Cornubite
Encyclopedia
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
.
is distinct in two directions, both perpendicular to the enlarged faces of the tabular crystals, intersecting at about 70o. 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.
; 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.
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.
, 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.
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
.
is Wheal Carpenter, Gwinear
, Cornwall, UK. The type material is conserved at the Natural History Museum, London, reference BM.1958,122
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
arsenate mineral
Arsenate mineral
Arsenate minerals usually refer to the naturally occurring orthoarsenates, possessing the 3- anion group and, more rarely, other arsenates with anions like AsO32- or [AsO22]- . Arsenite minerals are much less common...
with formula: Cu5(AsO4)2(OH)4. It was first described for its discovery in 1958 in Wheal Carpenter, Gwinear
Gwinear
For the civil parish of Gwinear-Gwithian, see Gwinear-Gwithian. For the village of Gwinear see Gwinear, CornwallGwinear was a saint of the early Celtic period, one of only two early Cornish saints whose biographies survived the Reformation...
, Cornwall
Cornwall
Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...
, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
, UK. The name is from Cornubia, the medieval Latin name for Cornwall. It is a dimorph of Cornwallite, and the arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
analogue of pseudomalachite
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, Cu22. Both are green coloured secondary minerals found in oxidised zones of copper deposits, often associated with...
.
Physical properties
CleavageCleavage (crystal)
Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the...
is distinct in two directions, both perpendicular to the enlarged faces of the tabular crystals, intersecting at about 70o. Cornubite is fairly soft, with hardness
Mohs scale of mineral hardness
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in...
4, the same as fluorite
Fluorite
Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...
, and specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
4.64, which is similar to another copper arsenate, clinoclase
Clinoclase
Clinoclase is a hydrous copper arsenate mineral, Cu3AsO43. 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 dark greenish blue colored crystals and botryoidal...
, at 4.38, but much denser than quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, at 2.66.
Optical properties
All triclinic minerals are biaxialBirefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
; cornubite is biaxial (-). Its refractive indices
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
are quite high, close to 1.9, similar to zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
and garnet
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
. 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 triclinic crystal class , space group P, meaning that it has a very low symmetry, with only a center of symmetry and no mirror planesReflection symmetry
Reflection symmetry, reflectional symmetry, line symmetry, mirror symmetry, mirror-image symmetry, or bilateral symmetry is symmetry with respect to reflection. That is, a figure which does not change upon undergoing a reflection has reflectional symmetry.In 2D there is a line of symmetry, in 3D a...
or axes of symmetry
Rotational symmetry
Generally speaking, an object with rotational symmetry is an object that looks the same after a certain amount of rotation. An object may have more than one rotational symmetry; for instance, if reflections or turning it over are not counted, the triskelion appearing on the Isle of Man's flag has...
. In 1984 Sieber, Hofmeister, Tillmans and Abraham reported new data from microprobe analysis
Electron microprobe
An electron microprobe , also known as an electron probe microanalyzer or electron micro probe analyzer , is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials...
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.
Structure
The structure is made up of sheets of edge-shared Cu(O,OH)6 octahedraOctahedron
In geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex....
, with a copper atom Cu in the middle of the octahedron and either oxygen O or hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
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 drusyDruse (geology)
In geological usage druse or druzy is a coating of fine crystals on a rock fracture surface, vein or within a vug or geode.- Occurrence :Druse occurs worldwide, the most common is perhaps quartz druse within voids in chert or agates...
quartz associated with malachite
Malachite
Malachite is a copper carbonate mineral, with the formula Cu2CO32. This green-colored mineral crystallizes in the monoclinic crystal system, and most often forms botryoidal, fibrous, or stalagmitic masses. Individual crystals are rare but do occur as slender to acicular prisms...
, Cu2(CO3)(OH)2, olivenite
Olivenite
Olivenite is a copper arsenate mineral, formula Cu2AsO4OH. It crystallizes in the monoclinic system , and is sometimes found in small brilliant crystals of simple prismatic habit terminated by domal faces...
, Cu2(AsO4)(OH), cuprite
Cuprite
Cuprite is an oxide mineral composed of copper 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, or in combinations. Penetration twins frequently occur...
, Cu2O, cornwallite, Cu2+5(AsO4)2(OH)2, and liroconite
Liroconite
Liroconite is a complex mineral: Hydrated copper aluminium arsenate hydroxide, with the formula Cu2Al[4|AsO4]·4. It is a vitreous monoclinic mineral, colored bright blue to green, often associated with malachite, azurite, olivenite, and clinoclase...
, 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
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
or as balls and crusts on cornwallite, also as coatings on clinoclase
Clinoclase
Clinoclase is a hydrous copper arsenate mineral, Cu3AsO43. 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 dark greenish blue colored crystals and botryoidal...
and pseudomorphs after parnauite. Other associations are with chalcophyllite
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...
, chenevixite, pseudomalachite
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, Cu22. Both are green coloured secondary minerals found in oxidised zones of copper deposits, often associated with...
, bayldonite
Bayldonite
Bayldonite is a rare secondary mineral with the chemical formula PbCu322H2O. It was first discovered in Penberthy Croft Mine, Cornwall, England, UK. It is named after its discoverer, John Bayldon....
, tyrolite
Tyrolite
Tyrolite is a hydrated calcium copper arsenate carbonate mineral with formula: CaCu52CO34·6H2O. Tyrolite forms glassy blue to green orthorhombic radial crystals and botryoidal masses. It has a Mohs hardness of 1.5 to 2 and a specific gravity of 3.1 to 3.2...
, azurite
Azurite
Azurite is a soft, deep blue copper mineral produced by weathering of copper ore deposits. It is also known as Chessylite after the type locality at Chessy-les-Mines near Lyon, France...
and chrysocolla
Chrysocolla
Chrysocolla is a hydrated copper silicate mineral with formula 2H2Si2O54·nH2O.-Properties:Chrysocolla has a blue-green color and is a minor ore of copper, having a hardness of 2.5 to 3.5.-Name and discovery:...
.
Type locality
The type localityType locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
is Wheal Carpenter, Gwinear
Gwinear
For the civil parish of Gwinear-Gwithian, see Gwinear-Gwithian. For the village of Gwinear see Gwinear, CornwallGwinear was a saint of the early Celtic period, one of only two early Cornish saints whose biographies survived the Reformation...
, Cornwall, UK. The type material is conserved at the Natural History Museum, London, reference BM.1958,122