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 T ₂ sites in natural leucite are deficient in aluminum: g ₁≈0.39, g ₂≈0.16, and g ₃≈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: g ₁≈0.36, g ₂≈0.20, and g ₃≈0.42. For analcite, two distinctly different Si, Al distributions are obtained: (A) g ₁=g ₃≈0.09, g ₂≈0.78 and (B) g ₁=g ₃≈0.46, g ₂≈0.04. Additional NMR measurements on an ion-exchanged sample or an accurate determination of unit-cell dimensions could resolve this ambiguity.