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 H_2O), including a colorless and a pale orange grossular sample from Asbestos, Quebec, and a colorless grossular sample from Lelatema Hills, Tanzania. Synthetic hydrogrussular powder, Ca_3Al_2(O_4H_4)_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 indicate that in the Tanzanian garnet the dominant cluster size is two protons; in the colorless grossular from Asbestos a mixture of two proton and four proton clusters is in evidence; in the hydrogrossular powder, the 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 case of the Tanzanian sample, the broad band constitutes ~85% of the area in the NMR spectrum. Moment analysis shows that although the synthetic hydrogrossular has an average interproton separation of 2.16 Å in the interacting cluster, it is 1.69 Å in the grossular.