Formula: Cu5(AsO4)2(OH)4
Anhydrous arsenate containing hydroxyl, monoclinic paramorph of cornubite, which is triclinic
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 (Dana)
Common impurities: P

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 (Mindat) associated with olivenite, cornubite, arthurite, clinoclase, chalcophyllite, strashimirite, lavendulan, tyrolite, spangolite, austinite, conichalcite, brochantite, azurite and malachite (HOM).

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 (AJM 03.1.58).

At the Telfer gold mine, Western Australia, cornwallite is associated with chalcocite, chrysocolla, agardite, malachite and cornubite (AJM 12.1.36-37)

At the San Rafael mine, Nye county, Nevada, USA, cornwallite has been found with olivenite and mimetite (R&M 85.6.517).

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 (JRS.12.53)

The Activity-pH diagram below was calculated at 298.2 K for the main Cu2+ and Pb2+ arsenate minerals. Boundaries are calculated for constant activities (roughly equivalent to concentrations) of Pb2+ and Cl- ions in solution, over a range of values of pH and of Cu2+ activity. Higher chloride activites make the stability fields of bayldonite and duftite vanish, with respect to the encroachment of that of mimetite (LMW p269).
stability Cu Pb.jpg

The arsenate mineral formulae are:
philipsbornite PbAl3(AsO4)(AsO3OH)(OH)6
mimetite Pb5(AsO4)3Cl
duftite PbCu(AsO4)(OH)
bayldonite Cu3PbO(AsO3OH)2(OH)2
olivenite Cu2(AsO4)(OH)
cornwallite Cu5(AsO4)2(OH)4
clinoclase Cu3(AsO4)(OH)3

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