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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]

Dissolution and epitaxial growth on dolomite [CaMg(CO3)2] cleavage surfaces under well-defined aqueous solution conditions were studied in real-time by atomic force microscopy (AFM). Step-retreat speeds in undersaturated solutions were used to predict the growth speed for ordered dolomite utilizing a theoretical step-growth model. Monolayer growth on dolomite under highly supersaturated conditions was observed, but multilayer growth was inhibited due to film disorder enhanced solubility. Ordered dolomite film growth is predicted in supersaturated solutions close to equilibrium, but observed step speeds were immeasurably low under the current laboratory low-temperature conditions. [ABSTRACT FROM AUTHOR]

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]

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]