Cornubite from Majuba Hill, Nevada, USA. Specimen size 5 cm
|Color||Light or dark green|
|Crystal habit||Fibrous, botryoidal, globular or massive, also rare tabular crystals|
|Crystal system||Triclinic 1|
|Cleavage||Distinct in two directions|
|Mohs scale hardness||4|
|Diaphaneity||Translucent to transparent|
|Optical properties||Biaxial (-)|
|Refractive index||Nα = 1.87, Nβ not determined, Nγ = 1.90|
|Other characteristics||Not radioactive|
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.
The structure is made up of sheets of edge-shared Cu(O,OH)6 octahedra, with a copper atom Cu in the middle of the octahedron and either oxygen O or hydroxyl OH at each of the six vertices. These sheets are connected together by AsO4 groups with an arsenic As atom in the middle and O at each of the four vertices. In the AsO4 groups three of the O's are shared with octahedra in one octahedral sheet, and the fourth O is shared with an octahedron in the adjacent sheet. Hydrogen bonds also connect the sheets together.
Crystals, which are very rare, are tabular, often in aggregates as rosettes. Cornubite is generally fibrous, botryoidal, globular or massive.
Cornubite is a secondary mineral found in the oxidised zone of copper deposits. In Cornwall it is found in drusy quartz associated with malachite, Cu2(CO3)(OH)2, olivenite, Cu2(AsO4)(OH), cuprite, Cu2O, cornwallite, Cu2+5(AsO4)2(OH)2, and liroconite, Cu2+2Al(AsO4)(OH)4·4H2O. Some of the world’s best specimens come from the Majuba Hill Mine in the Antelope District, Nevada, USA. Cornubite occurs there as light green botryoidal crusts on rhyolite or as balls and crusts on cornwallite, also as coatings on clinoclase and pseudomorphs after parnauite. Other associations are with chalcophyllite, chenevixite, pseudomalachite, bayldonite, tyrolite, azurite and chrysocolla.
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- Gaines et al (1997) Dana’s New Mineralogy Eighth Edition, Wiley
- Claringbull, Hey and Davis, American Mineralogist (1959) 44: 1321
- American Mineralogist (1985)70: 1333
- Castor and Ferdock (2004) Minerals of Nevada, Nevada Bureau of Mines and Geology with the University of Nevada Press