Single-crystal NMR studies of low-concentration hydrous species in minerals: Grossular garnet

Herman Cho
A.A Noyes Laboratory of Chemical Physics
California Institute of Technology
Pasadena, CA 91125

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


    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.

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