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| Cornwallite | |
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| Category: | Arsenate mineral |
| Formula: | Cu5(AsO4)2(OH)4 |
| Imasymbol: | Cnw[1] |
| Strunz: | 8.BD.05 |
| Dana: | 41.4.2.2 |
| System: | Monoclinic |
| Class: | Prismatic (2/m) (same H-M symbol) |
| Symmetry: | P21/c |
| Unit Cell: | a = 17.33 Å, b = 5.82 Å, c = 4.60 Å; β = 92.22°; Z = 2 |
| Color: | Verdigis green, blackish-green, emerald-green |
| Habit: | Microcrystalline radial fibrous, botryoidal to globular crusts |
| Cleavage: | Distinct in one direction |
| Fracture: | Conchoidal |
| Tenacity: | Brittle |
| Mohs: | 4.5 |
| Luster: | Sub-vitreous, resinous, waxy |
| Streak: | Apple green |
| Diaphaneity: | Translucent to opaque |
| Gravity: | 4.17 |
| Opticalprop: | Biaxial (+/−) |
| Refractive: | nα = 1.810 – 1.820 nβ = 1.815 – 1.860 nγ = 1.850 – 1.880 |
| Birefringence: | δ = 0.040 – 0.060 |
| 2V: | Measured: 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.
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]