Determination of molar absorptivity of IR fundamental OH-stretching vibration in rhyolitic glasses

Molar absorptivity of the infrared (IR) fundamental OH-stretching vibration band at 3550 cm^-1 was determined for rhyolitic glasses. Five obsidian samples, unheated and heated at 500-700°C using an internally heated pressure vessel, were used to evaluate the dependence of the molar absorptivity and final quenched H₂O speciation on H₂O contents and temperature. Water contents of the obsidians were measured by Karl-Fischer titration first, then the amount of unextracted H₂O was calibrated by IR spectroscopy and a conventional vacuum extraction method. Total H₂O contents of the obsidians were determined to be 0.24-1.25 wt%. IR spectra of the unheated and heated obsidian samples were obtained using an FT-IR microspectrometer. We determined the molar absorptivity for the 3550 cm^-1 band to be 75 ± 4 L/mol/cm without significant dependence on the H₂O contents and heating temperature. This value can be used to determine precise H₂O contents up to 1.25 wt% in rhyolitic volcanic glasses.