SI-29 MAS NMR OF THE ALUMINOSILICATE MINERAL KYANITE - RESIDUAL DIPOLAR COUPLING TO AL-27 AND NONEXPONENTIAL SPIN-LATTICE RELAXATION

The Si-29 MAS NMR signal of the mineral kyanite (Al2SiO5) is split into two closely spaced resonances corresponding to the two nonequivalent silicon sites, but only at high magnetic field strengths. Broadening and disappearance of this splitting at lower field (4.7 T), under conditions that give a very sharp signal from quartz present in the same sample, is consistent only with residual dipolar coupling to Al-27, hitherto an elusive phenomenon in Si-29 MAS NMR, and demonstrates the value of very high magnetic field strength, even with I = 1/2 nuclei. This small line broadening effect is usually overwhelmed by larger effects, such as broadening arising from Si,Al disorder, which are absent in kyanite. Si-29 spin-lattice relaxation in natural kyanite samples with a range of paramagnetic impurities does not follow the commonly assumed exponential decay pattern, and this is consistent with lack of spin diffusion.