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The chapter deals with relations between genesis and crystal-chemical aspects of microporous heterosilicate minerals (MHM) with mixed octahedral-tetrahedral frameworks and containing 6 or 5 coordinated transition elements (mainly Ti, Nb, Zr, Fe, Mn, Zn) which have been reviewed and discussed by Chukanov and Pekov (2005).
Natural occurrences of microporous silicates with transition elements are very localized: 113 out 122 known MHM (Chukanov and Pekov 2005, Tables 2⇓–4) occur in postmagmatic derivatives of peralkaline rocks. Most of them are known only in this geological setting together with zeolites and zeolite-like beryllo- and borosilicates. In alkaline pegmatites and hydrothermalites, zeolites and MHM may represent up to 90–95% of a rock. Similar diversity and concentrations of microporous silicates are unknown in other geological situations.
Almost all chemical elements present in high-alkaline systems can be incorporated in MHM as either species-forming or important components of isomorphous substitutions; these elements enter into the structure either as framework or extra-framework constituents. The following elements are known as species-forming constituents: O, H, Si, Al, Be, B, P, Zr, Ti, Nb, Sn, Fe, Mn, Zn, Mg, Li, Na, K, Cs, Ca, Sr, Ba, Y, Ce, La, Th, W, F, Cl, C; Ta, Hf, Rb; Nd, Sm, Gd, Dy, Er, Yb, U, Pb, S can be present in MHM with concentrations higher than 1 wt%. In alkaline rocks, relatively high concentrations of some rare elements can be achieved only in microporous minerals thanks to the topological and compositional variety of their structural frameworks and cavities. However, characteristics such as chemical bonds polarization and Lewis acidity of active centers play a role too. As a result, sites having high selectivity towards certain elements occur in the crystal structures of MHM and determine the important role of these minerals for the geochemistry of rare and transition elements …