High-speed proton MAS-NMR of minerals

James P. Yesinowski, Hellmut Eckert
Division of Chemistry and Chemical Engineering
California Institute of Technology, Pasadena, CA 91125

George R. Rossman
Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, CA 91125


Proton magic-angle spinning (MAS) NMR at 200 and 500 MHz and at high spinning speeds (ca. 8 kHz) has been used to characterize hydrous species, both stoichiometric and nonstoichiometric, in a variety of minerals. High-resolution 1H MAS-NMR spectra of minerals containing stoichiometric hydroxyl groups as the only hydrous species are obtainable provided that the hydrogen density in the sample is less than about 15 atoms/nm3. Structurally isolated water molecules in analcite, NaAlSiaO^H^O, and gypsum, CaSO4·2H2O, yield characteristic 1MAS-NMR spectra with numerous spinning sidebands extending over a range of about 100 kHz, reflecting the strong, largely inhomogeneous character of the homonuclear dipolar coupling. The Held dependence of both line widths and spinning sideband patterns provides evidence about the nature of the broadening interactions. Lawsonite and hemimorphite, minerals containing stoichiometric amounts of both OH and H2O groups, yield spectra with numerous intense spinning sidebands; strong dipolar interactions preclude discrimination of OH and H2O, Nonstoichiometric hydrogen in nominally anhydrous minerals (feldspars, nepheline, quartz, and grossular garnet) is found to occur in a variety of forms: mobile H2O in fluid inclusions, anisotropically constrained, isolated H2O molecules, and clustered species consistent with H4O44- groups in a hydrogarnet substitution.