Calculated solution energies of heterovalent cations in forsterite and diopside: Implications for trace element partitioning

Solution energies are calculated for a wide range of heterovalent impurities in forsterite and diopside, using atomistic simulation techniques and a consistent set of interatomic potentials to represent the non-Coulombic interactions between the ions. The calculations allow explicitly for ionic relaxation. Association between a charged defect and its compensating defect(s) cannot be neglected at low temperatures; however, at concentrations of 10-100 ppm a large proportion will be dissociated at temperatures above 1000 K. The variation of calculated solution energy with ion size reflects the variation in the relaxation energies, and often shows a parabolic variation with ionic radius. For the pure mineral, the calculated solution energies always show a minimum at a radius corresponding to that of the host cation; for impure clinopyroxene (with <1 Ca per formula unit) the optimum cation radius varies with composition, as observed experimentally. A marked variation in the calculated solution energies for trivalent trace elements is predicted depending on which alkali-metal cation is the compensating defect, At the M1 site in diopside the lowest calculated solution energy is for trivalent ions coupled with the substitution of a Na+ ion on the M2 site, i.e. M3+(M1)/Na+(M2); at M2 it is X3+(M2)/Na+(M2). X3+(M2)/Li+(M1) is the lowest energy pairing for forsterite. Copyright (C) 1997 Elsevier Science Ltd.