In this work, we present a phase field model to simulate the crystallization of Fe3O4 in FeOx–SiO2 melts under oxidizing conditions. The diffusion of FeO and Fe2O3 is considered, because the FeO/Fe2O3 ratio in the melt can be related to the local oxygen potential of the melt. A boundary condition is developed to ensure conservation of Fe while the ratio of FeO/Fe2O3 is in equilibrium with the oxygen fugacity of the atmosphere. Two-dimensional simulations are performed with different nucleation densities of Fe3O4 and varying oxygen fugacity in the atmosphere. The results show that, for the considered nucleation densities, the crystallization of the melt has a larger effect on the oxidation state than the oxygen fugacity of the atmosphere. Furthermore, for the considered composition range, the growth velocities of the spinel crystals increase with decreasing oxygen fugacity.