Cornwallite Locality: Pastrana , Mazarrón-Águilas, Murcia , Spain ( Locality at mindat.org ) Size: 4.3 x 3.7 x 2.2 cm. Cornwallite is an uncommon Copper Arsenate which gets its name from the classic English mining district. This specimen is from Spain and features some small yet beautiful emerald green Cornwallite aggregates on matrix. — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Cornwallite
Cornwallite is an uncommon copper arsenate mineral with formula Cu5(AsO4)2(OH)4. It forms a series with the phosphate pseudomalachite and is a dimorph of the triclinic cornubite. It is a green monoclinic mineral which forms as radial to fibrous encrustations.
Intro adapted from Wikipedia (CC BY-SA).
Cornwallite is an IMA-recognized mineral species in the arsenate class, with the chemical formula Cu₅(AsO₄)₂(OH)₄.
It crystallizes in the monoclinic crystal system (space group P2_1/c).
Key properties: Mohs hardness 4.5; specific gravity 4.17; refractive index 1.839; optically biaxial.
IMA symbol Cnw, grandfathered status — known before the IMA’s founding, named in 1847.
Found as a rare secondary mineral in oxidized copper deposits.
It is found associated with Olivenite, cornubite, arthurite, clinoclase, chalcophyllite, strashimirite, lavendulan, tyrolite, spangolite, austinite, conichalcite, brochantite, azurite.
The name refers to its occurrence in the County of Cornwall, England.