The hydrous component in garnets: pyralspites

Roger D. Aines and George R. Rossman
Division of Geological and Planetary Sciences, 170-25, California Institute of Technology,
Pasadena, California 91125, U.S.A.


Natural pyralspite garnets have been found to commonly contain a hydrous component, ranging in concentration from 0.02 to 0.25 wt.% as H2O.  Anhydrous pyralspites of crustal origin are rare. Of  forty crustal garnets examined, only two were anhydrous. The most-hydrous garnets were spessartines from igneous pegmatites. Metamorphic garnets had lower water contents, and frequently also contained hydrous inclusions. The infrared absorptions of the hydrous comonent in the garnet end members are characteristic, and consist of 2 to 4 hnarrow bands centered at 3640 cm-1 in spessartine, 3500 cm-1 in almandine, and 3670 cm-1 in pyrope.  The IR spectra indicate that the hdyrous comonent in  not in the form of molecular H2O; the most likely form is H4O44- substitutiong for SiO44- but other substitutions involving multiple OH- groups on one site are consistent with the data. The concentration of OH (as H2O) in garnets may be determined from the integral absorptivity (K) in the 3700-3400 cm-1 region, and although K varies with the chemistry from 3700-6000 (l molH2O-1 cm-2) in end members to 120-600 in intermediate compositions.