¹⁷O Multiple-Quantum MAS NMR Study of Pyroxenes

Abstract Two-dimensional triple-quantum 17O (I = 5/2) MAS NMR powder spectra were obtained for orthoenstatite, clinoenstatite, and protoenstatite (MgSiO3 polymorphs), and diopside (CaMgSi2O6). The spectra were analysed to yield the 17O isotropic chemical shifts (δCS) and quadrupolar coupling parameters (PQ = CQ (1+η2/3)1/2) of the oxygen sites in each sample. High-resolution isotropic projections exhibit resolved 17O peaks equal in number to the number of crystallographically inequivalent oxygen sites. Bridging and non-bridging oxygens were distinguished by their characteristic PQ values. Further assignments were possible by comparing the isotropic 17O NMR spectra for this series of similar compounds with that reported previously for forsterite (Mg2SiO4). Full assignments of the three oxygen sites in protoenstatite and diopside and of the six sites in orthoenstatite and clinoenstatite are suggested.

Two-dimensional triple-quantum 17O MAS NMR spectra of (a) orthoenstatite, (b) clinoenstatite, and (c) protoenstatite, recorded using phase-modulated split-t1 experiments.