We have determined infrared molar absorptivities for water absorption bands in Fe-bearing and Fe-free andesitic glasses. Water dissolves in andesitic glasses as both hydroxyl groups and molecular water as observed in other silicate glasses. Concentrations of molecular water and hydroxyl species are a strong function of total water content. IR molar absorptivities for Fe-bearing andesite are ε3570 = 62.32 ± 0.42 L/mol·cm, ε4500 = 0.79 ± 0.07 L/mol·cm, ε5200 = 1.07 ± 0.07 L/mol·cm, and ε1630 = 42.34 ± 2.77 L/mol·cm. Molar absorptivities for Fe-free andesite are 69.21 ± 0.52 L/mol·cm for ε3570, 0.89 ± 0.07 L/mol·cm for ε4500, 1.46 ± 0.07 L/mol·cm for ε5200, and 52.05 ± 2.85 L/mol·cm for ε1630. Molar absorptivities show significant compositional dependencies that can be predicted based on tetrahedral cation (Si+4, Al+3)/total cation fraction.

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