Cornwallite

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 × 5.1 × 4.7 cm)

Cornwallite
Cornwallite in a vug from Pastrana, Murcia, Spain (size: 4.3 × 3.7 × 2.2 cm)
General
Category	Arsenate mineral
Formula	Cu5(AsO4)2(OH)4
IMA symbol	Cnw[1]
Strunz classification	8.BD.05
Dana classification	41.4.2.2
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 17.33 Å, b = 5.82 Å, c = 4.60 Å; β = 92.22°; Z = 2
Identification
Color	Verdigis green, blackish-green, emerald-green
Crystal habit	Microcrystalline radial fibrous, botryoidal to globular crusts
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	[2][3][4][5]

Discovery and occurrence

It was first described in 1846, for an occurrence in Wheal Gorland, St Day United Mines of the St Day District, Cornwall, England.^[4] It occurs as 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.^[3]

See also

• Kernowite, another mineral named after Cornwall