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Fourier transform infrared (FTIR) spectroscopy can be used to determine the concentration and of dissolved water in silicate glasses if the molar absorptivity coefficients (e) are known. that are thin and/or water-poor typically require the use of the mid-IR 3500 cm–1 total water (H2Ot) and 1630 cm–1 molecular water (H2Om) absorbance bands, from which hydroxyl water (OH) must determined by difference; however, accurate determination of H2Ot and OH is complicated because 3500 varies with water speciation, which is not usually known a priori. We derive an equation that uses -member e3500 values to find accurate H2Ot and OH concentrations from the 3500 cm–1 absorbance samples where only the H2Om concentration need be known (e. g. , from the 1630 cm–1 band). We this new species-dependent e3500 method against published data sets and new analyses of glass. We use published data to calculate new end-member e3500 values of e3500OH = 79 ± 11 and 3500H2Om = 49 ± 6 L/mol∙cm for Fe-bearing andesite and e3500OH = 76 ± 12 and e3500H2Om = 62 ± 7 /mol∙cm for Fe-free andesite. These supplement existing end-member values for rhyolite and albite. We demonstrate that accounting for the species-dependence of e3500 is especially important hydrated samples, which contain excess H2Om, and that accurate measurement of OH concentration, conjunction with published speciation models, enables reconstruction of original pre-hydration contents. Although previous studies of hydrous silicate glasses have suggested that values of e with decreasing tetrahedral cation fraction of the glass, this trend is not seen in the four sets of -member e3500 values presented here. We expect that future FTIR studies that derive end-member 3500 values for additional compositions will therefore not only enable the species-dependent e3500 to be applied more widely, but will also offer additional insights into the relationship between of e and glass composition.
McIntosh et al. (Tue,) studied this question.