The evidence for a miscibility gap in the Fe3O4-ZnFe2O4 system—A review

There is an increasing amount of evidence indicating the existence of a miscibility gap in the solid solution of the spinels zinc ferrite (franklinite) (ZnFe₂O₄) and magnetite (Fe₃O₄). Several types of information on the solid state chemistry of this system have been assessed, including measurements of the dependence of activity, lattice constant, conductivity, magnetization, and Mössbauer spectrum on the composition of the solid solution. The indicated miscibility gap is imminent at a temperature of about 1100 K and, for several hundred degrees below this, is bounded by two phases that contain 0.12 and 0.30 mole fraction magnetite. These characteristics of the solvus curve are confirmed by the analysis of a geological specimen. The cation distribution as a function of composition has been calculated from the lattice parameter data and is confirmed by Mössbauer spectroscopic measurements. The miscibility gap was found to occur between two largely normal spinels that contain little tetrahedrally coordinated Fe³⁺. Due to the existence of this incipient immiscibility during the roasting of zinc concentrate, there are greater losses of zinc to the ferrite than would otherwise occur.