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Abstract: Experimental observation of step (i.e., a linear discontinuity in a surface) growth on calcium carbonate and barium sulfate surfaces was performed using in situ scanning probe microscopy (SPM) in aqueous solutions. Step speeds at very short step lengths (<300nm) were found to increase with step length, but scatter in the data precluded comparison of different theoretical models with regard to the experimental data. Analysis of experimental errors revealed that the data scatter was not unusual and further suggested that significant experimental bias in the determination of step properties such as the step free energy and kinetics of kink propagation and nucleation are introduced if the instrument bandwidth is not appropriate for the specific dynamics of the step. In light of step growth models based on either equilibrium assumptions or non-equilibrium assumptions, these biases tend to favor one model over the other. [Copyright &y& Elsevier]

Provides information on a study which investigated the rate and mechanism of periclase (001) surface dissolution. Methodology; Results; Discussion of the findings.

Stepvelocities of monolayer-height steps on the (101̅4)magnesite surface have been measured as functions of the aqueous magnesium:carbonateratio and saturation index (SI) using a hydrothermal atomic forcemicroscope. At SI ≤ 1.9 and 80-90 °C, step velocitieswere found to be invariant with changes in the magnesium:carbonateratio, an observation in contrast with standard models for growthand dissolution of ionically bonded, multicomponent crystals. However, at high saturation indices (SI = 2.15), step velocities displayeda ratio dependence, maximized at magnesium:carbonate ratios slightlygreater than 1:1. Traditional affinity-based models could not describegrowth rates at the higher saturation index. Step velocities alsocould not be modeled solely through nucleation of kink sites, in contrastto other minerals whose bonding between constituent ions is also dominantlyionic in nature, such as calcite and barite. Instead, they could bedescribed only by a model that incorporates both kink nucleation andpropagation. On the basis of observed step morphological changes atthese higher saturation indices, the step velocity maximum at SI =2.15 is likely due to the rate of attachment to propagating kink sitesovercoming the rate of detachment from kink sites as the latter becomesless significant under far from equilibrium conditions. [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this work was to examine the effects of polished crystal-surface orientation and degree of solution undersaturation on the dissolution kinetics of calcite. Crystallographic surface orientations utilized in this study included natural calcite specimens polished approximately parallel to the (104) plane, giving rise to surfaces with flat terraces with few steps, as well as fully kinked surfaces created by sectioning approximately parallel to the (001) plane. Results from inductively coupled plasma optical emission spectroscopy (ICP-OES) and vertical scanning interferometry (VSI) investigations revealed how crystallographic orientations of calcite with initially higher energy surface morphologies were associated with greater extent of reaction, greater surface retreat, and therefore, greater initial transient dissolution rates than those with lower energy initial surface morphologies. However, both the ICP-OES and atomic force microscopy (AFM) results confirm that the effects of crystal orientation become minimal under long-term conditions because (1) variously oriented calcite surfaces exhibited similar “long-term” rates and (2) orientations with high initial reactive site densities developed lower energy morphologies. Estimating a step rate coefficient from the experimental data from a miscut sample suggest that these coefficients may be highly morphology dependent. Results from this study are significant for predicting long term calcite dissolution rates because they suggest that the “long-term” dissolution rate of calcite will be governed by the rates of the rhombohedral faces and microfacets and the time-scale for approaching such conditions is of the order of days at room temperature and slightly alkaline solution conditions. [Copyright &y& Elsevier]

To better understand the role of spectator ions in barite growth, the kinetics of step edge growth on barite (001) surfaces were studied under various salt solutions. Hydrothermal atomic force microscopy (HAFM) was used to investigate the effect of background electrolytes (NaCl, NaBr, and NaNO3) as a function of saturation index and ionic strength (I) on barite growth sourced at dislocations at 108 °C. Results demonstrate that hillock morphology is affected by I, as well as type of anion, where the prevalence of steps aligned on the [010] direction is highest under Cl-. There is a modest increase in kinetic coefficient of 55-130% with a 10-fold increase in I for each salt. In comparing the kinetic coefficients of the salts at low ionic strength (0.01 M), there is a moderate difference, suggesting that the anion may play a role in barium attachment. [ABSTRACT FROM AUTHOR]