DIOPSIDE DISSOLUTION RATES AT EARTH SURFACE CONDITIONS IN ALKALINE SOLUTIONS

Understanding changes of the concentration of CO₂ in the atmosphere due to changes in the Phanerozoic carbon cycle requires knowledge of the weathering rate of Ca-Mg silicates. The present study examines the role of pH on the dissolution rate of diopside. Experiments were performed by reacting ground diopside in alkaline pH buffer solutions in batch reactors open to atmospheric CO₂. Rates of diopside dissolution were determined by measuring the concentration of elements in solution as a function of time, correcting for the removal of sampling solution. These experiments indicate the far from equilibrium steady-state dissolution rate of diopside is pH independent at alkaline pH as reported by other investigators. This behavior is unlike that of most other silicates that have been measured. Most silicates show an increase in the log of the far from equilibrium rate of dissolution of 0.3 for each unit increase of pH in alkaline solutions (Brady and Walther, 1989). It is argued that the alkaline, pH independent dissolution rate of diopside is due to stabilization of diopside surface sites by attachment of carbonate species from solution as proposed by Wogelius and Walther (1991) for their olivine dissolution experiments. Increased numbers of rate controlling negatively charged Si surface sites with increased pH are not exposed at the surface as with other silicates as stable Mg-Ca carbonate complexes form on the surface instead. This idea was tested by performing diopside dissolution experiments in nearly CO₂ free alkaline solutions by vigorously boiling the buffer solutions, cooling them to room temperature and running the experiments in a CO₂ purged glove box by introducing a steady flow of N₂. Between a pH of 9 and 13, these far from equilibrium steady-state log rates of diopside dissolution increased by 0.25±0.03 for each unit pH increase, consistent with the model of silicate dissolution in alkaline solutions proposed by Brady and Walther.