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Experimental determination of siderite stability and application to Martian Meteorite ALH84001

University of Dayton, Dayton, Ohio 45469-2364, U.S.A.

Correspondence: ^* E-mail: koziol{at}notes.udayton.edu

The pressure-temperature equilibrium curve of the reaction siderite + hematite = magnetite + CO₂ was determined in the range 5–12 kbar and 480–650°C by piston-cylinder experiments, with NaCl as a pressure medium. Silver oxalate was used as a CO₂ source and samples were buffered at hematite-magnetite oxygen fugacity. Reaction progress was monitored by extent of CO₂ gas loss and by X-ray diffraction (XRD) analysis.

The data define a univariant curve, which is described by P = –14.599 + 0.025 T + 0.000027 T² with P in kbar and T in°C. Calculations based on these data give H_f⁰ (298K) siderite = –760.6 ± 0.9 kJ (kilojoules) from the oxides. The formation of siderite requires a specific range of ambient oxygen and carbon dioxide fugacities, dependent upon temperature and pressure. The stable assemblage of siderite and magnetite, at a given temperature and pressure, implies more restrictive ranges of oxygen and carbon dioxide fugacities, defined by reactions among siderite, magnetite, graphite, and hematite. Experimental and thermodynamic investigation of the Fe-C-O system indicates that the formation of magnetite along with Ca-Fe-Mg carbonate globules by inorganic processes is possible and may be relevant to Martian meteorite ALH84001 Decarbonation of the siderite component of the carbonate, either by a transient heating event or by a change in oxygen fugacity of a coexisting fluid, may have formed the observed grains, although this study does not address the size or morphology of magnetite grains formed by this mechanism.

This article has been cited by other articles:

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