Direct Determination of the Crystal Structure of Cornetite, Cu3PO4(OH)3, by the Monte Carlo Method

Abstract

The crystal structure of the peacock-blue mineral cornetite, Cu3PO4(OH)3, orthorhombic, with a0=10.845, b0=14.045, c0=7.081 Å, space group Pbca, has been determined. The phase problem has been solved by the direct determination of the heavy atom positions through the application of the Monte Carlo and optimal shift methods. Subsequent three-dimensional Fourier synthesis revealed all the light atom positions. The atomic parameters have been refined, using three-dimensional data, by the method of full-matrix least squares. Two of the three independent copper atoms have essentially fivefold tetragonal pyramidal coordination, the coordination plane being curved in a boat shape towards the fifth ligand. The third copper atom has a highly distorted tetragonal bipyramidal coordination. Cornetite contains two sets of dimeric cupric complex ions. The first set [Cu2(OH)4O2] is formed by two different copper coordination planes sharing an edge, and the second [Cu2(OH)4O4] by two tetragonal pyramids, related by a center of symmetry, sharing a pyramidal edge. The [Cu2(OH)4O2] dimers are stacked upon each other along the c axis in a zigzag fashion, so that each dimer has two (OH) groups common with two other dimers above and below it. The dimeric chains thus formed are held together in a three-dimensional network, through isolated [Cu2(OH)4O4] dimers, PO4 groups, and O–H···O bonds.