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 impu rities in forsterite and diopside, using atomistic simulation techniqu es and a consistent set of interatomic potentials to represent the non -Coulombic interactions between the ions. The calculations allow expli citly for ionic relaxation. Association between a charged defect and i ts compensating defect(s) cannot be neglected at low temperatures; how ever, at concentrations of 10-100 ppm a large proportion will be disso ciated at temperatures above 1000 K. The variation of calculated solut ion energy with ion size reflects the variation in the relaxation ener gies, 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 clino pyroxene (with <1 Ca per formula unit) the optimum cation radius varie s with composition, as observed experimentally. A marked variation in the calculated solution energies for trivalent trace elements is predi cted 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 M 2 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 E lsevier Science Ltd.