Thermodynamics of mixing and ordering in solid solutions with coupled substitutions: A computer simulation approach

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The chemical variability of many rock-forming minerals - solid solutions, is determined by the coupled-substitution mechanism. This mechanism permits a cation to enter the lattice of a host mineral even when its charge does not match the average charge of the particular site. The termodynamic behaviour of such solid solutions is complicated due to the development of cationic order both at inermediate and at the end-member compositions. Static lattice energy calculations (SLEC) and Monte Carlo simulations are the only tools which permit relatistic modeling of these phase. The success of this approach is determined by the quality of the empirical interatomic potentials used to constrain the SLEC. We propose to make the use of an already developed self-consistent set of interatomic potentials for the system Si4+, Al3+, Mg2+, Ca2+, Na+, K+, Fe2+, Fe3+, Mn2+, Ti4+, ZR4+, Y3+, P5+, O2-, which shows good performance in predicting magnitudes of the mixing and ordering enthalpy in a variety of minerals and industrially important compounds. The main outcomes of the project will be the development of realistic activity-composition models for solid solutions with coupled substitutions such as CaMg, = (Na,K)(Al, Fe3+) in clinopyroxene, Mg,Si=Al,Al in clinopyroxene and perovskite, (Na,K)Si=CaAl ind feldspar and Si,Zr4+=P5+, Y3+ in zircon.