A basic carbonate of copper with the chemical formula Cu₃(OH)₂(CO₃)₂. Azurite is normally associated with copper ores and often occurs with malachite. Azurite is monoclinic. It may be massive or may occur in tabular, prismatic, or equant crystals (see illus.). Invariably blue, azurite was originally used extensively as a pigment. Hardness is 3.5–4 (Mohs scale) and specific gravity is 3.8. It can be synthesized by gentle heating of cupric nitrate or sulfate solutions with calcium carbonate in a closed tube. Notable localities for azurite are at Tsumeb, Southwest Africa, and Bisbee, Arizona.