Cornubite
| Cornubite | |
|---|---|
Cornubite from Majuba Hill, Nevada, USA. Specimen size 5 cm
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| General | |
| Category | Arsenate minerals |
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Formula (repeating unit) |
Cu5(AsO4)2(OH)4 |
| Strunz classification | 8.BD.30 |
| Dana classification | 41.04.02.01 |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) (same H-M symbol) |
| Space group | P1 |
| Identification | |
| Color | Light or dark green |
| Crystal habit | Fibrous, botryoidal, globular or massive, also rare tabular crystals |
| Cleavage | Distinct in two directions |
| Mohs scale hardness | 4 |
| Luster | Vitreous |
| Streak | Light green |
| Diaphaneity | Translucent to transparent |
| Specific gravity | 4.64 |
| Optical properties | Biaxial (-) |
| Refractive index | Nα = 1.87, Nβ not determined, Nγ = 1.90 |
| Birefringence | r>v |
| Other characteristics | Not radioactive |
| References | |
Cornubite is a rare secondary copper arsenate mineral with formula: Cu5(AsO4)2(OH)4. It was first described for its discovery in 1958 in Wheal Carpenter, Gwinear, Cornwall, England, UK. The name is from Cornubia, the medieval Latin name for Cornwall. It is a dimorph of Cornwallite, and the arsenic analogue of pseudomalachite.
Cleavage is distinct in two directions, both perpendicular to the enlarged faces of the tabular crystals, intersecting at about 70°. Cornubite is fairly soft, with hardness 4, the same as fluorite, and specific gravity 4.64, which is similar to another copper arsenate, clinoclase, at 4.38, but much denser than quartz, at 2.66.
All triclinic minerals are biaxial; cornubite is biaxial (-). Its refractive indices are quite high, close to 1.9, similar to zircon and garnet. It is green, as are many copper minerals, usually translucent, with a vitreous luster and a light green streak.
Cornubite belongs to the triclinic crystal class 1, space group P1, meaning that it has a very low symmetry, with only a center of symmetry and no mirror planes or axes of symmetry. In 1984 Sieber, Hofmeister, Tillmans and Abraham reported new data from microprobe analysis of cornubite, which gave unit cell parameters a = 6.121 Å, b = 6.251 Å, c = 6.790 Å, α = 92.93°, β = 111.3°, γ = 107.47° and Z=1.
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