Diopside and anthophyllite dissolution at 25° and 90°C and acid pH

Dissolution experiments of diopside and anthophyllite were conducted in continuously stirred flow-through reactors at temperatures of 25° and 90°C at acid pH. Stoichiometric steady state was reached at ∼2700 h (at 25°) or ∼1000 h (90°C). Specific surface area significantly increased after dissolution of diopside but not anthophyllite. Extensive etch pits were observed on some of the reacted surfaces of diopside grains. The observed etch pits are preferentially developed along the exsolution lamellae boundaries between diopside and pigeonite or hypersthene. The surface morphology observed on our laboratory-leached samples is very similar to that observed on naturally weathered pyroxene grains by previous workers. No etch pits were observed on the surface of anthophyllite. Presumably, dissolution of anthophyllite results in the disintegration of fibrous anthophyllite without observable etching. Under the experimental conditions, dissolution rates of diopside and anthophyllite are pH-dependent, and the pH-dependence increases with increasing temperature. For diopside, the reaction order n with respect to H+ increases from 0.19±0.06 at 25°C to 0.76±0.08 at 90°C, while for anthophyllite, the value of n increases from 0.24±0.03 at 25°C to 0.63±0.04 at 90°C. This result indicates that when temperature increases, the rate of increase in pH-dependence is faster for diopside than for anthophyllite. Activation energies (Ea) have been estimated for diopside and anthophyllite dissolution based on dissolution at the two temperatures. The values of pH-independent Ea are 22.8±1.6 and 19.4±0.9 kcal mol−1 for diopside and anthophyllite, respectively. Both of the activation energies are well above the Ea of transport in solution (5 kcal mol−1), and significantly smaller than the Ea of breaking bonds in crystals.