The hydrous components in garnets: Grossular-hydrogrossular

George R. ROSSMAN, Roger D. AINES

Division of Geological and Planetary Science, California Institute of Technology
Pasadena, California 91125, U.S.A.


Grossular garnets from 33 localities were examined for indications of OH- or H2O in their infrared spectrum. All contained OH . Classical hydrogrossular with more than 5 wt% H2O displays systematic spectroscopic behavior consistent with the hydrogarnet substitution consisting of two absorption bands at 3598 and 3662 cm-1. These spectroscopic characteristics were generally not observed in other grossular samples. Instead, 20 distinct absorption bands have been identified in the spectra of common grossular, occurring in groups of four to ten bands. Both the number and intensities of these bands show a large variation, which does not correspond with the garnet's composition. Seven classes of spectra were identified in the OH region based on the position of the most intense absorption band. The spectroscopic data suggest that in addition to the tetrahedral site, OH groups exist in multiple other environments. The OH content of grossular garnets can be obtained from infrared spectra using the equation H2O wt% = 0.0000786 x integrated absorbance per cm in the OH region near 3600 cm-1. The OH content of macroscopic grossular crystals (expressed as weight percent H2O) ranged from 12.8% down to less than 0.005%. Macroscopic grossular typically contains less than 0.3 wt% H2O. Grossular from rodingites and low-temperature alteration vugs contained much more than that from skarn or contact metamorphic environments.

American Mineralogist, Volume 76, pages 1153-1164, 1991