Cornwallite

What is cornwallite? A copper arsenate named for Cornwall

Cornwallite is Cu5(AsO4)2(OH)4, a green copper arsenate that forms crusts rather than free crystals. The Handbook of Mineralogy cornwallite sheet gives monoclinic, point group 2/m, space group P21/a, as microcrystalline or radial fibrous botryoidal to globular crusts, to 1 mm thick. Hardness is 4.5; density 4.17 measured against 4.64 calculated, a gap the sheet does not explain. It is verdigris-green to blackish green or pale green, emerald-green in transmitted light, semitransparent, with conchoidal fracture; biaxial, (+) or (−), α 1.81–1.820, β 1.815–1.860, γ 1.85–1.88. The sheet prints no streak, lustre or cleavage line.

What the records say. The IMA list: Cornwallite, Cu5(AsO4)2(OH)4, status G, 1847, United Kingdom. The sheet calls it a rare secondary mineral in oxidized copper deposits, dimorphous with cornubite, and names it for its occurrence in the County of Cornwall. Its type material is at The Natural History Museum, London, MI 1985,1754; the sheet marks no type locality, so this page names none (what a type locality is).

What grows with it. The association line names olivenite, cornubite, arthurite, clinoclase, chalcophyllite, strashimirite, lavendulan, tyrolite, spangolite, austinite, conichalcite, brochantite, azurite and malachite.

Is cornwallite the arsenate of pseudomalachite? The arsenic–phosphorus swap

By formula, yes: cornwallite and pseudomalachite differ only in arsenic for phosphorus. A field scan of the September 2026 IMA list for formulas exactly of the form Cu5(XO4)2(OH)4, cross-checked by a raw line scan, finds 5 rows. With arsenic: cornwallite (G, 1847) and cornubite (A, 1962 s.p.). With phosphorus: pseudomalachite (G, 1813), reichenbachite (A, 1985-044) and ludjibaite (A, 1987-009); the pseudomalachite sheet calls those three trimorphous. The cornwallite sheet itself does not mention pseudomalachite, and no sheet read here calls the two a series, so this page claims none.

What the swap does to the numbers. Cornwallite’s formula mass is 663.59 and pseudomalachite’s 575.70; the difference, 87.90, is twice arsenic (74.922) minus phosphorus (30.974). Copper is 47.88% of cornwallite and 55.19% of pseudomalachite. The same swap one size down links olivenite, Cu2(AsO4)(OH), and libethenite, Cu2(PO4)(OH).

The phosphorus column, worked. The sheet prints three columns. Cornwall, analysis (1): P2O5 2.71, As2O5 30.47, CuO 59.95, H2O 8.23. Divided by the oxide weights, that is 0.0382 mol phosphorus and 0.2651 mol arsenic per 100 g, so phosphorus is 12.6% of the arsenic plus phosphorus, and copper per (As + P) is 2.48. Read without the phosphorus it would be 2.84 copper per arsenic, well off the formula’s 2.5. Wheal Carpenter, analysis (2), has no phosphorus and gives 2.56 from its oxides; the sheet’s own formula for it, Cu4.85(AsO4)1.91(OH)4.12, gives 2.54. The copper-to-arsenic ladder of the plain copper arsenate hydroxides is on the cornubite page and is not repeated here.

Cornwallite or pseudomalachite? Where the sheets overlap

Documented failure: the two sheets give no physical test that reliably separates cornwallite from pseudomalachite. Both are green, botryoidal or fibrous crusts. Hardness is 4.5 against 4.5–5. Worked as 20 − 20/D, a 20.00 g piece reads 15.20 g in water at cornwallite’s measured density and 15.18 to 15.40 g as pseudomalachite, so the weight falls inside the phosphate’s range. Cornwallite’s calculated density gives 15.69 g, but its measured figure is the one a crust will show. Their β ranges, 1.815–1.860 and 1.835–1.86, and γ ranges, 1.85–1.88 and 1.845–1.88, overlap. Arsenic against phosphorus is a chemical question. Method: how to measure specific gravity at home.

Where the neighbours do separate. olivenite is hardness 3 with an olive-green to brown streak; clinoclase is 2.5–3, dark greenish blue to greenish black, streak bluish green; cornubite is triclinic, about 4, apple-green, streak pale green. Pseudomalachite’s sheet prints a pale green to pale bluish green streak; cornwallite’s prints none, so a streak cannot be compared on the sheets. Streak method: how to use a streak plate.

Where is cornwallite found? Gwennap, Gwinear and Caldbeck

The cornwallite sheet’s Distribution field starts in England. In Cornwall, from Wheals Gorland and Unity, Gwennap, at Wheal Carpenter, Gwinear, and other mines; in the Potts Gill mine, Caldbeck Fells, Cumbria. Then Sainte-Marie-aux-Mines and Triembach-au-Val, Haut-Rhin, and the Cap Garonne mine near le Pradet, Var, France; the Clara mine near Oberwolfach, Neubulach, and Reichenbach near Bensheim, Germany; L’ubietová, Slovakia; Bou Skour and Ounein, Morocco; Chouichica, Tunisia; the Mammoth and Centennial Eureka mines, Tintic district, and crystallized at the Gold Hill mine, Utah; the Majuba Hill mine, Nevada; the Guerra al Tirano mine, Chihuahua, Mexico; and in Australia, Broken Hill, the Ethiudna mine near Olary, and Western Australia. It adds that a number of other localities are known.

Which analysis is from where. The X-ray powder pattern on the sheet is from Wheal Carpenter, Cornwall, as is analysis (2); analysis (1) is given only as Cornwall, England.

Cornwallite is 22.58% arsenic and 47.88% copper by mass; see toxic minerals in a collection, and nothing here is health advice. More species are at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.

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