High-resolution silicon-29 nuclear magnetic resonance spectroscopic study of rock-forming silicates

Abstract

High-resolution silicon-29 magic-angle sample-spinning NMR spectroscopic studies of a wide range of natural and synthetic silicates indicate (l) considerable overlap among the ranges of isotropic chemical shifts for crystals of different polymer types. This expands upon the work of Lippmaa et a/. (1980), who found well-separated ranges; (2) a wide range of chemical shift anisotropies (CSA) and asymmetry parameters (q) that are related to symmetry and structure; (3) a relatively poor correlation between isotropic chemical shift and average Si-O bond length; (4) a better correlation between isotropic chemical shift and total cation-oxygen bond strength for the four oxygens ofeach silicon tetrahedron;,and (5) discrepancies between the NMR results and crystal structure refinements for kyanite and wollastonite. These results indicate that both the Si-O bond length-chemical shift and bond strength-chemical shift relationships are useful tools for investigating the structures of crystalline silicates and, perhaps more importantly, silicate glasses, clays, and zeolites that cannot be examined by single crystal X-ray or neutron diffraction methods.