Cornetite

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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
Phosphate minerals
Cu3PO4(OH)3
Cne
8.BE.15
41.03.02.01
Orthorhombic
Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Pbca
a = 10.845(10) Å,b = 14.045(10) Å,c = 7.081(5) Å; Z = 8
336.63 g/mol
Dark blue to green-blue
Crystals are short prismatic
On {h0l}
None observed
4.5
Vitreous
Transparent to translucent
(Measured) 4.10
Biaxial (−)
nα = 1.765 nβ = 1.810 nγ = 1.820
δ = 0.055 max
Non-pleochroic
Measured: 33°, Calculated: 48°
None
cold HCl

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

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

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

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.

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