Magnesite step growth rates as a function of the aqueous magnesium:carbonate ratio

Step velocities of monolayer-height steps on the (1014) magnesite surface have been measured as functions of the aqueous magnesium:carbonate ratio and saturation index (SI) using a hydrothermal atomic force microscope. At SI ≤ 1.9 and 80-90°C, step velocities were found to be invariant with changes in the magnesium:carbonate ratio, an observation in contrast with standard models for growth and dissolution of ionically bonded, multicomponent crystals. However, at high saturation indices (SI = 2.15), step velocities displayed a ratio dependence, maximized at magnesium:carbonate ratios slightly greater than 1:1. Traditional affinity-based models could not describe growth rates at the higher saturation index. Step velocities also could not be modeled solely through nucleation of kink sites, in contrast to other minerals whose bonding between constituent ions is also dominantly ionic in nature, such as calcite and barite. Instead, they could be described only by a model that incorporates both kink nucleation and propagation. On the basis of observed step morphological changes at these higher saturation indices, the step velocity maximum at SI = 2.15 is likely due to the rate of attachment to propagating kink sites overcoming the rate of detachment from kink sites as the latter becomes less significant under far from equilibrium conditions. (Graph Presented).