CORNWALLITE

    Class : Phosphates, arsenates, vanadates
    Subclass : Anhydrous arsenates
    Crystal system : Monoclinic
    Chemistry : Cu5(AsO4)2(OH)4
    Rarity : Uncommon

Cornwallite is an arsenate from the oxidation zone of copper deposits, it is the dimorph of cornubite. It takes its name from its region of discovery : Cornwall. It is a mineral which usually occurs in botryoidal or spheroidal crusts, with a fibroradiated or microcrystalline texture, not exceeding 1 mm thick, accompanied by numerous other secondary copper minerals (malachite, olivenite, etc...). Cornwallite comes in different shades of green, from light green to blackish green.

Main photo : Twinned cornwallite from Clara Mine, Germany © Michael Förch

Cornwallite from Clara Mine, Germany © Stephan Wolfsried
Cornwallite from La Atalaya, Murcia, Spain © Fran Garcia
Cornwallite from Cap Garonne, Var, France © Robert Pecorini
Twinned cornwallite from Clara Mine, Germany © Stephan Wolfsried

Cornwallite in the World

Cornwallite is relatively common in the gossans of the copper veins of English Cornwall (Gwennap, Gwinear...), and in that of Caldbeck Fells in Cumbria. It is found in the superficial areas of American copper deposits, notably those of Utah (Gold Hill, Eureka, Mammoth), Nevada (Majuba Hill), the Gold Hill deposit having yielded the only known millimeter crystals. Cornwallite is also present at the top of polymetallic veins in the Black Forest in Germany (Clara, Neubulach, etc...) and in many Australian locations (Arkaroola, Ethiudna, etc...).

Cornwallite in France

In France, cornwallite is present in Longefay (Rhône), Cap Garonne (Var), Faymont (Vosges), Ste-Marie-aux-Mines (Haut-Rhin), Rabejac (Hérault) and Villefranche-de -Rouergues (Aveyron).

Twinning

A cyclic snowflake twin is known to have 7 or 14 individuals.

Fakes and treatments

No fake identified for this mineral species but can be difficult to differentiate from the malachite with which it shares its deposits.


Hardness : 4.5
Density : 4.17
Fracture : Irregular to conchoidal
Streak : Apple green

TP : Opaque to translucent
RI : 1.810 to 1.880
Birefringence : 0.050
Optical character : Biaxial (+/-)
Pleochroism : None
Fluorescence : None

Solubility : Nitric acid


Magnetism : None
Radioactivity : None

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