pyrites may be obtained in the laboratory in the wet way by shaking a weakly ammoniacal solution of cuprous chloride with potassium ferric sulphide until the solution no longer contains copper: -

K₂S.Fe₂S₃ + 2CuCl = 2KCl + Cu₂S.Fe₂S,

The crystals obtained are practically identical with the natural copper pyrites.

Another method consists in heating copper carbonate and ferric sulphate with water containing hydrogen sulphide. The reaction is carried out in a closed tube, and the heating prolonged for several days.

The internal structure of copper pyrites, CuFeS₂, has been studied by means of their X-ray interference phenomena.