Impurity cations in the bulk and the {001} surface of muscovite: an atomistic simulation study

Solution energies are calculated for monovalent and divalent impurities in the bulk and the {001} surface of muscovite, using atomistic simulation techniques and a consistent set of interatomic potentials. Cs+ is the most soluble alkali metal cation. There are marked differences between the bulk and surface solution energies for the smaller univalent cations, indicating appreciable segregation to the surface. Without deprotonation of an OH group, none of the Group 1 cations are able to enter the hexagonal cavity on the mica basal plane. Small divalent cations are predicted to substitute for octahedral Al with the most favourable charge-balance mechanism involving the additional substitution of Al for Si. The solution mechanism for larger, less soluble divalent cations involves substitution for K+, with a compensating Al/Si exchange. Where possible, results are calculated for two models, the first assuming complete ordering of Al and Si on the tetrahedral aluminosilicate sheets and the second completely random ordering. There is little difference between the values from the two models, with the exception of the {001} surface energy.