Christopher Barton (Committee Member), David Dolson (Committee Member), David Grossie (Committee Member), Steven Higgins (Advisor), Andrew Stack (Committee Member)
Doctor of Philosophy (PhD)
A fundamental understanding of mineral growth kinetics is necessary to predict mineral reactivity in geologic environments. We use hydrothermal atomic force microscopy (HAFM) to measure step advance speeds and morphological evolution of the sparingly-soluble minerals magnesite (MgCO3), barite (BaSO4) and celestite (SrSO4) while systematically varying the concentrations of their constituent cations and anions in solution. For all three minerals, a maximum step velocity is reached at an aqueous cation:anion concentration (r) near unity and decreases with extreme r despite order of magnitude differences in the water exchange rate for the cations comprising the minerals. Affinity based models fail to reproduce the observed trends in which step velocities vary with changes in the cation-to-anion ratio. A process based model developed by Zhang and Nancollas (1990, 1998) does not fit the peak shape in experimental measurements of step speed versus r and underestimates step velocities which may arise from the model assumption that the forward reaction is tied to the back reaction through the solubility product. Step velocities as a function of r on all three minerals can be modeled well using the Stack and Grantham (2010) kink site nucleation + propagation model. While the growth of the minerals as a function of r can be described using the same model, there are some significant differences in the behavior of the three minerals. First, model derived detachment rates of ions from a step to propagate kink sites is zero for barite and celestite, but nonzero for magnesite. Significant morphological changes are also observed for barite as a function of r, but not for celestite and magnesite. Finally, step velocities on celestite are non-linear as a function of saturation state, whereas they are linear for barite and magnesite. Together, these results suggest that current models which are utilized to predict mineral reactivity in environmental settings (e.g. reactive transport models) are missing a key parameter necessary to accurately predict mineral growth.
Department or Program
Department of Earth and Environmental Sciences
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