Cornetite

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.