Chalcopyrite

Chalcopyrite (CuFeS₂) is a mixed sulfide of copper and iron that crystallizes in the tetragonal system, although it often exhibits pseudocubic forms that make it easily confused with pyrite. Its hardness of 3.5–4 on the Mohs scale and characteristic brittleness distinguish it from harder minerals. It forms in highly diverse geological contexts: in high-temperature hydrothermal environments as a primary mineral associated with magnetite and bornite, in granitic pegmatites, and especially in porphyry copper deposits where it represents the principal mineral of economic interest. Chalcopyrite is also notable for the phenomenon of iridescence—a changeable blue-violet coloration that appears on the oxidized surface—due to the formation of thin layers of secondary oxides. Historically, it was confused with gold by inexperienced prospectors, earning it the nickname "fool's gold." Its economic importance is enormous: from a single kilogram of chalcopyrite, approximately 340 grams of pure copper can be extracted.

Chalcopyrite crystallizes in the tetragonal system, space group I4̄2d, with lattice parameters a = 5.29 Å and c = 10.44 Å. The structure is derived from sphalerite with doubling of the unit cell along the c-axis due to alternating ordering of Cu and Fe in tetrahedral sites. Density: 4.1–4.3 g/cm³. Refractive index (reflected light): marked anisotropy with color variations from yellow-bronze to copper-yellow. Semiconducting electrical conductivity (p-type). Raman spectroscopy shows characteristic bands at ~290 cm⁻¹ (Cu-S stretching) and ~350 cm⁻¹ (Fe-S stretching). XRD analysis allows distinction from pyrite (cubic, a = 5.42 Å) and bornite (cubic, a = 10.94 Å). Chalcopyrite is paramagnetic at room temperature. Under surface oxidation it forms covellite (CuS) and chalcocite (Cu₂S). Typical mineral associations: magnetite, hematite, bornite, pyrite, sphalerite, galena, quartz. Soluble in dilute nitric acid with formation of yellow elemental sulfur.