Cornwallite - Encyclopedia
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 in the World
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