SI-AL ORDERING IN LEUCITE GROUP MINERALS AND ION-EXCHANGED ANALOGS - AN MAS NMR-STUDY

Two series of leucite group materials, with K, Rb, and Cs as extra-fra mework cations, have been synthesized by ion exchange from a natural w ell-ordered analcite and a natural disordered leucite. Si-29 and Al-27 MAS NMR data for the analcite-derived series provide complementary in formation on tetrahedral cation ordering. The ordering in terms of the number of Al next-nearest neighbors, Q(4)(nAl) (short-range order), d oes not change sig nificantly during ion exchange, indicating that Al and Si remain essentially fixed in their original positions. In contra st, the ordering of Al over T1, T2, and T3 (long-range order) for the analcite-derived series changes dramatically with changing alkali cati on; the Al occupancies for the three analcite-derived samples expresse d as T1:T2:T3 are approximately 0.25:0.50:0.25 for KAlSi2O6, 0.40:0.20 :0.40 for RbAlSi2O6, and 0.15:0.70:0.15 for CsAlSi2O6. During the ion exchange, at temperatures above the cubic-tetragonal phase transitions , only one symmetrically distinct T site is present. It is proposed th at on cooling through the cubic-tetragonal phase transition the struct ure collapses around the non-frame work cations to give the lowest ene rgy Si-Al distribution over the three T sites irrespective of the orig inal T-site ordering in the starting material. Our data suggest that t he identity of the cation in the W site affects the orientation of the framework distortions associated with the cubic-tetragonal phase tran sition and leads to the possibility that a particular tetrahedral cati on site can take on the characteristics of a T1, T2, or T3 site. The d ata and their interpretation have important implications for the mecha nism of this type of structural phase transition.