Cornubite is a rare secondary

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

arsenate mineral Arsenate minerals usually refer to the naturally occurring orthoarsenates, possessing the (AsO4)3− anion group and, more rarely, other arsenates with anions like AsO3(OH)2− (also written HAsO42−) (example: pharmacolite Ca(AsO3OH).2H2O) or ( ...

with formula: Cu₅(AsO₄)₂(OH)₄. It was first described for its discovery in 1958 in Wheal Carpenter, Gwinear,

Cornwall Cornwall (; kw, Kernow ) is a historic county and ceremonial county in South West England. It is recognised as one of the Celtic nations, and is the homeland of the Cornish people. Cornwall is bordered to the north and west by the Atlantic ...

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

, UK.Claringbull, Hey and Davis, ''American Mineralogist'' (1959) 44: 1321 The name is from Cornubia, the medieval Latin name for Cornwall. It is a dimorph of

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 ...

, and the

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...

analogue of

pseudomalachite Pseudomalachite is a phosphate of copper with hydroxyl, named from the Greek for “false” and “malachite”, because of its similarity in appearance to the carbonate mineral malachite, Cu2(CO3)(OH)2. Both are green coloured secondary minerals ...

Physical properties

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 In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...

4, the same as

fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon. The Mohs sca ...

, and

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...

4.64, which is similar to another copper arsenate,

clinoclase Clinoclase is a hydrous copper arsenate mineral, Cu3AsO4(OH)3. Clinoclase is a rare secondary copper mineral and forms acicular crystals in the fractured weathered zone above copper sulfide deposits. It occurs in vitreous, translucent dark blue to ...

, at 4.38, but much denser than

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

, at 2.66.

Optical properties

All

triclinic 180px, Triclinic (a ≠ b ≠ c and α ≠ β ≠ γ ) In crystallography, the triclinic (or anorthic) crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal i ...

minerals are

biaxial In crystal optics, the index ellipsoid (also known as the ''optical indicatrix'' or sometimes as the ''dielectric ellipsoid'') is a geometric construction which concisely represents the refractive index, refractive indices and as ...

; cornubite is biaxial (-). Its

refractive indices In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

are quite high, close to 1.9, similar to

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of t ...

and

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

. It is green, as are many copper minerals, usually translucent, with a vitreous luster and a light green streak.

Unit cell

Cornubite belongs to the

crystal class , space group P, meaning that it has a very low symmetry, with only a center of symmetry and no mirror planes or

axes of symmetry In mathematics, reflection symmetry, line symmetry, mirror symmetry, or mirror-image symmetry is symmetry with respect to a reflection. That is, a figure which does not change upon undergoing a reflection has reflectional symmetry. In 2D ther ...

. In 1984 Sieber, Hofmeister, Tillmans and Abraham reported new data from microprobe analysis of cornubite, which gave

unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...

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.American Mineralogist (1985)70: 1333

Structure

The structure is made up of sheets of edge-shared Cu(O,OH)₆

octahedra In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...

, with a copper atom Cu in the middle of the octahedron and either oxygen O or

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...

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.

Crystal habit

Crystals, which are very rare, are tabular, often in aggregates as rosettes. Cornubite is generally fibrous, botryoidal, globular or massive.

Environment

Cornubite is a secondary mineral found in the oxidised zone of copper deposits. In Cornwall it is found in

drusy In geology, druse refers to a coating of fine crystals on a rock fracture surface or vein or within a vug or geode. See also * Crystal habit In mineralogy, crystal habit is the characteristic external shape of an individual crystal or crys ...

quartz associated with

malachite Malachite is a copper carbonate hydroxide mineral, with the formula Cu2CO3(OH)2. This opaque, green-banded mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses, in fracture ...

, Cu₂(CO₃)(OH)₂,

olivenite Olivenite is a copper arsenate mineral, formula Cu2 As O4O H. It crystallizes in the monoclinic system (pseudo-orthorhombic), and is sometimes found in small brilliant crystals of simple prismatic habit terminated by domal faces. More commonl ...

, Cu₂(AsO₄)(OH),

cuprite Cuprite is an oxide mineral composed of copper(I) oxide Cu2O, and is a minor ore of copper. Its dark crystals with red internal reflections are in the isometric system hexoctahedral class, appearing as cubic, octahedral, or dodecahedral forms, ...

, Cu₂O,

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 ...

, Cu²⁺₅(AsO₄)₂(OH)₂, and

liroconite Liroconite is a complex mineral: Hydrated copper aluminium arsenate hydroxide, with the formula Cu2 Al As O4�4(H2O). It is a vitreous monoclinic mineral, colored bright blue to green, often associated with malachite, azurite, olivenite, and cli ...

, Cu²⁺₂Al(AsO₄)(OH)₄·4H₂O. Some of the world’s best specimens come from the Majuba Hill Mine in the Antelope District, Nevada, US. Cornubite occurs there as light green botryoidal crusts on rhyolite or as balls and crusts on cornwallite, also as coatings on

and pseudomorphs after parnauite.Castor and Ferdock (2004) Minerals of Nevada, Nevada Bureau of Mines and Geology with the University of Nevada Press Other associations are with

chalcophyllite Chalcophyllite is a rare secondary copper arsenate mineral occurring in the oxidized zones of some arsenic-bearing copper deposits. It was first described from material collected in Germany. At one time chalcophyllite from Wheal Tamar in Cornwall ...

, chenevixite,

bayldonite Bayldonite (BAIL-done-ite) is a rare secondary mineral with the chemical formula PbCu3(AsO4)2(OH)2. It was first discovered in Penberthy Croft Mine, Cornwall, England, United Kingdom. It is named after its discoverer, John Bayldon (1837(8) – 187 ...

tyrolite Tyrolite is a hydrated calcium copper arsenate carbonate mineral with formula: Ca Cu5( As O4)2 CO3(O H)4·6 H2O. Tyrolite forms glassy blue to green orthorhombic radial crystals and botryoidal masses. It has a Mohs hardness of 1.5 to 2 and a spe ...

azurite Azurite is a soft, deep-blue copper mineral produced by weathering of copper ore deposits. During the early 19th century, it was also known as chessylite, after the type locality at Chessy-les-Mines near Lyon, France. The mineral, a basic carb ...

and

chrysocolla Chrysocolla ( ) is a hydrated copper phyllosilicate mineral and mineraloid with formula (x<1) or . The structure of the mineral has been questioned, as a 2006 spectrographic study suggest material identified as ...

Type locality

The type locality is Wheal Carpenter, Gwinear, Cornwall, UK. The

type material In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to anchor or centralizes t ...

is conserved at the

Natural History Museum, London The Natural History Museum in London is a museum that exhibits a vast range of specimens from various segments of natural history. It is one of three major museums on Exhibition Road in South Kensington, the others being the Science Museum an ...

, reference BM.1958,122

References

{{Reflist

External links

Jmol: http://rruff.geo.arizona.edu/AMS/viewJmol.php?id=13776 Arsenate minerals Copper(II) minerals Triclinic minerals Minerals in space group 2