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

Two (17)O-enriched hydrous magnesium silicates, the minerals hydroxyl-chondrodite (2Mg(2)SiO(4).Mg(OH)(2)) and hydroxyl-clinohumite (4Mg(2)SiO(4).Mg(OH)(2)), were synthesized. High-resolution "isotropic" (17)O (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 (17)O 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 (Mg(2)SiO(4)). The (17)O resonances of the protonated (hydroxyl) sites were recorded and assigned with the aid of (17)O [(1)H] cross-polarization and comparison with the spectrum of (17)O-enriched brucite (Mg(OH)(2)). Using all of 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 assignments are supported by the observation of a correlation between (17)O isotropic chemical shift and Si-O bond length. The (29)Si MAS NMR spectra of the two minerals were also obtained.