Magic-angle sample-spinning (MAS) 29Si high-resolution NMR spectra are presented for a natural sapphirine sample from Aileron, Australia, synthetic sapphirine samples crystallized from a gel of 7MgO·9Al2O3·3SiO2 composition and subjected to various heat treatments, and a more siliceous synthetic sample. A 27Al NMR spectrum was also obtained from a synthetic sample.
The most intense peak in the 29Si spectra is near −73.1 ppm for all samples. The synthetic samples also show distinct maxima near −75.5 and −78.0 ppm and a weak peak near −80.8 ppm. The similarity between the envelopes of these spectra and that of the natural sample suggests that the local Si environments are similar in all samples. The 27Al spectrum showed only one maximum attributable to tetrahedral Al.
The 29Si spectra are best fitted with a short-range ordering pattern in which one Q3 site is almost completely occupied by Si and the other by Al. The location of the remaining Si is not well constrained. The Si concentration in one Q3 site appears to maximize the number of Si-O-Al linkages in the tetrahedral chains.
Fourier transform infrared spectra over the wavenumber range 400–1300 cm−1 were obtained for one of the synthetic sapphirine samples and five natural sapphirine samples with different Si/Al ratios. All spectra were similar in general envelope. A progressive loss of resolution with increasing Si content suggests a concomitant increase in disorder in the natural samples. A heat-treated natural sample gave a spectrum that was badly resolved for its Si content. These data support the conclusions of the NMR study.