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Published September 2001 | public
Journal Article

Thermodynamic properties of the Pt-Fe system


We determined activity-composition relationships for the Pt-Fe system by equilibrating Fe-oxides with Pt-Fe alloys at temperatures in the range of 1200–1400 °C and oxygen fugacities from 1.6 to 7.7 log units above the iron-wüstite (IW) buffer. The system is characterized by strong negative deviations from ideality throughout the investigated temperature range (e.g., γ^(alloy)_Fe <0.02 for X^(alloy)_Fe <0.3). Our data are consistent with an asymmetric regular solution of the form: RTln γ^(alloy)_Fe = [W_(G1)+2(W_(G2)-W_(G1))X^(alloy)_Fe](X^(alloy)_(Pt))^2 where W_(G1) = –138.0 ± 3.3 kJ/mol and W_(G2) = –90.8 ± 24.0 kJ/mol (1σ). Based on experiments at 1200–1400 °C, variations in the activity coefficients at a given composition are consistent with ln γ^(alloy)_Fe(T_1)/ln γ^(alloy)_Fe(T_2)=T_2/T_1. The Pt-Fe alloy composition in equilibrium with a FeO-bearing silicate liquid can be obtained from: log_(10)f_(O_2) = log{exp[lna^(liq)_(Fe2SiO4) - lna^(liq)_Si)2 - 2 lna^(alloy)_(Fe) -(-ΔG^(0)_(r)/RT)]} where ΔG^(0)_(r) is the standard state free energy for the reaction 2Fe^(alloy) + O_2 + SiO^(liq)_2 = Fe_(2)SiO^(liq)_4. We obtained values of a^(alloy)_(Fe) from our model and used the program MELTS together with the thermodynamic properties of these elements to evaluate activities of SiO_2 and Fe_(2)SiO_4 components in the liquid and ΔG^(0)_(r). We provide sample calculations showing how to predict the optimum Fe concentrations for pre-saturation of Pt-bearing containers to reduce Fe loss from the charge during experiments on magmatic liquids at high temperatures and pressures from 1 atm to 40 kbar.

Additional Information

© 2001 Mineralogical Society of America. Manuscript received September 11, 2000; manuscript accepted May 25, 2001; manuscript handled by John H. Jones. This work was supported by NASA grant NAG5-10423. Discussions with M. Baker and R. Mendybaev greatly improved the quality of this study. Ma Chi is thanked for his help with the analytical work. C. Capobianco and J. Jones are thanked for their constructive reviews.

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