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]