Very fast MAS and MQMAS NMR studies of the spectroscopically challenging minerals kyanite and andalusite on 400, 500, and 800 MHz spectrometers

The well-characterized minerals kyanite and andalusite have long presented great challenges in using solid state Al-27 NMR to determine the isotropic chemical shift delta(CS), quadrupole coupling constant e(2)qQ/h, and asymme try parameter eta for each of the inequivalent aluminum sites in these mine rals. Indeed, these minerals have frequently been used to test advances in instrumentation. Recent advances in magnet technology (up to 18.8 T = 800 M Hz H-1) and in MAS probe technology (spinning up to 35 kHz and considerably stronger rf) and refinements of the two-dimensional, multiple quantum magi c angle spinning (MQMAS) technique suggested that these developments could be profitably used to study kyanite and andalusite by solid state Al-27 NMR . The benefit of being able to study kyanite both by MAS and MQMAS techniqu es on 400, 500, and 800 MHz spectrometers is demonstrated. The two octahedr al aluminum sites with the largest (and nearly equal) e(2)qQ/h values give overlapping 1D MAS or 2D 3QMAS signals at all three field strengths. Nevert heless, quantitatively accurate 34 signal intensities at 9.4 T for all four octahedral aluminum sites (with e(2)qQ/h values up to 10 MHz) allow more d etailed analysis. Even if the 3Q signal intensities are not quantitative, t heir isotropic shifts provide an approach (if accurate e(2)qQ/h and eta val ues are available) other than deconvolution of the MAS spectrum for calcula ting delta(CS) values. For andalusite, 34 kHz MAS on the 800 MHz spectromet er significantly narrows the extremely broad signal for the octahedral alum inum, and only slight difficulties are encountered in quantitating the rela tive amounts of AlO5 and AlO6 present. Even with e(2)qQ/h = 15.3 MHz, the o ctahedral aluminum in andalusite gives a signal in a MQMAS experiment, albe it of reduced intensity. As appropriate, we discuss some of the benefits an d limitations of these advances in instrumentation and of different experim ental approaches for studying non-integral spin quadrupolar nuclei in solid s. (C) 1999 Elsevier Science B.V, All rights reserved.