Experimental study of epidote dissolution rates from pH 2 to 11 and temperatures from 25 to 200 °C

The dissolution rates of Green Monster Mine epidote at temperatures of 25, 100 and 200 °C, and over the pH range 2–11, were determined from far-from equilibrium experiments performed in both batch and mixed-flow reactors. Epidote dissolution rates based on silicon release decrease with increasing pH to pH ∼ 8, then increase in response to further pH increases. The apparent relative metal release rates from epidote at chemical steady-state were found to vary with pH and temperature, in part due to secondary phase formation. At 100 and 200 °C, Al is preferentially released relative to Si at alkaline pH, but preferentially retained by the solid at 2 < pH < 8. This behaviour is interpreted to stem from the precipitation and re-dissolution of Al oxy-hydroxides such as boehmite, diaspore, and gibbsite, as observed in solids recovered after the experiments. Calcium tends to be preferentially released compared to Si and Al at acidic and near to neutral pH. Iron was found to be retained in the solid phase due to the prompt formation of secondary Fe minerals. Nevertheless, neither the dissolution of 20 wt% of the initial epidote mass nor the formation of secondary phases on the epidote surface substantially changed epidote dissolution rates. As epidote (i) is almost three times more Ca-rich than a typical basaltic glass, (ii) preferentially releases Ca at pH typical of CO₂-charged water injections, and (iii) exhibits increasing dissolution rates as pH increases at alkaline conditions, it seems likely that the presence of epidote in altered basalts may be beneficial for the formation of calcite and other Ca-carbonates during mineral carbonation efforts. In addition, the relatively rapid rates of epidote dissolution at acidic conditions suggest that epidote has a significant role in the weathering of altered mafic rocks.