Cornubite, AzuriteCu5(AsO4)2(OH)4, Cu3(CO3)2(OH)2Locality:Cap…

Cornubite, AzuriteCu5(AsO4)2(OH)4, Cu3(CO3)2(OH)2Locality:Cap…

Cornubite, Azurite

Cu₅(AsO₄)₂(OH)₄, Cu₃(CO₃)₂(OH)₂

Locality:

Cap Garonne Mine, Le Pradet, Var, Provence-Alpes-Côte d’Azur, Franc

Field of View: 4 mm

Green cornubite spheres with blue azurite.

Collection and photo: Stephan Wolfsried

Cornubite is a rare secondary copper arsenate mineral.  It is green, as are many copper minerals, usually translucent, fairly soft with a vitreous luster and a light green streak. Cornubite is a secondary mineral found in the oxidised zone of copper deposits.

Azurite owes its name to its beautiful azure-blue color, which makes it a very popular and well-known mineral. Azurite is unstable in open air with respect to malachite, and often is pseudomorphically replaced by malachite. This weathering process involves the replacement of some the carbon dioxide (CO₂) units with water (H₂O), changing the carbonate: hydroxide ratio of azurite from 1:1 to the 1:2 ratio of malachite:

2 Cu₃(CO₃)₂(OH)₂ + H₂O → 3 Cu₂(CO₃)(OH)₂ + CO₂

Azurite is not a useful pigment because it is unstable in air. It was however used as a blue pigment in antiquity. Depending on the degree of fineness to which it was ground, and its basic content of copper carbonate, it gave a wide range of blues. When mixed with oil it turns slightly green. When mixed with egg yolk it turns green-grey. Older examples of azurite pigment may show a more greenish tint due to weathering into malachite.