A SOLID SOLUTION APPROACH TO STANDARD STATE THERMODYNAMIC MODELS OF MINERAL SURFACES

Mineral reactivity and stability is inherently controlled by the presence, properties, and areas of mineral surfaces. Considerable progress has been made relating mineral dissolution/precipitation kinetics, surface adsorption, and solubility to mineral surface area, usually cast in terms of area of mineral surfaces per mole or unit of mass. However, simultaneous consideration of the thermodynamics of surface area-controlled processes and heterogeneous reactions between minerals, aqueous solutions, and other fluids in Earth’s crust is hampered by the lack of suitable standard state descriptions of the presence and abundance of surfaces. A novel approach to this problem will be presented in which all minerals are considered to be solutions (in a structural, rather than chemical sense) between their surfaces (including the near-surface portions of crystals affected by the presence of the surface) and the bulk phase. The standard state thus adopted is unit activity of the pure phase (bulk mineral, mineral surface) at all temperatures and pressures. Mole fractions of the solution members may be calculated as a function of grain size through geometric models. Geometric calculations demonstrate that the mole fractions of the surface and bulk endmembers vary non-linearly with surface area. Given the typically nearly-linear dependence of enthalpies of reaction (e.g., heats of solution) with surface area for small grains, these models thus predict that mixing between the surface and the bulk is non-ideal. Calculations of the mixing and standard state thermodynamic properties of surface and bulk endmembers of titania and silica minerals demonstrate the veracity of this approach, and permit prediction of mineral stability as a function of grain size, temperature, pressure, and chemical potential. Specifically, observed properties of nano-phase minerals are predicted by these models, including the grain sizes at which the surface endmember is negligible (observed in heat of solution experiments) and stability crossovers at small grain sizes leading to initial formation of metastable minerals per the Ostwald Step Rule.