COORDINATION-GEOMETRY STRUCTURAL PATHWAYS IN Cu2+ OXYSALT MINERALS

PETER C. BURNS* and FRANK C. HAWTHORNE
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2
* Current address: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K.

Abstract

Examination of available Cu2+ oxysalt mineral structures has shown that a number contain Cu2+ n ( = O2-, OH-, H2O; n = 4, 5, 6) coordination polyhedra that are transitional between the usual (4 + 2)-distorted octahedral, square-pyramidal, triangular-bipyramidal and square-planar geometries. The structural pathways between (4 + 2)-distorted octahedral and square-pyramidal, (4 + 2)-distorted octahedral and square-planar, and square-pyramidal and square- planar are represented in Cu2+ oxysalt minerals. Hartree-Fock molecular-orbital calculations for (Cu2+ n)-n+2 model clusters using the STO<0150>3G* basis-set were used to examine cluster geometries and the energetics of possible structural pathways. The calculations show the presence of a small energy-barrier along the pathway from (4 + 2)-distorted octahedral to square-planar, and a large energy-barrier along the pathway from (4 + 2)-distorted octahedral to triangular-bipyramidal. There are no energy-barriers along the other possible structural pathways between the normal coordination geometries. The results of these calculations are in good accord with Cu2+ n geometries observed in minerals. This work shows that Cu2+ coordinations should be interpreted in terms of structural pathways between holosymmetric coordination-geometries.

Keywords: copper, copper oxysalt mineral, molecular orbital, Hartree-Fock, structural pathway, coordination geometry, theoretical mineralogy.