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 x 5.1 x 4.7 cm)
Cornwallite
Cornwallite in a vug from Pastrana, Murcia, Spain (size: 4.3 x 3.7 x 2.2 cm)
General
CategoryArsenate mineral
Formula
(repeating unit)
Cu5(AsO4)2(OH)4
Strunz classification8.BD.05
Dana classification41.4.2.2
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/c
Unit cella = 17.33 Å, b = 5.82 Å,
c = 4.60 Å; β = 92.22°; Z = 2
Identification
ColorVerdigis green, blackish-green, emerald-green
Crystal habitMicrocrystalline radial fibrous, botryoidal to globular crusts
CleavageDistinct in one direction
FractureConchoidal
TenacityBrittle
Mohs scale hardness4.5
LusterSub-vitreous, resinous, waxy
StreakApple green
DiaphaneityTranslucent to opaque
Specific gravity4.17
Optical propertiesBiaxial (+/-)
Refractive indexnα = 1.810 - 1.820 nβ = 1.815 - 1.860 nγ = 1.850 - 1.880
Birefringenceδ = 0.040 - 0.060
2V angleMeasured: 30° to 50°
References[1][2][3][4]

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.[3] 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.[2]

References

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