DISSOLUTION RATES OF PREHNITE, EPIDOTE, AND ALBITE

Dissolution rates of prehnite and epidote in aqueous solutions were measured in the temperature range 25 to 90-degrees-C, and as a function of pH. The dissolution rate of albite was measured at pH 1.4 at temperatures between 25 and 90-degrees-C. Batch experiments using low ionic strength pH buffers in constant temperature water baths or ovens provided data on cumulative element release as a function of time. Steady state Si, Ca, Na, and Al release data obtained from these experiments were used to obtain limiting dissolution rates, where the term limiting rate denotes dissolution of a bulk mineral under conditions where it is far from equilibrium with the fluid. At 90-degrees-C and at pH 1.4 to 6, prehnite and epidote dissolution rates decrease and are proportional to approximately -0.3 pH for prehnite and -0.2 pH for epidote. Above pH 6, prehnite dissolution becomes pH independent, but epidote dissolution increases with rates that are proportional to between +0.3 and +0.6 pH. Prehnite and epidote dissolution is linear and stoichiometric at low pH. At pH greater than 7, both minerals initially display preferential release of Si and Al relative to Ca; however, with increasing reaction dissolution becomes stoichiometric. This suggests that a Ca-enriched layer forms but reaches a steady state thickness which does not impede subsequent linear stoichiometric release. At pH 1.4, the limiting dissolution rate for albite is linear and stoichiometric. At pH 1.4, the activation energies are 18.12 +/- 0.81 kcal mol-1 for prehnite, 19.76 +/- 1.2 kcal mol-1 for epidote and 17.07 +/- 1.6 kcal mol-1 for albite. At pH 6.5, the activation energy for prehnite dissolution is 20.73 +/- 3.2 kcal mol-1.