Temperature-induced changes in the NIR spectra of hydrous albitic and rhyolitic glasses between 300 and 100 K

Near-infrared(NIR) absorption bands related to total water (4000 and 7050 c m(-1)), OH groups (4500 cm(-1)) and molecular H2O (5200 cm(-1)) were studie d in two polymerised glasses, a synthetic albitic composition and a natural obsidian. The water contents of the glasses were determined using Karl Fis cher titration. Molar absorption coefficients were calculated for each of t he bands using albitic glasses containing between 0.54 and 9.16 wt.% H2O an d rhyolitic glasses containing between 0.97 and 9.20 wt.% H2O. Different co mbinations of baseline type and intensity measure (peak height/area) for th e combination bands at 4500 and 5200 cm(-1) were used to investigate the ef fect of evaluation procedure on calculated hydrous species concentrations. Total water contents calculated using each of the baseline/molar absorption coefficient combinations agree to within 5.8% relative for rhyolitic and 6 .5% relative for albitic glasses (maximum absolute differences of 0.08 and 0.15 wt.% H2O, respectively). In glasses with water contents >1 wt.%, calcu lated hydrous species concentrations vary by up to 17% relative for OH and 11% relative for H2O (maximum absolute differences of 0.33 and 0.43 wt.% H2 O, respectively). This variation in calculated species concentrations is ty pically greater in rhyolitic glasses than albitic. In situ, micro-FTIR anal ysis at 300 and 100 K was used to investigate the effect of varying tempera ture on the NIR spectra of the glasses. The linear and integral molar absor ption coefficients for each of the bands were recalculated from the 100 K s pectra, and were found to vary systematically from the 300 K values. Linear molar absorption coefficients for the 4000 and 7050 cm(-1) bands decrease by 16-20% and integral molar absorption coefficients by up to 30%. Dependin g on glass composition and baseline type, the integral molar absorption coe fficients for the absorption bands related to OH groups and molecular H2O c hange by up to -5.8 and +7.4%, respectively, while linear molar absorption coefficients show less variation, with a maximum change of similar to 4%. U sing the new molar absorption coefficients for the combination bands to cal culate species concentrations at 100 K, the maximum change in species conce ntration is 0.08 wt.% H2O, compared with 0.39 wt.% which would be calculate d if constant values were assumed for the combination band molar absorption coefficients. Almost all the changes in the spectra can therefore be inter preted in terms of changing molar absorption coefficient, rather than inter conversion between hydrous species.