KINETICS OF THE REACTION H2O + O # 2OH IN RHYOLITIC GLASSES UPON COOLING: GEOSPEEDOMETRY AND COMPARISON WITH GLASS TRANSITION

Abstract

Controlled cooling rate experiments of hydrous rhyolitic glasses have been carried out to examine the relation between the apparent equilibrium temperature of the quenched glass (or the final quenched speciation of molecular H2Om and OH groups), the cooling rate, and the total H2O content (H2Ot). The experimental data are highly reproducible and internally consistent. Original band intensities are used to represent the data. A523 and A452 (absorbances of the 523 and 452 mm-1 bands in terms of peak height per mm sample thickness) are used as proxy for H2Om and OH, respectively. Q' (= A2452/A523) is used as proxy for the equilibrium constant of the reaction. For a given cooling rate, ln Q' and ln [A523 + A452>] are linearly related. When combined with air-quenched and water-quenched experiments reported by Silver et al. (1990), an abrupt change in slope is apparent. The experimental results provide a geospeedometer to calculate quench rates of natural hydrous rhyolitic glasses. The apparent equilibrium temperature for the reaction is roughly the same as the viscosity-defined glass transition temperature. An approximate reaction rate law for the reaction has been inferred.