Amthauer G, Rossman GR
Division of Geological and Planetary Sciences
California Institute of Technology, Pasadena, CA 91125-2500, U.S.A.
Twenty-two andradite samples from a variety of geological environments and two synthetic hydroandradite samples were studied by Fourier transform IR spectroscopy. Their spectra show that H enters andradite in the form of OH. Amounts up to 6 wt% H2O occur in these samples; those from low-temperature formations contain the most OH. Some features in the absorption spectra indicate the hydrogarnet substitution (SiO4)4- - (O4H4)4- whereas others indicate additional types of OH incorporation. The complexity of the spectra due to multi-site distribution of OH increases with increasing complexity of the garnet composition.