Molecular dynamics simulations of the growth inhibiting effect of Fe2+, Mg2+, Cd2+, and Sr2+ on calcite crystal growth

Molecular dynamics simulations were employed to investigate the initial stages of growth of calcium, magnesium, iron, cadmium, and strontium carbonates at two experimentally observed calcite steps. The calculated energies suggest that in a solution containing all five cations MgCO3, FeCO3, and SrCO3 grow onto the steps in preference to CdCO3 and especially CaCO3. Initial incorporation of all impurity ions is more exothermic at the more open, obtuse step than at the acute step edge (by 44-86 kJ mol(-1)), although growth next to an existing CaCO3 unit occurs preferentially at the acute step. Growth of full rows of impurity carbonates is highly exothermic for the first row at -25 to -155 kJ mol(-1) per MCO3 depending on M (M = Mg, Fe, Cd, Sr), but is much less so for a subsequent row (-5 to -80 kJ mol(-1) per MCO3), due to increasing mismatch between the rows of MCO3 at the surface and the underlying calcite lattice, especially for the large strontium ion. Calcite growth, which is a slightly endothermic process in its pure form (on average +1.8 to +35 kJ mol(-1) per CaCO3), is severely hindered by the presence of impurity ions at the step edges, when the average enthalpies of the CaCO3 growth process increase to +15 to +75 kJ mol(-1) per CaCO3, depending on the type of cation decorating the step. The results of these molecular dynamics simulations therefore suggest that growth of impurity carbonates at the calcite steps is initially an energetically very favorable process, hence competing effectively with growth of pure calcite. Subsequent incorporation of impurities, however, becomes more and more endothermic, until finally the presence of steps decorated with rows of impurities inhibits further crystal growth, in agreement with experimental evidence, which shows for example that the presence of magnesium and iron ions inhibits calcite growth.