17O Multiple-Quantum MAS NMR Study of High-Pressure Hydrous Magnesium Silicates

Two O-17-enriched hydrous magnesium silicates, the minerals hydroxyl-chondrodite (2Mg(2)SiO(4). Mg(OH)(2)) and hydroxyl-clinohumite (4Mg2SiO(4). Mg(OH)(2)), were synthesized. High-resolution "isotropic" O-17 (I = 5/2) NMR spectra of the powdered solids were obtained using three- and five-quantum MAS NMR at magnetic field strengths of 9.4 and 16.4 T. These multiple-quantum (MQ) MAS spectra were analyzed to yield the O-17 isotropic chemical shifts (delta (CS)) and quadrupolar parameters (C-Q, eta and their "product" P-Q) of the distinct oxygen sites resolved in each sample. The values obtained were compared with those found previously for forsterite (Mg2SiO4). The O-17 resonances of the protonated (hydroxyl) sites were recorded and assigned with the aid of O-17 {H-1} cross-polarization and comparison with the spectrum of O-17-enriched brucite (Mg(OH)(2)). Using all of these data, complete assignments of the five crystallographically inequivalent oxygen sites in hydroxylchondrodite and of the nine such sites in hydroxyl-clinohumite are suggested. The validity of these assignments are supported by the observation of a correlation between O-17 isotropic chemical shift and Si-O bond length. The Si-29 MAS NMR spectra of the two minerals were also obtained.