Cornwallite

Cornwallite
Cornwallite in a vug from Pastrana, Murcia, Spain (size: 4.3 × 3.7 × 2.2 cm)
General
CategoryArsenate mineral
FormulaCu5(AsO4)2(OH)4
IMA symbolCnw[1]
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[2][3][4][5]

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.[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

References

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Mineralienatlas
  3. ^ a b Cornwallite in the Handbook of Mineralogy
  4. ^ a b Cornwallite on Mindat.org
  5. ^ Cornwallite data on Webmineral
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