Figure 4: Homologous series of iron oxides described by the common formula nFeO·mFe₂O₃.

The structures may be described as assembled from two building blocks, FeO6 octahedra and trigonal prisms (prisms could be two-capped but they are not shown for simplicity). Prisms connect to each other through triangular faces, while octahedra share edges, so that they form parallel columns of face-shared prisms and edge-shared octahedra arranged in different motifs as seen in the figure with structures viewed from the top of the columns. Increasing Fe²⁺ content favours octahedral packing over mixed octahedral and prismatic packing. This requires denser packing of FeO6 octahedra and as a result columns of octahedra condense in slabs by sharing common edges. In particular, η-Fe₂O₃ has ordinary columns of prisms and octahedra with a chequerboard-like arrangement; Fe₅O₇ has ordinary and doubled columns of octahedra; the HP-Fe₃O₄ (ref. 41) possesses only doubled columns; Fe₇O₉ (ICSD reference number CSD-430601)⁴² has doubled columns and tripled columns organized in zigzag slabs; Fe₄O₅ (ref. 20) possesses only tripled and Fe₅O₆ (ref. 21) only quadruple zigzag slabs. The end-member of the homologous series wüstite (FeO) consists of octahedra with a maximum (12) number of edge-shared neighbours.