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¹⁷O multiple-quantum MAS NMR study of silicate minerals

A.J. Berry, S.E. Ashbrook¹ and S. Wimperis^{1 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.