THE CRYSTAL CHEMISTRY OF SILICATE MINERALS WITH CHAINS OF (TiO6) OCTAHEDRA

Chains of (TiO6) octahedra occur in several crystal structures as fundamental building blocks, confirming the tendency of self-polymerization for (TiO6) octahedra. There are two topologically distinct types of chains based on linkage of (TiO6) octahedra, corner-sharing chains and edge-sharing chains. In this paper, we focus on the diversity of linkages between chains of (TiO6) octahedra and (SiO4) tetrahedra. In Ti-silicate structures based on chains of corner-sharing (Ti4+ 6) octahedra, the chains are neither branched nor looped; they are topologically simple [Ti 5] chains. The chemical formulae of such structures may be written in a very general way as Na2a (TiO)a [Sic O2(a+c)] (H2O)n and Naa (Ti{OH})a [Sic O2(a+c)] (H2O)n, where a and c are integers. These are not arbitrary formulae; the bond topology is such that all anions obey the valence-matching principle. The formulae of batisite, narsarsukite, titanite, the minerals of the labuntsovite group and quartz (Ti-free) are in accord with this general formula. In structures based on chains of edge-sharing (Ti4+ 6) octahedra, the chains may be simple, branched or looped, and there is usually another complicating factor to the bond topology: additional components [e.g., (PO4), Cl, [4]Al, Cr3+] are common. None of the resultant structures have cubic, hexagonal or trigonal symmetry. There are numerous silicate minerals containing chains of (TiO6) octahedra. In contrast, layers of (TiO6) octahedra are rare and occur only in four structure types, and frameworks of (TiO6) octahedra are not known in silicate minerals.