Dolomite represents 30% of sedimentary carbonates; however, there are no reports of laboratory synthesis in ambient conditions. A common strategy to force crystal growth from solution is to use a supersaturated solution that thermodynamically prefers to separate into solvent and crystal. However, even 1,000-fold supersaturated solutions left for 32 years failed to precipitate dolomite. Such a failure contradicts the substantial natural deposits observed.
Sun and colleagues used a combination of materials modelling and electron microscopy to explain this. Ruling out nucleation inhibition by using density functional theory to show the activation barrier for nucleation is low, they instead investigated a growth inhibition mechanism. CaMg(CO3)2 is a layered compound with alternating Ca(II) and Mg(II) layers along the (0001) direction. The favoured growth surface, (\(10\bar{1}4\)), has step edges for growth that are composed of alternating Ca and Mg ions. There is a considerable entropic penalty for the growth of an ordered surface, ion by ion, over other disordered surfaces where Ca(II) binds to Mg(II) or vice versa. The ordered surface is most stable, and over time should form following a dissolution–reprecipitation process, only then enabling growth of the next layer. To semiquantitatively determine this, Sun and colleagues performed kinetic Monte Carlo (KMC) simulations, parameterized by density functional theory calculations of activation energy ΔEa of the microkinetic steps, of the growth of ordered step edges following ion dissolution from a disordered surface (with cation ordering of 0; top two panels), followed by reprecipitation and formation of a more ordered surface (with cation ordering of 1). The time for each step was proportional to ΔEa/kT, where k is Boltzmann’s constant and T temperature, while step edge growth rates were referenced to experimental step edge velocities.
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