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American Mineralogist; September 2001; v. 86; no. 9; p. 1003-1014
© 2001 Mineralogical Society of America
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Thermodynamic properties of the Pt-Fe system

Ronit Kessel*, John R. Beckett and Edward M. Stolper

Division of Geological and Planetary Sciences, MC 170-25, California Institute of Technology, Pasadena, California 91125, U.S.A.

Correspondence: * E-mail: ronit{at}gps.caltech.edu

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., {gamma}Fealloy<0.02 for XFealloy <0.3). Our data are consistent with an asymmetric regular solution of the form:


Formula

where WG1 = –138.0 ± 3.3 kJ/mol and WG2 = –90.8 ± 24.0 kJ/mol (1{sigma}). Based on experiments at 1200–1400 °C, variations in the activity coefficients at a given composition are consistent with ln {gamma} Fealloy(T1) / ln {gamma}Fealloy(T2) = T2 / T1.

The Pt-Fe alloy composition in equilibrium with a FeO-bearing silicate liquid can be obtained from:


Formula

where {Delta}G0r is the standard state free energy for the reaction 2Fealloy + O2gas+ SiO2liq = Fe2SiO4liq. We obtained values of a Fealloy from our model and used the program MELTS together with the thermodynamic properties of these elements to evaluate activities of SiO2 and Fe2SiO4 components in the liquid and {Delta}G0r. 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.




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