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Cornwallite in a vug from Pastrana, Murcia, Spain (size: 4.3 x 3.7 x 2.2 cm)
Category Arsenate mineral
(repeating unit)
Strunz classification 8.BD.05
Dana classification
Crystal symmetry Monoclinic prismatic
H-M symbol: 2/m
Space group: P21/a
Unit cell a = 17.33 Å, b = 5.82 Å, c = 4.60 Å; β = 92.22°; Z=2
Color Verdigis green, blackish-green, emerald-green
Crystal habit Microcrystalline radial fibrous, botryoidal to globular crusts
Crystal system Monoclinic
Cleavage Distinct in one direction
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 4.5
Luster Sub-vitreous, resinous, waxy
Streak Apple green
Diaphaneity Translucent to opaque
Specific gravity 4.17
Optical properties Biaxial (+/-)
Refractive index nα = 1.810 - 1.820 nβ = 1.815 - 1.860 nγ = 1.850 - 1.880
Birefringence δ = 0.040 - 0.060
2V angle Measured: 30° to 50°
References [1][2][3]

Cornwallite is an uncommon copper arsenate mineral with formula Cu5(AsO4)2(OH)4. It forms a series with the phosphate pseudomalachite and is a dimorph of the triclinic cornubite. It is a green monoclinic mineral which forms as radial to fibrous encrustations.

Botryoidal, green cornwallite in a gossan with white baryte from Caldbeck Fells, Cumbria (7.6 x 5.1 x 4.7 cm)

Discovery and occurrence[edit]

It was first described in 1846 for an occurrence in Wheal Gorland, St Day United Mines of the St Day District, Cornwall, England.[2] It occurs as a secondary mineral in the oxidized zone of copper sulfide deposits. Associated minerals include olivenite, cornubite, arthurite, clinoclase, chalcophyllite, strashimirite, lavendulan, tyrolite, spangolite, austinite, conichalcite, brochantite, azurite and malachite.[1]