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 (e.g., Ni, Mn, Co) substitute on both octahedral sites, and the second is appropriate for quadrilateral solutions, where substitution is limited to a single site (e.g., Ca on M2). The ternary model is calibrated for Ni-bearing olivines from consideration of site occupancy determinations and heterogeneous exchange equilibria. Ca-bearing olivines are calibrated from consideration of miscibility gaps. Both the models for (Ni,Mg,Fe)2SiO4 and (Ca,Mg,Fe)2SiO4 olivines are internally consistent with Sack and Ghiorso's (1989) analysis of ferromagnesian olivines and with the standard state data base of Berman (1988). 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. Predictions of Ni-Fe exchange between olivines and FeNi alloys agree with experimentally determined distributions. Predicted mixing properties of calcian olivines on the Ca-poor side of the solvus are similar to those calculated from the formulation of Davidson and Lindsley (1989), but small differences arise from adoption here of a larger Fe-Mg interaction energy.