The hydrous component in andradite garnet

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% H_2O occur in these samples; those from low-temperature formations contain the most OH-. Some features in the absorption spectra indicate the hydrogarnet substitution (SiO_4)^(4-)↔(O_4H_4)^(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