A silicon-29 nuclear magnetic resonance study of silicon-aluminum ordering in leucite and analcite

Silicon-29 magic-angle-spinning NMR spectroscopy has been used to investigate the silicon-aluminum distribution in natural samples of analcite and leucite (before and after heat treatment) as well as a leucite synthesized from a gel. Three different simulation programs have been developed to fit the experimental spectra. For two we assume a different aluminum occupancy fraction g i for each of the three crystallographically distinct tetrahedral sites T i in leucite and some degree of aluminum avoidance, but an otherwise random arrangement of tetrahedral cations. A third program interchanges Al and Si cations on a lattice of 3×3×3 unit cells to generate an optimized fit. All models predict that the T2 sites in natural leucite are deficient in aluminum: g1≈0.39, g2≈0.16, and g3≈0.42 for the fractional Al occupancy at each site, with apparently strict aluminum avoidance. Heat treatment of the sample at 1673 K for a week has little effect on the g i values but may create some Al-O-Al linkages. In the gel-synthesized leucite, Al occupancies are slightly more uniform than in natural leucite: g1≈0.36, g2≈0.20, and g3≈0.42.