Si-Al order in leucite revisited; new information from an analcite-derived analogue

Abstract

The Si-AI ordering in t\VOcontrasting samples of leucite was studied using MAS NMR. One sample was synthesized by performing an ion exchange, in molten KCl, on a natural analcite specimen that had a small distribution of Al next-nearest neighbors (NNN). The other leucite sample was a typical natural specimen from the Alban Hills. In the specimen derived by ion exchange, 45-50% of the Al is on T2 sites with the remainder distributed approximately equally bet\veen the T 1 and T3 sites. The 29Sipeak positions in this sample resolve a debate in the literature over assignment of the more complex 29Si spectra of naturalleucite specimens. It is possible to simulate the 29Sispectrum of Alban Hills leucite with a wide variety of Al distributions, including either Al ordering onto T2 or Al ordering onto Tl + T3. By analogy with the analcite-derived sample and considering previous neutron and X-ray data, Al ordering onto T2 is considered somewhat more probable than the alternative schemes. ~ite-ordering information obtained from