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(1) Structure of FeO, Fe2O3, and Fe3O:
(1) FeO: This oxide is non-stoichiometric and has a composition FexO (Generally ‘x’ varying from0.92 to 0.97). The oxide ions form a cubic close packing. The octahedral voids are occupied by Fe2+ but a small number of Fe2+ is replaced by Fe3+ ions. Three Fe2+will be replaced by two Fe3+ to maintain electrical neutrality but then what we obtain is an iron-deficient crystal.

(2) Fe2O3: If all Fe2+ are replaced by Fe3+, the ratio between Fe:O will be 2 : 3 (since 3 Fe2+ are replaced by 2Fe3+) and not 1 : 1. As such, we obtain Fe2O3.
(3) Fe3O4: This is obtained by replacing exactly two thirds of Fe2+ by Fe3+ (in FeO structure).The remaining Fe2+ ions and 50% of Fe3+ ions occupy the octahedral voids. The remaining Fe3+ ions occupy tetrahedral voids. If in the structure of Fe3O4, the Fe2+ ions are replaced by divalent cations such as Mg2+, Zn2+, etc., the compounds obtained are called ferrites. In ferrites, divalent cations occupy tetrahedral voids and trivalent cations occupy octahedral voids. This structure is called spinel structure.
Example of Spinel is a MgAl2O4.( mineral) In it oxide ions (O-2) are arranged in ccp with Mg+2 ions occupying tetrahedral voids and Al+3 ions in a set of octahedral voids.
Many ferrites (such as ZnFe2O4) also possess spinel structure. These are very important magnetic materials and are used in telephone and memory loops in computers.
In Fe3O4, Fe+2 and Fe+3 ions are present in the ratio 2:1. it may be considered as having composition FeO.Fe2O3. In Fe3O4 Oxide arranged in ccp. Fe+2 ions occupy octahedral voids while Fe+3 ions are equally distributed between octahedral and tetrahedral voids
MgFe2O4 also has structure similar to magnetite. In this Mg+2 ions are present in place of Fe+2 ion in Fe3O4. Magnetite has inverse spinet structure.

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