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| - 17O Multiple-Quantum MAS NMR Study of High-Pressure HydrousMagnesium Silicates
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| - Two 17O-enriched hydrous magnesium silicates, the minerals hydroxyl-chondrodite (2Mg2SiO4.Mg(OH)2) and hydroxyl-clinohumite (4Mg2SiO4.Mg(OH)2), were synthesized. High-resolution “isotropic” 17O (I= 5/2) NMR spectra of the powdered solids were obtained using three- and five-quantum MAS NMR at magneticfield strengths of 9.4 and 16.4 T. These multiple-quantum (MQ) MAS spectra were analyzed to yield the 17Oisotropic chemical shifts (δCS) and quadrupolar parameters (CQ, η and their “product” PQ) of the distinct oxygensites resolved in each sample. The values obtained were compared with those found previously for forsterite(Mg2SiO4). The 17O resonances of the protonated (hydroxyl) sites were recorded and assigned with the aid of17O {1H} cross-polarization and comparison with the spectrum of 17O-enriched brucite (Mg(OH)2). Using allof these data, complete assignments of the five crystallographically inequivalent oxygen sites in hydroxyl-chondrodite and of the nine such sites in hydroxyl-clinohumite are suggested. The validity of these assignmentsare supported by the observation of a correlation between 17O isotropic chemical shift and Si−O bond length.The 29Si MAS NMR spectra of the two minerals were also obtained.
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