17O Multiple-Quantum MAS NMR Study of Pyroxenes

Two-dimensional triple-quantum O-17 (l = (5)/(2)) MAS NMR powder spectra were obtained for orthoenstatite, clinoenstatite, and protoenstatite (MgSiO3 polymorphs) and diopside (CaMgSi2O6). The spectra were analyzed to yield the O-17 isotropic chemical shifts (delta(CS)) and quadrupolar coupling parameters (P-Q = C-Q(1 + eta(2)/3)(1/2)) of the oxygen sites in each sample. High-resolution isotropic projections exhibit resolved O-17 peaks equal in number to the number of crystallographically inequivalent oxygen sites. Bridging and nonbridging oxygens were distinguished by their characteristic P-Q values. Further assignments were possible by comparing the isotropic O-17 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.