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 (super -) . Amounts up to 6 wt% H 2 O occur in these samples; those from low-temperature formations contain the most OH (super -) . Some features in the absorption spectra indicate the hydrogarnet substitution (SiO 4 ) (super 4-) <-->(O 4 H 4 ) (super 4-) whereas others indicate additional types of OH (super -) incorporation. The complexity of the spectra due to multi-site distribution of OH (super -) increases with increasing complexity of the garnet composition.
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