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17O multiple-quantum MAS NMR study of silicate minerals

A.J. Berry, S.E. Ashbrook1 and S. Wimperis1 1 School of Chemistry, University of Exeter

Two-dimensional triple-quantum 17O (I = 5/2) MAS NMR powder spectra were obtained for hydroxyl-chondrodite, hydroxyl-clinohumite, orthoenstatite, clinoenstatite, protoenstatite, and diopside. The spectra were analysed to yield the 17O isotropic chemical shifts and quadrupolar coupling parameters of the oxygen sites in each sample. The values obtained were compared with those found previously for forsterite. The 17O resonances of the hydroxyl sites were identified using 17O {1H} cross-polarisation and assigned by comparison with the spectrum of 17O-enriched brucite. For the pyroxenes, bridging and non-bridging oxygens were distinguished by their characteristic quadrupolar parameters. Using all this data, complete assignments of the five crystallographically-inequivalent oxygen sites in hydroxyl-chondrodite, the nine such sites in hydroxyl-clinohumite, the three sites in protoenstatite and diopside, and of the six sites in orthoenstatite and clinoenstatite, have been suggested. The validity of these assignments are supported by the observation of a correlation between 17O isotropic chemical shift and Si-O bond length. It is hoped that the present work will serve as a model for future NMR studies aimed at determining the hydroxyl site in nominally anhydrous mantle minerals.