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 colorless and a pale orange grossular sample from Asbestos, Quebec, and a colorless grossular sample from LeLatema Hills, Tanzania. Synthetic hydrogrossular 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 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 site is four protons.

    Solid echo envelope decay experiments show that for all the natural samples a two-stop decay process occurs, implying the existence of two distinct proton environments. The area rations 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 1.60 Å in the grossular.

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American Mineralogist, Volume 78, pages 1149-1164, 1993