Mixed-Framework Microporous Natural Zirconosilicates

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Abstract

Among the rare elements in the Earth Crust, Zr is the most abundant one. Zr-minerals mainly occur in alkaline rocks, except zircon ZrSiO4 which was described in various types of rocks. Zirconium silicates revealed widely in nature and their formation is mainly connected with hydrothermal conditions (200–500°C). The most recent statistics of the IMA Commission of New Minerals, Nomenclature and Classification (CNMNC) shows that among Zr containing minerals Zr silicates form the largest class (73 from the total number of 94 mineral species). The crystal structures of Zr-silicates and Ti-silicates in the beginning of 70th contributed the theory of mixed frameworks formed by TO4 tetrahedra (T=P,Si) and MO6 octahedra (M=Zr,Ti). Zirconosilicates which mixed frameworks are characterized by general formula [ZrmSinO3m+2n]−2m form the largest group within the family of Zr-silicates (53 min. sp.). Most of these compounds exhibit technologically important alkali-ion mobility and ion exchange properties. The mixed frameworks in the structures of these compounds are characterized by the formation of the almost equivalent bonds Si-O-Si or Si-O-Zr, which determine the stability of such polyhedral configurations. ZrO6-octahedra in the structures of zirconosilicates with mixed frameworks do not show the tendency to condensation (unlike TiO6- and NbO6-octahedra in the structures of titano- and niobosilicates) (Pyatenko et al., 1999). That’s why there are no natural zirconosilicates with the ratio Si:Zr<1. The lowest Si:Zr ratio occurs in keldyshite, parakeldyshite and khibinskite (Si:Zr = 2) and some other related zirconosilicates in which structures isolated ZrO6-octahedra are connected with Si2O7 pyrogroups (Pekov and Chukanov, 2005).