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Abstract: Dissolution kinetics at the aqueous solution-calcite interface at 50 °C were investigated using in situ atomic force microscopy (AFM) to reveal the influence of magnesium concentration and solution saturation state on calcite dissolution kinetics and surface morphology. Under near-equilibrium conditions, dissolved displayed negligible inhibitory effects on calcite dissolution even at concentrations of . Upon the introduction of , the solution saturation state with respect to calcite, , acted as a “switch” for magnesium inhibition whereby no significant changes in step kinetics were observed at , whereas a sudden inhibition from was activated at . The presence of the -switch in dissolution kinetics indicates the presence of critical undersaturation in accordance with thermodynamic principles. The etch pits formed in solutions with exhibited a unique distorted rhombic profile, different from those formed in Mg-free solutions and in de-ionized water. Such unique etch pit morphology may be associated with the anisotropy in net detachment rates of counter-propagating kink sites upon the addition of . [ABSTRACT FROM AUTHOR]

Abstract: In situ Atomic Force Microscopy (AFM) and Lateral Force Microscopy (LFM) studies on dolomite (101̄4) were performed during exposure to supersaturated aqueous solutions (supersaturated in dolomite, calcite, aragonite, vaterite, huntite and magnesite) at pH = 9 at various Ca2+/Mg2+ aqueous ion activity ratios. At high saturation ratios, rapid growth of a single layer (∼3 Å thick) of a carbonate followed by much slower growth of a second layer was observed. Growth of the second layer was highly inhibited, suggesting that the first layer was essentially self-limited, and inhibited further layer-by-layer growth. The growth of the first layer was observed over a wide range of Ca2+/Mg2+ ratios, suggesting that the dolomite surface is favorable to formation of a range of Ca-Mg carbonates. LFM data revealed contrast in the tip-surface frictional forces on the first grown layer, but this contrast was only observed in layers grown from middle to high Ca2+/Mg2+ solutions. Thus, LFM may have detected or responded to differences in the structure and/or composition between the first layer relative and the dolomite substrate. Dissolution of the first layer occurred from significantly supersaturated solutions relative to ordered stoichiometric dolomite permitting an estimate of the excess interfacial strain energy of up to 10 mJ/m2. [Copyright &y& Elsevier]

Sr partitioning on calcite crystals growing from Ca 2 + –Sr 2 + –CO 3 2 − solutions was studied by means of friction force microscopy (FFM). Experiments were performed with various Sr 2 + (aq) /Ca 2 + (aq) concentration ratios and total Sr 2 + (aq) concentration in order to examine conditions under which Sr-calcite growth is self-limiting (e.g., the so-called “template effect”) and also to investigate continuous Sr-calcite growth, where spiral growth predominates and Sr incorporation is sector-dependent. In these latter experiments, the goal was to evaluate the utility of friction force microscopy to discriminate sector zoning. Results from the experiments show that friction increases with the incorporation of Sr into the growing calcite layers. The maximum increase in friction was measured at high Sr 2 + (aq) /Ca 2 + (aq) , although a quantitative link between a specific amount of increase in friction to a specific amount of Sr incorporation was not possible to determine due to experimental uncertainties. Nevertheless, it was possible to establish that no change in friction is detectable when Sr incorporation yields a solid composition of Sr 0.05 Ca 0.95 CO 3 . Friction was found to increase during growth of several layers in an incremental fashion. The increase can be linked either to an incremental increase in Sr content in the newly formed calcite, controlled by the thermodynamics of the strained layers necessitated by the substitution of larger Sr cations into the calcite, or to the incremental increase in layer thickness which in turn leads to increases in the probe-surface contact area. No difference in friction could be observed between acute and obtuse sectors under any of the experimental conditions, which was primarily due to the limits of the friction measurement sensitivity. [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]

Abstract: Direct measurements of calcite faces were performed using in situ atomic force microscopy (AFM) to reveal the dissolution processes as a function of solution saturation state and temperature. Time-sequential AFM images demonstrated that step velocities at constant temperature increased with increasing undersaturation. The anisotropy of obtuse and acute step velocities appeared to become more significant as solutions approached equilibrium and temperature increased. At saturation state Ω >0.02, a curvilinear boundary was formed at the intersection of two acute steps and the initially rhombohedral etch pit exhibited a nearly triangular shape. This suggests that the and steps may not belong to the calcite-aqueous solution equilibrium system. Further increase in the saturation state (Ω ⩾0.3) led to a lack of etch pit formation and dissolution primarily occurred at existing steps, in accordance with . Analysis of step kinetics at different temperatures yielded activation energies of 25±6kJ/mol and 14±13kJ/mol for obtuse and acute steps, respectively. The inconsistencies in etch pit morphology, step anisotropy, and step activation energies from the present study with those of studies far-from-equilibrium can be explained by increased influence of the backward reaction, or growth, near-equilibrium. We propose that the backward reaction occurs preferentially at the acute–acute kink sites. The kinetics and effective activation energies of near-equilibrium calcite dissolution presented in this work provide accurate experimental data under likely CO2 sequestration conditions, and thus are crucial to the development of robust geochemical models that predict the long-term performance of mineral-trapped CO2. [Copyright &y& Elsevier]