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 3 SiBO 9 , 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+ or -0.1 MHz), the asymmetry parameter (0.6+ or -0.05), and the isotropic chemical shift (55+ or -2 ppm) were derived by spectral simulation. These spectroscopic parameters are discussed in terms of the crystallographic geometry of the fivefold-coordinated site.

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