Chalcopyrite

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Chalcopyrite (CuFeS₂) is the principal ore mineral of copper. The gold colour, metallic lustre, hardness (3.5) and high density are the chief diagnostic properties. Chalcopyrite is often artificially treated with sulphuric acid to cause an iridescent blue/purple tarnish and then sold as “peacock ore”. However, natural chalcopyrite rarely has this tarnish. The term “peacock ore” more formally refers to bornite (Cu₅FeS₄), a related copper-iron sulphide.