Chemical potentials of oxygen for three‐phase assemblages of CaSiO 3 (s) + Ca 3 Si 2 O 7 (s) + {CaO + SiO 2 + Fe x O} melt and Ca 3 Si 2 O 7 (s) + Ca 2 SiO 4 (s) + {CaO + SiO 2 + Fe x O} melt

By employing an electrochemical technique involving stabilized zirconia as solid electrolyte and Mo + MoO 2 mixture as reference electrode, the equilibrium oxygen partial pressures for three‐phase assemblages of CaSiO 3 (s) + Ca 3 Si 2 O 7 (s) + {CaO + SiO 2 + Fe x O} melt and Ca 3 Si 2 O 7 (s) + Ca 2 SiO 4 (s) + {CaO + SiO 2 + Fe x O} melt were determined as: ‐ log { P O2 (CS + C 3 S 2 + L)/bar} = ‐ 3.22 13000/( T /K) ± 0.05 ‐ log { P O2 (C 3 S 2 + C 2 S + L)/bar} = ‐ 0.92 16400/ ( T /K) ± 0.04. respectively, where CS, C 3 S 2 and C 2 S indicate CaSiO 3 (s), Ca 3 Si 2 O 7 (s). and Ca 2 SiO 4 (s), respectively.