Stability relations in the system Na-Fe-Mg-Si-C-O-H at low temperature and pressure. At constant temperature riebeckite is stable to lower fO 2 conditions than hematite and higher pH values than magnetite or hematite. Where riebeckite coexists with carbonates, its stability field is enlarged by a decrease of total carbonate at constant pH. Riebeckite assemblages in the Dales Gorge Member can be divided into two groups, carbonate-free and carbonate-bearing.--Modified journal abstract.

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