SI-AL ORDER IN LEUCITE REVISITED - NEW INFORMATION FROM AN ANALCITE-DERIVED ANALOG

The Si-Al ordering in two contrasting samples of leucite was studied u sing MAS NMR. One sample was synthesized by performing an ion exchange , in molten KCI, on a natural analcite specimen that had a small distr ibution of Al next-nearest neighbors (NNN). The other leucite sample w as a typical natural specimen from the Alban Hills. In the specimen de rived by ion exchange, 45-50% of the Al is on T2 sites with the remain der distributed approximately equally between the TI and T3 sites. The Si-29 peak positions in this sample resolve a debate in the literatur e over assignment of the more complex Si-29 spectra of natural leucite specimens. It is possible to simulate the Si-29 spectrum of Alban Hil ls leucite with a wide variety of Al distributions, including either A l ordering onto T2 or Al ordering onto T1 + T3. By analogy with the an alcite-derived sample and considering previous neutron and X-ray data, Al ordering onto T2 is considered somewhat more probable than the alt ernative schemes. Site-ordering information obtained from (27)A1 NMR i n an earlier study is not considered reliable because of complications arising from the differing (and uncertain) electric-field gradients a t these sites.