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.

[4] It occurs as secondary mineral in the oxidized zone of copper sulfide deposits.

Associated minerals include olivenite, cornubite, arthurite, clinoclase, chalcophyllite, strashimirite, lavendulan, tyrolite, spangolite, austinite, conichalcite, brochantite, azurite and malachite.