Atomistic simulations of trace element incorporation into the large site of MgSiO₃ and CaSiO₃ perovskites

We have used computer simulation techniques to study the mechanisms of trace element incorporation into the large sites of MgSiO₃ and CaSiO₃ perovskites on the atomistic level. Both relaxation and solution energies corresponding to the incorporation of isovalent and heterovalent defects can be fitted using the 'lattice strain' model. This underlines the importance of crystal chemistry in the partitioning of trace elements between perovskite and melt. As expected, solution energies, which take some account of possible melt components, approximate more closely experimental perovskite-melt partitioning observations than do relaxation energies. With calculated solution energies we find that the optimum site radius r ₀ decreases with increasing defect charge while the apparent Young's modulus E_α of the large site increases with charge in both perovskites, consistent with experimental perovskite-melt partitioning data. For a given trace element charge r ₀ is smaller for MgSiO₃ than for CaSiO₃, as observed experimentally. For a given trace element the difference in calculated solution energy between the two perovskites is reflected in the difference in experimental partition coefficients. For all the charge balancing mechanisms we have considered the solution energies for REE³⁺, and 4+ cations (including U⁴⁺ and Th⁴⁺) are much lower in CaSiO₃ perovskite than in MgSiO₃ perovskite, which provides an explanation for the observed ease with which CaSiO₃ perovskite accommodates 3+ and 4+ cations.