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~ for each of the three crystallographically distinct tetrahedral sites T~ in leucite and some degree of aluminum avoidance, but an otherwise random arrangement of tetrahedral cations. A third program interchanges A1 and Si cations on a lattice of 3 x 3 x 3 unit cells to generate an optimized fit. All models predict that the T2 sites in natural leucite are deficient in aluminum: g ~ 0 . 3 9 , g2~0.16, and g3~0.42 for the fractional A1 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 gz values but may create some A 1 O A 1 linkages. In the gel-synthesized leucite, A1 occupancies are slightly more uniform than in natural leucite: gl ~ 0.36, g2 ~ 0.20, and g3 ~ 0.42. For analcite, two distinctly different Si, A1 distributions are obtained: (A) g l=g3~0.09, g2~0.78 and (B) g l=g3 ~0.46, g2~0.04. Additional NMR measurements on an ion-exchanged sample or an accurate determination of unitcell dimensions could resolve this ambiguity.