The role of local heteropolyhedral substitutions in the stoichiometry, topological characteristics and ion-migration paths in the eudialyte-related structures: a quantitative analysis

Topological analysis of the heteropolyhedral MT framework (where M and T are octahedral and tetrahedral cations, respectively) in the eudialyte-type structure and its derivatives was performed based on a natural tiling analysis of the 3D cation. To analyze the migration paths of sodium cations in these structures, the Voronoi method was used. The parental eudialyte-type MT framework is formed by isolated ZO₆ octahedra, six-membered [M(1)₆O₂₄] rings of edge-sharing M(1)O₆ octahedra, and two kinds of rings of tetrahedra, [Si₃O₉] and [Si₉O₂₇]. Different occupancies of M(2), M(3) and M(4) sites with variable coordination numbers by the additional Q, T* and M* cations, respectively, result in 12 different types of the MT framework. Based on the results of natural tilings calculations as well as theoretical analysis of migration paths, it is found that Na⁺ ions can migrate through six- and seven-membered rings, while all other rings are too small for the migration. In eight types of MT frameworks, Na⁺-ion migration and diffusion is possible at ambient temperature and pressure, while in four other types cages are connected by narrow windows and, as a result, the Na⁺ diffusion in them is complicated at ambient conditions because of the window diameter, but may be possible either at higher temperatures or under mild geological conditions for long periods of time.

New data on the crystal structures of natural zirconosilicates: structure refinements and ion-exchange behavior

A review of the mineralogical, structural and crystal-chemical studies carried out on a large group of natural zirconosilicates is reported. These zirconosilicates belong to a large number of distinct structural types, and those with mixed frameworks of isolated Zr-centred octahedra and silicate complexes are described according to the degree of condensation of SiO4tetrahedra.The microporous heteropolyhedral nature of these phases is considered in connection with their ion exchange properties. The application of X-ray diffractometers equipped with CCD detectors, use of the Rietveld method, and electron microprobe and IR spectroscopic data provide new insights into the structural peculiarities of these minerals. The results stimulate the search for new Zr—Si-compounds with potential applications in the areas of catalysis, separation of molecules, ion exchange, etc.