Quantitative Determination of Iron Redox State in Copper–Iron–Silicate Slags by Electron Probe Microanalysis

Iron‐bearing silicate systems are fundamental components of oxide melts, glasses, and ceramic materials, where multivalent iron strongly influences structure, thermodynamic properties, and redox behavior. Accurate determination of iron oxidation state remains a central challenge in studies of high‐temperature silicate systems because ferric to ferrous ratio control oxygen activity, defect chemistry, and phase equilibria. In this study, a quantitative approach for estimating the Fe 3+ /(Fe 3+ + Fe 2+ ) ratio is presented based on direct measurement of oxygen concentration in oxide system by electron probe microanalysis (EPMA). Two sets of samples in the Cu─Si─Fe─O system were synthesized using equilibration and quenching techniques, producing phase assemblages involving slag/copper/tridymite or slag/copper/magnetite (or wüstite) phases. Equilibration experiments were conducted at 1250°C and 1300°C, over oxygen partial pressures ranging from 10 −9 to 10 −6.5 atm, followed by rapid quenching of the samples to preserve high‐temperature equilibrium states. Elemental phase compositions, including metal cations and oxygen, were analyzed by EPMA. The measured oxygen concentrations were used to calculate valence states of metal cations (Cu + , Si 4+ , Fe 2+ , and Fe 3+ , respectively), enabling direct determination of the ferric to ferrous ratio in the samples. The experimental results have been compared with calculated and literature data, demonstrating that accurate oxygen quantification by EPMA provides a reliable approach for oxidation state determination in complex oxide systems. Beyond metallurgical slags, the proposed methodology is applicable to silicate melts, glasses, and ceramic materials, where multivalent cations control structure, defect chemistry, and thermodynamic behavior, offering a robust experimental tool for studies redox‐dependent properties in high‐temperature oxide materials.