Chalcopyrite

Chalcopyrite (/ˌkælkəˈpaɪˌraɪt, -koʊ-/ KAL-kə-PY-ryte, -⁠koh-) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS₂ and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green-tinged black.

On exposure to air, chalcopyrite tarnishes to a variety of oxides, hydroxides, and sulfates. Associated copper minerals include the sulfides bornite (Cu₅FeS₄), chalcocite (Cu₂S), covellite (CuS), digenite (Cu₉S₅); carbonates such as malachite and azurite, and rarely oxides such as cuprite (Cu₂O). It is rarely found in association with native copper. Chalcopyrite is a conductor of electricity.

Copper can be extracted from chalcopyrite ore using various methods. The two predominant methods are pyrometallurgy and hydrometallurgy, the former being the most commercially viable.

The name chalcopyrite comes from the Greek words chalkos, which means copper, and pyrites, which means striking fire. It was sometimes historically referred to as "yellow copper".

Chalcopyrite is often confused with pyrite and gold since all three of these minerals have a yellowish color and a metallic luster. Some important mineral characteristics that help distinguish these minerals are hardness and streak. Chalcopyrite is much softer than pyrite and can be scratched with a knife, whereas pyrite cannot be scratched by a knife. However, chalcopyrite is harder than gold, which, if pure, can be scratched by copper. Additionally, gold is malleable, while chalcopyrite is brittle. Chalcopyrite has a distinctive black streak with green flecks in it. Pyrite has a black streak and gold has a yellow streak.

Natural chalcopyrite has no solid solution series with any other sulfide minerals. There is limited substitution of zinc with copper despite chalcopyrite having the same crystal structure as sphalerite.

Minor amounts of elements such as silver, gold, cadmium, cobalt, nickel, lead, tin, and zinc can be measured (at parts per million levels), likely substituting for copper and iron. Selenium, bismuth, tellurium, and arsenic may substitute for sulfur in minor amounts. Chalcopyrite can be oxidized to form malachite, azurite, and cuprite.

Chalcopyrite is a member of the tetragonal crystal system. Crystallographically the structure of chalcopyrite is closely related to that of zinc blende ZnS (sphalerite). The unit cell is twice as large, reflecting an alternation of Cu⁺ and Fe³⁺ ions replacing Zn²⁺ ions in adjacent cells. In contrast to the pyrite structure chalcopyrite has single S²⁻ sulfide anions rather than disulfide pairs. Another difference is that the iron cation is not diamagnetic low spin Fe(II) as in pyrite.