Cornwallite

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	Cnw1
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	2345

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.

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