- © The Mineralogical Society Of America
Steady progress in methods of synthesis and investigation of physical properties and crystal-chemical features of silicate microporous materials with transition elements (so-called zeolite-like amphoterosilicates) took place during the last decade. Materials of this type are promising ion exchangers, sorbents, catalysts or catalyst carriers and, accordingly, can be used in chromatography, catalysis, water purification, etc. Unlike common zeolites being aluminosilicates, frameworks of amphoterosilicates with transition metals are built of both tetrahedral fragments and “strong” cations (Ti, Nb, Zr, Ta, Sn, W, Fe, Mn, Zn, etc., see Chukanov et al. 2004) with coordination numbers 6 or 5. Structural and chemical diversity of synthetic and natural amphoterosilicates is a basis for the wide variety of their properties. On the other hand, the number of different combinations of structure types and compositions of non-zeolite microporous minerals is still larger than the number of different synthetic materials of this type. In addition, whereas minerals are often available as crystals suitable for structural investigations, synthetic materials are often microcrystalline. Thus, natural titanosilicates, niobosilicates and zirconosilicates can be considered as prototypes of new materials with ion exchange, sorptional and catalytic properties (cf. Rocha and Lin 2005). The brightest examples are members of the eudialyte, labuntsovite, lovozerite, and hilairite groups, elpidite, zorite, gaidonnayite, penkvilksite. These minerals were formed in late formations related to alkaline massifs (alkaline pegmatites, hydrothermalites and metasomatites; cf. Pekov and Chukanov 2005).
GENERAL CHARACTERIZATION AND CLASSIFICATION OF MICROPOROUS HETEROSILICATE MINERALS
Among about 4100 known mineral species, several hundreds minerals have open-framework structures but only about one hundred are classified as proper zeolites (Bish and Ming 2001) with frameworks consisting of atoms having only 4-fold coordination. In particular, a large number of zeolite-like microporous heterosilicate minerals (MHM) is known whose frameworks contain transition elements (mainly Ti, Nb and Zr, but also Fe, Mn, Zn, Ta, Sn, W) having 6-fold, rarely 5-fold coordination (see …