Cornetite

Cornetite
Cornetite from the type locality, Star of the Congo Mine, Lubumbashi, Haut-Katanga District, Democratic Republic of the Congo. 5.7 × 3.9 × 3.9 cm in size
General
Category	Phosphate minerals
Formula (repeating unit)	Cu3PO4(OH)3
Strunz classification	8.BE.15
Dana classification	41.03.02.01
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pbca
Unit cell	a = 10.845(10) Å, b = 14.045(10) Å, c = 7.081(5) Å; Z = 8
Identification
Formula mass	336.63 g/mol
Color	Dark blue to green-blue
Crystal habit	Crystals are short prismatic
Twinning	On {h0l}
Cleavage	None observed
Mohs scale hardness	4.5
Luster	Vitreous
Diaphaneity	Transparent to translucent
Specific gravity	(Measured) 4.10
Optical properties	Biaxial (-)
Refractive index	nα = 1.765 nβ = 1.810 nγ = 1.820
Birefringence	δ = 0.055 max
Pleochroism	non-pleochroic
2V angle	Measured: 33° , Calculated: 48°
Dispersion	none
Solubility	cold HCl
References

Cornetite is a phosphate of copper with hydroxyl named after the geologist Jules Cornet. It was discovered in 1917.

Type Locality

Cornetite is most notably found in the Star of Congo mine, near Lubumbashi.

Environment

Cornetite is a rare secondary mineral in some hydrothermal copper deposits.

Structure

Unlike related phases such as Pseudomalachite, the copper atoms are all five-fold coordinated by oxygen. There are three unique copper sites that are all quite distorted from ideal symmetry. Two are in approximate tetragonal pyramids and the third is essentially a trigonal bipyramidal coordination. Edge sharing polyhedra lead to copper-copper dimer formation, and the overall structure is a three-dimensional network of copper-oxygen polyhedra.