Modeling Dynamic Properties of Mineral Surfaces

We describe the application of atomistic simulation techniques to investigate mineral surface properties, such as the effect water adsorption on the structure and stability of baryte (BaSO₄) surfaces, the energetics of calcite (CaCO₃) growth at surface steps and the determination of the surface vibrational properties of Fe₂O₃,. Hydration stabilises all baryte surfaces, although the {210} surface remains the dominant surface in agreement with experiment. The interaction of water molecules with both calcite and baryte surfaces is predominantly through coordination of the water molecule's oxygen atom to the surface cation, rather than hydrogen-bonding to surface oxygen atoms. On baryte the hydration energies (-53 to -66 kJmol^-1) indicate physisorption of the water molecules, while on calcite the energies (-79 to -139 kJmol^-1) are more in the region of chemisorption. The rate determining step of calcite growth from two step edges is the creation of the kink sites on the edges when the first CaCO₃ or MgCO₃ unit is introducted. Pure calcite growth is calculated to occur preferentially from the obtuse step edge, where introduction of the kink sites costs 82.0 kJmol^-1 as opposed to 235.4 kJmol^-1 on the acute edge. Addition of MgCO₃ to both purely calcite edges and edges decorated by magnesium is an exothermic process and at all stages energetically more favourable than addition of CaCO₃ units. Hence in a mixed solution magnesium will be preferentially incorporated into the growing calcite crystal and pure calcite growth is thus inhibited. Finally, simulations on hematite surfaces suggest that the surface free-energy is dominated by the potential and zero point energies and that the contribution of vibrational entropy is minimal.