Feldspar dissolution kinetics and equilibrium: Implications for natural and engineered systems
Beig, Mikala Stephanie
Doctor of Philosophy
The chemical composition of groundwater is controlled largely by subsurface mineral/fluid interactions. Experimental measurement of plagioclase dissolution rates provides a critical means of understanding the relationship between groundwater composition and reservoir characteristics. This thesis focuses on two areas in which understanding of crystal dissolution is poor, i.e., dissolution kinetics as a function of undersaturation under near-equilibrium conditions, and the relationship between dissolution rate, mechanism, crystallography and surface reactivity. We employ a powerful approach: quantification of crystal surface normal retreat rates using vertical scanning interferometry (VSI), which provides direct observation of changes in the mineral surface due to dissolution reactions. This thesis presents results from three experimental studies during which we measured the dissolution rates of albite and anorthite single-crystal, cleavage surfaces as a function of solution saturation state during flow-through experiments over a wide range of temperatures (25--200°C). In addition, we identified specific dissolution mechanisms on reacted crystal surfaces using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The first study compared the dissolution rates of a fine-grained albite powder with those of two albite cleavage surfaces [(010) and (001)]. The dissolution rates of fine-grained albite powders are substantially enhanced compared to those prevailing on large single crystal cleavage surfaces. The second study compared the dissolution rates on previously dissolved albite surfaces exhibiting etch pits and pristine surfaces lacking dissolution features. Experimental results document an up to 2 orders of magnitude difference in dissolution rate between the differently pretreated surfaces, which are dominated by different dissolution mechanisms. The rate difference, which persists over a range of solution saturation state, indicates that the dissolution mechanisms obey different Gibbs free energy difference (DeltaG) dependencies. We propose that the rate gap is the direct consequence of a kinetic bifurcation in dissolution rate and mechanism as DeltaG varies. The existence of the kinetic bifurcation indicates that a single, continuous function describing the relationship between dissolution rate and DeltaG is insufficient. The third study focused on the dissolution kinetics of anorthite with respect to DeltaG and surface reactivity. Our results indicate that dissolution experiments conducted with mineral powders have limited relevance to natural systems.