The distribution of ferric and ferrous ions within the quaternary Fe2+–Fe3+–Zn–Cr spinel solid solution series is documented using Mössbauer spectroscopy and X-ray diffraction. Along the binary joins FeCr2O4–Fe3O4, ZnCr2O4–Fe3O4 and ZnFe2O4–Fe3O4 the cation distribution changes from normal to inverse. From the different trends of iron distribution versus composition observed along these three joins and at other points within the Fe–Zn–Cr solid solution series, it was found that octahedral site ferric/ferrous charge hopping may be the most important phenomenon stabilizing the inverse ion distribution. For spinels whose compositions make charge hopping impossible or statistically unlikely, cation distribution is governed by the intrinsic spinel site preferences (tetrahedral site: Zn > Fe2+ Fe3+ >> Cr, octahedral site: Cr > Fe3+ > Fe2+ > Zn), which result from the size and bonding characteristics of these atoms.
Near the ZnFe2O4–Fe3O4 edge of this spinel quadrilateral, the normal to inverse transition proceeds essentially to the maximum extent allowed by the Zn content, because the octahedral Fe2+ produced by inversion can share electrons with the surrounding octahedral Fe3+ ions. Inversion along the FeCr2O4–Fe3O4 edge of the quadrilateral, however, is inhibited until there is a sufficient octahedral Fe3+ population to make electron sharing (with inversion produced octahedral Fe2+) statistically likely. A rapid change of site preference of iron ions occurs in the central range of this composition subspace, offering a physical reason for the miscibility gap found along the chromite–magnetite join.