STRUCTURAL RELATIONS IN COPPER OXYSALT MINERALS .1. STRUCTURAL HIERARCHY

A hierarchical structural classification is developed for the copper oxysalt minerals, based on the polymerization of coordination polyhedra of higher bond valences, and focusing specifically on [3]-, [4]-, [5]- and [6]-coordinate polyhedra. The nature of copper oxysalt structures is complicated by the extremely distorted coordinations often occurring around the Cu2+ cation, a result of the well-known Jahn-Teller effect. Although this is widely recognized at the qualitative level, it has led to a very inconsistent assignment of coordination numbers to Cu2+ in minerals. While this seems like a minor point, it has made intercomparison of copper oxysalt structures generally difficult to impossible. Here we re-examine these structures, assigning octahedral coordination to Cu2+ wherever possible; in the majority of structures this can be done. This allows a consistent hierarchical organization scheme to be developed for these structures, and also allows structural comparison with other triangular/tetrahedral/octahedral structures. Cu2+ shows octahedral, square-pyramidal, triangular-bipyramidal, square-planar, trigonal-prismatic and augmented-octahedral ([7]-fold) coordinations in oxysalt minerals. Octahedral coordination is by far the most common. The distribution of octahedral bond lengths is distinctly bimodal, with maxima at 1.97 and 2.44 angstrom and a frequency of 2:1; this is compatible with the usual Jahn-Teller argument concerning bond-length distributions in Cu2+ compounds, the [4+2]-coordination. There are a few instances of regular octahedral coordination, sometimes (but not always) forced by symmetry. Such examples are either at a position of very low multiplicity, or are associated with partially occupied sites. Only one example of [2+4]-coordination occurs, again at a site of low multiplicity. These unusual coordinations are generally associated with some type of disorder. Square-pyramidal and triangular-bipyramidal coordinations are also common, but do not show the distortions from regularity (with regard to bond length) that characterize octahedral coordination. The overall distribution of compositions is centered on an o:t (octahedra:tetrahedra or triangles) ratio of 1:1. For chain and sheet structures, o:t < 1:1; the framework structures show a wider distribution, but o:t > 1:1 predominates.