O-17 multiple-quantum MAS NMR study of high-pressure hydrous magnesium silicates

Two O-17-enriched hydrous magnesium silicates, the minerals hydroxyl-chondr odite (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 spe ctra 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 w ere compared with those found previously for forsterite (Mg2SiO4). The O-17 resonances of the protonated (hydroxyl) sites were recorded and assigned w ith the aid of O-17 {H-1} cross-polarization and comparison with the spectr um of O-17-enriched brucite (Mg(OH)(2)). Using all of these data, complete assignments of the five crystallographically inequivalent oxygen sites in h ydroxylchondrodite and of the nine such sites in hydroxyl-clinohumite are s uggested. The validity of these assignments are supported by the observatio n of a correlation between O-17 isotropic chemical shift and Si-O bond leng th. The Si-29 MAS NMR spectra of the two minerals were also obtained.