Wiktionary
cornwallite
n. (context mineral English) A basic copper arsenate mineral with chemical formula Cu5(AsO4)2(OH)4.
Wikipedia
Cornwallite
Cornwallite is an uncommon copper arsenate mineral with formula Cu(AsO)(OH). 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.