Jean Lehmann, Jacques Ronx
In this study, a model is developed for calculating activities in multisite solutions, particularly focusing on spinels. The Gibbs free energy is expressed as that of a solution with ideal cation mixing on each sublattice, adjusted by appropriate higher order terms. The activities of the ith end-member are expressed in a simplified manner involving site occupancy fractions and constant parameters, but no explicit function is provided for general macroscopic composition. The site occupancy fractions that minimize Gibbs free energy are determined by solving a series of nonlinear equations to meet internal equilibrium conditions. The practical application of these calculations is illustrated through the (Fe2+, Mg)(Al, Cr, Fe3+) spinel systems. This research provides derived activity expressions for multicomponent AB204 spinels based on the ideal multisite model and discusses various existing approximations in the literature for simplifying activity-concentration relationships. The relationships between site occupancy at equilibrium are further examined, leading to multiple equivalent expressions for activities, thereby enriching the understanding of thermodynamic properties in complex mineral solutions.
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title={Calculations of activity composition rel},
author={Jean Lehmann and Jacques Ronx},
year={2026},
language={en}
}TY - JOUR TI - Calculations of activity composition rel AU - Jean Lehmann AU - Jacques Ronx PY - 2026 LA - en ER -
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