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 bythe coupled-substitution mechanism. This mechanism permits a cation to enter the lattice of ahost 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 ofcationic order both at inermediate and at the end-member compositions. Static lattice energycalculations (SLEC) and Monte Carlo simulations are the only tools which permit relatisticmodeling of these phase. The success of this approach is determined by the quality of theempirical interatomic potentials used to constrain the SLEC. We propose to make the use ofan 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 goodperformance in predicting magnitudes of the mixing and ordering enthalpy in a variety ofminerals and industrially important compounds. The main outcomes of the project will be thedevelopment of realistic activity-composition models for solid solutions with coupledsubstitutions such as CaMg, = (Na,K)(Al, Fe3+) in clinopyroxene, Mg,Si=Al,Al inclinopyroxene and perovskite, (Na,K)Si=CaAl ind feldspar and Si,Zr4+=P5+, Y3+ in zircon.