Decomposition of siderite to magnetite at lower oxygen fugacities: a thermochemical interpretation and geological implications. Economic Geology 61

Siderite is decomposed to magnetite at lower oxygen fugacities according to the equation, 3FeCOs- FeaO4 + 3C + •O2 because of the decomposition of carbon dioxide to graphite and oxygen. Conse-quently, the assemblage of magnetite and siderite without graphite expressed as 3FeCOs + «02 = FeaO4 + 3CO2 is stable within a rather narrow temperature interval, for instance, 430o-470 ø K at a carbon dioxide fugacity of 1 atm. The development of magnetite from siderite in Precambrian iron-formations described by LaBerge.needs no addition of oxygen when they react with each other according to the former equation with elevation of temperature, or the latter with decrease in the fugacity of carbon dioxide