Overview
Cornwallite is a rare, deep green copper arsenate mineral named in 1846 after Cornwall, England, the historic mining region where it was first recognised. It is an aesthetically distinctive secondary species, typically forming velvety green crusts, botryoidal masses and fibrous coatings rather than large isolated crystals. Though never abundant, it holds an honoured place in mineralogy as one of the classic Cornish copper-arsenate minerals and is sought by collectors who specialise in arsenate species and in the rich oxidation suites of old copper mines.
Composition & structure
Cornwallite is a hydrated basic copper arsenate with the formula Cu5(AsO4)2(OH)4. It crystallises in the monoclinic system and is the monoclinic dimorph of cornubite, a triclinic mineral of identical composition. Cornwallite also forms a series with the phosphate mineral pseudomalachite, with arsenate progressively replaced by phosphate, so individual specimens can grade chemically between the two. These close relationships mean that visual identification alone is often insufficient to separate cornwallite from its dimorph and series partners.
| Formula | Cu5(AsO4)2(OH)4 |
| Crystal system | Monoclinic |
| Mohs hardness | Approximately 4.5 |
| Lustre | Vitreous to silky, sometimes dull |
| Colour | Emerald-green to blackish-green |
| Type locality | Wheal Gorland, Gwennap, Cornwall, England |
Formation & occurrence
Cornwallite is a secondary mineral that forms in the oxidised zones of copper deposits where copper and arsenic are both present. It develops as weathering alters primary copper-arsenic ores, precipitating as crusts and botryoidal coatings on fracture surfaces and in cavities alongside other oxidation-zone minerals. The arsenic in such systems is commonly supplied by the breakdown of arsenide and sulfarsenide ores. Because it needs this specific combination of elements under oxidising conditions, cornwallite is restricted to particular copper-arsenic mining districts rather than being a widespread species.
Identification & similar species
The rich green colour and botryoidal or velvety habit are first indicators, but cornwallite is easily confused with several green copper minerals. It closely resembles malachite, but malachite is a carbonate that effervesces in acid, whereas cornwallite is an arsenate. It is harder to distinguish from pseudomalachite (its phosphate series partner) and from its dimorph cornubite, both of which can look essentially identical; reliable separation usually requires chemical analysis or X-ray diffraction. The arsenate chemistry and association with other arsenic-bearing secondary minerals help confirm an identification in the field.
Notable localities & collecting
The type locality is Wheal Gorland, part of the historic copper-mining district around Gwennap in Cornwall, and Cornish material remains the most celebrated. Beyond Cornwall, cornwallite has been reported from oxidised copper deposits in France, Germany, Slovakia, Morocco, Tunisia, Australia, Mexico and the United States, among other localities. Specimens are generally small, valued for colour, the quality of botryoidal surfaces and association with other rare arsenates. Because cornwallite is uncommon and rarely forms showy crystals, well-documented examples from classic Cornish mines are especially prized by systematic collectors.