A ²⁷Al and ²⁹Si MAS NMR and infrared spectroscopic study of Al- Si ordering in natural and synthetic sapphirine

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 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 also obtained for one of the synthetic sapphirine samples and five natural sapphirine samples with different Si/Al ratios. -from Authors