Abstract

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 component in the garnet end members are characteristic, and consist of 2 to 4 narrow 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 hydrous component is not in the form of molecular H2O; the most likely form is H4O44 substituting 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, although K varies with chemistry from 37006000(1molH2O1cm2) in end members to 120–600 in intermediate compositions.

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