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

Abstract Two 17O-enriched hydrous magnesium silicates, the minerals hydroxyl-chondrodite (2Mg2SiO4.Mg(OH)2) and hydroxyl-clinohumite (4Mg2SiO4.Mg(OH)2), were synthesised. High-resolution "isotropic" 17O (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 analysed to yield the 17O isotropic chemical shifts (δCS) and quadrupolar parameters (CQ, η and their "product" PQ) of the distinct oxygen sites 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 of 17O {1H} cross-polarization and comparison with the spectrum of 17O-enriched brucite (Mg(OH)2). Using all this 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 17O isotropic chemical shift and Si-O bond length. The 29Si MAS NMR spectra of the two minerals were also obtained.

Isotropic projections of three-quantum 17O MAS NMR spectra of (a) forsterite, (b) chondrodite and (c) clinohumite recorded using a phase-modulated split-t1 experiments.