Thermodynamics of multicomponent olivines and the solution properties of (Ni,Mg,Fe)₂SiO₄ and (Ca,Mg,Fe)₂SiO₄ olivines

A general thermodynamic model is developed that describes the mixing properties of minor and trace divalent cations (Ni, Ca, Mn, Co) in ferromagnesian olivines in terms of octahedral site order-disorder and symmetric enthalpies of mixing. Two cases of this model are presented. The first is applicable to true ternary solutions, where minor components (eg Ni, Mn, Co) substitute on both octahedral sites, and the second is appropriate for quadrilateral solutions, where substitution is limited to a single site (eg Ca on M2). The resulting analysis of the mixing properties of nickel magnesium iron olivines indicates small negative deviations from ideality in Fe-free olivines and greater negative deviations in ferromagnesian olivines. -from Author