Structure Topology and Graphical Representation of Decorated and Undecorated Chains of Edge-Sharing Octahedra

Infinite chains of edge-sharing octahedra occur as fundamental building blocks (FBBs) in the structures of several hundred mineral species. Such chains consist of a backbone of octahedra to which decorating polyhedra may be attached. The general, stoichiometric formula of such chains may be written as c[MATxФz] where M is any octahedrally coordinated cation, T is any cation coordinated by a decoration polyhedron (regardless of coordination geometry), Ф is any possible ligand [O2–, (OH)–, (H2O), Cl–, or F–], and c indicates the configuration of backbone octahedra. In the minerals in which they occur, these types of chains will commonly (though not exclusively) form part of the structural unit (i.e., the strongly bonded part) of a mineral. Hence, investigating the topology, configuration, and arrangement of such chains may yield fundamental insights into the stability of minerals in which they occur. A discussion of the topological variability of chains is presented here, along with the formulae necessary for their characterization. It is shown that many aspects of chain topology can be efficiently communicated by a pair of values with the form ([x], [Bopqrst]), where [x] summarizes the symmetry operations necessary to characterize the configuration of backbone octahedra, B indicates the length of the topological repeat, and o through t indicate the number of individual decorations (related to B). A methodology for developing finite graphical representations for infinite chains is presented in detail, showing that for any given chain, a single, irreducible finite graph exists that contains all topological information. Such a graph, however, can correspond to multiple chain topologies, highlighting the importance of geometrical isomerism. The utility of the graphical approach in facilitating the development of a hierarchy of chains and chain-bearing structures is also discussed.