SINGLE-CRYSTAL NMR-STUDIES OF LOW-CONCENTRATION HYDROUS SPECIES IN MINERALS - GROSSULAR GARNET

A detailed solid-state proton NMR study has been performed on garnets containing low levels of OH (0.2-0.3 wt% as H2O), including a colorles s and a pale orange grossular sample from Asbestos, Quebec, and a colo rless grossular sample from Lelatema Hills, Tanzania. Synthetic hydrog rossular powder, Ca3Al2(O4H4)3, has been investigated as well. The NMR spectra of the three natural crystals have a broad line (40 kHz) with a weak sharper feature superimposed. Multiple-quantum spectra indicat e that in the Tanzanian garnet the dominant cluster size is two proton s; in the colorless grossular from Asbestos a mixture of two proton an d four proton clusters is in evidence; In the hydrogrossular powder, t he cluster size is four protons. Solid echo envelope decay experiments show that for all the natural samples a two-step decay process occurs , implying the existence of two distinct proton environments. The area ratios,indicate that the spectrum of each of the natural crystals is dominated by a broad line with a Gaussian shape and a line width that differs from the line width of the synthetic hydrogrossular. In the ca se of the Tanzanian sample, the broad band constitutes approximately 8 5% of the area in the NMR spectrum. Moment analysis shows that althoug h the synthetic hydrogrossular has an average interproton separation o f 216 angstrom in the interacting cluster, it is 1.69 angstrom in the grossular.