Formula: Cu5(AsO4)2(OH)4
Anhydrous arsenate containing hydroxyl, dimorph of cornubite
Forms a series with pseudomalachite
Specific gravity: 4.52
Hardness: 4½
Streak: Apple green
Colour: Verdigis green, blackish-green, emerald-green; emerald-green in transmitted light.
Solubility: Decomposed in oils containing As2S3. Soluble in nitric acid.

Hydrothermal environments

Cornwallite is a rare secondary mineral formed by the oxidation of ore containing both copper and arsenic (e.g, tennantite). At the type locality, Wheal Carpenter, Cornwall, England it occurs in copper bearing sulphide veins associated with olivenite, cornubite, arthurite, clinoclase, chalcophyllite, strashimirite, lavendulan, tyrolite, spangolite, austinite, conichalcite, brochantite, azurite and malachite.

Two specimens of cornwallite have been reported from the Kintore open cut, Broken Hill, New South Wales, Australia. One occurs on globular conichalcite, and the other shows all the stages of replacement by globular chrysocolla.

At the Telfer gold mine, Western Australia, cornwallite is associated with chalcocite, chrysocolla, agardite, malachite and cornubite.

At the San Rafael mine, Nye county, Nevada, USA, cornwallite has been found with olivenite and mimetite.

At Short Grain, Caldbeck Fells, Cumbria, England cornwallite occurs with chrysocolla and occasionally with supergene baryte in cavities in quartz or coating the exterior of blocks of altered veinstone.

Common impurities: P