The hydrous component in garnets: pyralspites

Abstract

Natural pyralspite garnets have been found to commonly contain a hydrous component, ranging in concentration from 0.02 to 0.25 wt.% as H_2O. Anhydrous pyralspites of crustal origin are rare. Of forty crustal garnets examined, only two were anhydrous. The mosthydrous 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 m the form of molecular H_2O; the most likely form is H_4O^(4-)_4 substituting for SiO^(4-)_4 but other substitutions involving multiple OH^- groups on one site are consistent with the data. The concentration of OH (as H_2O) in garnets may be determined from the integral absorptivity (K) in the 3700-3400 cm^(-1) region, although K varies with chemistry from 3700-6000 (l mol^(-1)_2O cm^(-2)) in end members to 120-600 in intermediate compositions.