Cornetite

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

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	Cu3PO4(OH)3
IMA symbol	Cne[1]
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	[2][3][4][5]

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.^[5]