Site Populations and short range order in aluminosilicates investigated by 27Al solid-state NMR

Site populations in Al2Si2O5 kyanite and short range order parameters in CaAl2Si2O8 anorthite have been determined by using Al-27 multiple quantum magic angle spinning NMR with quadrupolar Carr-Purcell-Meiboom-Gill echo train and FAM-II triple quantum to single quantum conversion pulses (MQ-QCPMG-MAS NMR). Significant increases in resolution in the isotropic dimension were obtained by using the increased signal intensities to extend t(1) acquisition times. Detailed calculations were used to optimize pulse sequences and then evaluate the accuracy of signal intensity to site abundances based on C-Q and resonance offset variation in the mineral kyanite. The same approach was used to investigate a well-ordered anorthite and a range of disordered anorthite samples annealed at 1400 degreesC between 1 min and 179 It. Because of intrinsic disorder accurate site populations were impossible to obtain for these samples but instead upper bounds for the order parameter were determined. Furthermore, the spectra of the annealed samples indicated that there was a distribution of EFG parameters for several sites (e.g. m000 and mz00), which suggests that these sites are important in the ordering process.