MAS NMR study of pentacoordinated magnesium in grandidierite

The 11.7 T 25 Mg, 27 Al, 29 Si, and 11 B MAS NMR spectra are reported for well-characterized grandidierite, (Mg,Fe)Al 3SiBO9, which contains both Al and Mg in fivefold coordination with oxygen. The 25 Mg spectrum is the first to be reported for 5 Mg, and exhibits a quadrupolar lineshape from which the nuclear quadrupolar coupling constant (3.8 6 0.1 MHz), the asymmetry parameter (0.6 6 0.05), and the isotropic chemical shift (55 6 2 ppm) were derived by spectral simulation. These spectroscopic parameters are discussed in terms of the crystallographic geometry of the fivefold-coordinated site.