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 Δ Hf0 (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.