Characterization of hydrous species in minerals by high-speed proton MAS-NMR

Abstract

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/nm^3. Structurally isolated water molecules in analcite, NaAlSi_2O_6•H_2O, and gypsum, CaSO_4•2H_2O, yield characteristic ^1H MAS-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 field 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 H_2O groups, yield spectra with numerous intense spinning sidebands; strong dipolar interactions preclude discrimination of OH and H_2O. Nonstoichiometric hydrogen in nominally anhydrous minerals (feldspars, nepheline, quartz, and grossular garnet) is found to occur in a variety of forms: mobile H_2O in fluid inclusions, anisotropically constrained, isolated H_2O molecules, and clustered species consistent with H_4O_4^(4-) groups in a hydrogarnet substitution.