The hydrous component in andradite garnet

Abstract

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 samles; those from low-temperature formations containing 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.