Surface chemistry and dissolution kinetics of glassy rocks at 25°C

The weathering rates and mechanisms of three types of glassy rocks were investigated experimentally at 25 °C, pH 1.0 to 6.2, and reaction times as much as to 3 months. Changes in major element chemistry were monitored concurrently as a function of time in the aqueous solution and within the near surface region of the glass. Leach profiles, obtained by a HF leaching technique, displayed near-surface zones depleted in major cations. These zones increased in depth with increasing time and decreasing pH of reactions. Release rates into the aqueous solution were parabolic for Na and K and linear for Si and Al. A coupled weathering model, involving surface dissolution with concurrent diffusion of Na, K, and Al, produced a mass balance between the aqueous and glass phases. Steady state conditions are reached at pH 1.0 after approximately 3 weeks of reaction. Steady-state is not reached even after 3 months at pH 6.2.An interdiffusion model describes observed changes in Na diffusion profiles for perlite at pH 1.0. The calculated Na self-diffusion coefficient of 5 × 10^?19 cm²·s^?1 at 25°C approximates coefficients extrapolated from previously reported high temperature data for obsidian. The self-diffusion coefficient for H₃O⁺, 1.2 × 10^?20 cm²·s^?1, is similar to measured rates of water diffusion during hydration of obsidian to form perlite.