High-Resolution O-17 Nmr of Solid Silicates

Several O-17-enriched silicates were studied by use of dynamic angle spinning (DAS) and double rotation (DOR) nuclear magnetic resonance spectroscopy. These methods average away second-order quadrupolar interactions by reorienting a sample about a time-dependent axis, thereby yielding high-resolution spectra of oxygen-17 nuclei. A narrow spectral line is observed for each distinct oxygen site at the sum of the isotropic chemical shift and the field-dependent isotropic second-order quadrupolar shift. Resolution is increased by up to 2 orders of magnitude compared to conventional magic angle spinning (MAS) spectra. Crystallographically inequivalent oxygens are now observable as distinct resonances in spectra of polycrystalline silicates such as diopside (CaMgSi2(17)O6), wollastonite (CaSi17O3), clinoenstatite (MgSi17O3), larnite (Ca2Si17O4),and forsterite (Mg2Si17O4).