Azurite

Azurite (Cu₃(CO₃)₂(OH)₂) is a secondary mineral that forms in the superficial oxidizing environment of copper deposits, where acidic solutions percolate through rocks containing primary copper minerals such as chalcocine and chalcocite. Its intense blue color, due to the presence of Cu²⁺ ions, has made it famous since antiquity: the Greeks and Romans pulverized it to obtain the precious blue pigment for frescoes and manuscripts. It crystallizes in the monoclinic system, developing prismatic or tabular habits, sometimes accompanied by malachite (Cu₂(CO₃)(OH)₂), its green mineral twin, with which it forms characteristic mineralogical associations. Its modest hardness (3.5–4 on the Mohs scale) and fragility due to perfect cleavage make it delicate: specimens of gemmological quality are rare and highly sought after by international mineral collectors.

Crystal system: monoclinic, space group P2₁/c. Unit cell parameters: a = 4.98 Å, b = 5.95 Å, c = 9.97 Å, β = 92.4°. Hardness: 3.5–4 Mohs; relative density: 3.77–3.89 g/cm³. Refractive index: nα = 1.730, nβ = 1.758, nγ = 1.838 (birefringence Δn ≈ 0.108, strong). Pleochroism: intense blue to light blue. Perfect cleavage along (11), high fragility. Soluble in dilute acids with effervescence (carbonate test). UV spectroscopy: weak fluorescence. Typical mineral associations: malachite, limonite, calcite, quartz. Formation: supergene alteration in oxidation zones of copper deposits (copper porphyries, veins). Decomposition temperature: ~220 °C (loss of H₂O), ~350 °C (conversion to malachite).