Anisotropy of Barite during Crystal Growth and the Uptake of Radium

The role of Ba2+ kink-site nucleation and step growth for the kinetics of anisotropic barite-(001) growth and Ra2+ kink-site nucleation for the activation energy of Ra uptake into the barite structure was investigated by using atomistic modeling approaches. The simulations show that Ba2+ kink-site nucleation at the barite-(001) water interface has higher activation energies at acute- and obtuse-low positions than at acute- and obtuse-high positions. The states at the low positions can be considered more relevant for barite crystal growth due to their higher stability. Kink-site nucleation at the obtuse step is preferred over that at the acute step due to the more open step geometry, which requires less dehydration. The simulations of the Ba2+─step growth processes indicate a slow and uniform growth of the acute step and a faster anisotropic growth of the obtuse step. Ra2+ kink-site nucleation at barite-(001) exhibits increased activation energies due to lattice distortion, which is partially compensated by the easier dehydration of Ra2+ compared to Ba2+. A small influence of Ra2+ on the kinetics of (Ba,Ra)SO4 growth can be assumed because the rate-limiting kink-site nucleation process occurs preferentially through Ba2+ attachment compared to Ra2+ attachment. However, a preferential uptake of Ra2+ over Ba2+ during step growth, which requires a higher dehydration than kink-site nucleation, is therefore likely.