Cornwallite - Misplaced Pages

Copper arsenate mineral

Cornwallite
Cornwallite in a vug from Pastrana, Murcia, Spain (size: 4.3 × 3.7 × 2.2 cm)
General
Category	Arsenate mineral
Formula (repeating unit)	Cu₅(AsO₄)₂(OH)₄
IMA symbol	Cnw
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	n_α = 1.810 – 1.820 n_β = 1.815 – 1.860 n_γ = 1.850 – 1.880
Birefringence	δ = 0.040 – 0.060
2V angle	Measured: 30° to 50°
References

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.

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. 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.