Site Populations and Short Range Order in Aluminosilicates Investigated by27Al Solid-State NMR

Abstract

Site populations in Al2Si2O5 kyanite and short range order parameters in CaAl2Si2O8 anorthite have been determined by using 27Al 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 t1 acquisition times. Detailed calculations were used to optimize pulse sequences and then evaluate the accuracy of signal intensity to site abundances based on CQ 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 °C between 1 min and 179 h. 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 sa...