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The problem of resolving the two aluminum sites in the  27 Al NMR spectrum of kaolinite has been unsuccessfully addressed for 30 years. A few years ago, it was shown that the two sites cannot be spectrally separated even by the use of high magnetic fields. Nevertheless, it is still possible to determine the NMR parameters of both sites. In this article, we present an alternative approach. We show that, at low magnetic field (7 T), the individual spinning sideband lineshapes of the outer satellite transitions are sensitive enough to differentiate information coming from the two aluminum sites. Thus, the isotropic chemical shift d, the quadrupolar constant C Q , and asymmetry parameter ηQ of each site can be obtained by accurately fitting the full  27 Al MAS spectrum acquired at low magnetic field. In return, this approach requires a carefully acquired and post-treated  27 Al spectrum. It is concluded that the two sets of parameters (δ = 7.5 ppm, C Q  = 3.4 MHz, η Q  = 0.8) and (d = 8.0 ppm, C Q  = 3.0 MHz, η Q  = 0.9) represent the best and the unique solution overall. Moreover, the accuracy of these experimental values is independently and fully supported by first-principles calculations of the electric field gradient.

american mineralogist

The two aluminum sites in the  27 Al MAS NMR spectrum of kaolinite: Accurate determination of isotropic chemical shifts and quadrupolar interaction parameters