High-resolution 17O MAS NMR spectroscopy of forsterite (α-Mg2SiO4) wadsleyite (β-Mg2SiO4), and ringwoodite (γ-Mg2SiO4)

The high sensitivity of the satellite-transition (ST) MAS NMR technique was exploited to obtain high-resolution O-17 MAS NMR spectra of the three polymorphs of Mg2SiO4: forsterite (alpha-Mg2SiO4) wadsleyite (beta-Mg2SiO4), and ringwoodite (gamma-Mg2SiO,). High NMR sensitivity was important in this application because O-17-enriched, Fe-free materials are required for O-17 NMR and high-pressure syntheses of the dense beta and gamma polymorphs result in a only a few milligrams of these solids. In all, eight distinct beta species were identified and assigned: three in forsterite, four in wadsleyite, and one in ringwoodite, in agreement with the number of beta sites in their crystal structures. The isotropic chemical shifts extracted are in excellent agreement with a previously published correlation with Si-O bond length. However, unexpectedly large quadrupolar coupling constants were found for the non-bridging O species in the dense polymorphs wadsleyite and ringwoodite.