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Impurity ions influence mineral growth rates through a variety of kinetic and thermodynamic processes that also affect partitioning of the impurity ion between the solid and solution. Here, the effect of an impurity ion, strontium, on Barite (BaSO4) (001) growth rates was studied using a combination of high-resolution in situ microscopy with ex situ chemical imaging techniques. In the presence of strontium, ⟨120⟩ steps roughened and bifurcated. The overall Barite growth rate also decreased with increasing aqueous strontium-to-barium ratio ([Sr]/[Ba]aq) < 1. Analysis of the reacted solids using chemical imaging techniques indicated strontium incorporated uniformly across all step orientations into the Barite growth hillock for [Sr]/[Ba]aq< 1. However, at [Sr]/[Ba]aq> 5, steps with an apparent [010] orientation were expressed and growth in the [010] step direction led to an increase in the overall growth rate of the surface. Strontium became preferentially incorporated into the [010] step direction, rather than being homogeneously distributed. The [Sr]/[Ba]sin the newly grown solid was found to correlate directly with that of solutions at [Sr]/[Ba]aq< 5, but not for higher [Sr]/[Ba]aq. Solid composition analyses indicate that thermodynamic equilibrium was not achieved. However, kinetic transport modeling successfully reproduces the shift in growth mechanism.

The dissolution kinetics of features on the magnesite (104) surface were studied in aqueous solutions from pH 4.2 to 2 and at temperatures between 60 and 90°C by hydrothermal atomic force microscopy (HAFM). At pH = 4.2, HAFM images showed magnesite step orientations that are comparable to the step orientations on calcite. Similar to calcite (104), there is anisotropy in the step velocity, but the magnitude of the anisotropy is much greater for magnesite. Furthermore, below pH = 4.2, changes in the dominant step orientation were observed. These results are discussed in terms of a nearest neighbor kink dynamic model, and the associated kink dynamics were tested with kinetic Monte Carlo (KMC) simulations. The KMC results suggest that the kink dynamic model does not account for the experimental observations and that further details such as second-nearest neighbor interactions or surface/edge diffusion cannot be excluded from the model. The dominant step orientations at low pH also point toward mechanisms stabilizing steps along periodic bond chain directions.

The pH and ionic strength dependence of light generated at a corundum-solution interface by the nonlinear optical process of second harmonic generation (SHG) is reported. A point of zero salt effect occurs in the pH range 5 to 6. The pH and ionic strength dependence of the SHG is qualitatively consistent with a model describing SHG from a charged mineral/water interface from Ong et al. (1992)and Zhao et al. (1993a, 1993b), but certain aspects of the model appear inadequate to describe the full range of our data. Atomic force microscopy (AFM) force-distance measurements, though imprecise, were consistent with a point of zero charge (p.z.c.) for the interface also in the pH range 5 to 6. The SHG (and AFM) results are different from expectation; the observed p.z.s.e. (and presumably also the p.z.c.) is considerably lower than the accepted point of zero charge of clean alumina powders (pH 8–9.4; Parks, 1965; Sverjenksy and Sahai, 1996). Although the reasons for this are unclear, SHG holds promise as a probe of oxide-water interfaces that is independent of interpretation of acid-base titration stoichiometry.

In contrast to most ionic minerals studied by SFM, the periclase (001) surface dissolves not by retreat of monolayer steps parallel to (001), but by retreat of a rough surface perpendicular to (001). At pH < 2, dissolution has an additional contribution from retreating macro-steps at the edges of nearly square pits. The macro-steps have heights up to 120 nm. In general, step direction is parallel to [110] and equivalent directions. During dissolution at low pH, a soft near-surface region is formed. Other investigators have shown that the near-surface region is protonated. Protonation is supposed to stabilize the (111) surface of periclase. Due to the structural similarities between periclase (111) and brucite (001), and similar dissolution rates of periclase and brucite at pH < 5, we conclude that during dissolution the periclase (001) surface is restructured into ‘‘(111) nano-facets’’ representing brucite (001)-like layers and appearing as a rough and soft surface in SFM images. The most probable reasons that the slopes of these macro-steps (up to 508) are lower than the slopes of perfect (111) facets are the likely poorly ordered structure of these layers, microtopography on these surface facets, and tip-surface convolution in SFM. By measuring the vertical position of the surface vs. time, we calculated the dissolution rate. At pH 1 and pH 2 we found the rates to be 17.1 ± 5.8 × 10-6and 5.7 ± 3.7 × 10-6mol/m2·s, respectively. These rates are in reasonable agreement with previously reported rates of periclase and brucite (001) dissolution, and are consistent with the idea that the MgO (001) surface consists of Mg(OH)2(001)-like layers.

The chemical dissolution of galena(001) surfaces immersed in acidic and neutral sulfate electrolyte solutions was investigated using Electrochemical Scanning Tunneling Microscopy (ECSTM). The nanometer-scale detail provided by the STM in this investigation provides new mechanistic information regarding surface dissolution processes at the mineral/water interface. In acidic solutions (pH = 2.7), we observe that dissolution occurs by selective removal of step edge species. This process is crystallographically anisotropic, resulting in steps with 〈110〉 orientations. Under neutral pH conditions (pH = 7), the dissolution rate is significantly lower, and no preferential step edge orientation is observed. The anisotropy observed in acidic solutions is attributed to different kinetic rates of dissolution along the 〈110〉 and 〈100〉 directions. Furthermore, we propose that the rate limiting step in the dissolution mechanism is the desorption of hydrosulfide (HS−) from step edges. The origin of the crystallographic anisotropy is discussed in terms of possible atomic structures of the step edges.