[5] FeO can be prepared by the thermal decomposition of iron(II) oxalate.
The procedure is conducted under an inert atmosphere to avoid the formation of iron(III) oxide (Fe2O3).
[6][7] Stoichiometric FeO can be prepared by heating Fe0.95O with metallic iron at 770 °C and 36 kbar.
The non-stoichiometry occurs because of the ease of oxidation of FeII to FeIII effectively replacing a small portion of FeII with two-thirds their number of FeIII, which take up tetrahedral positions in the close packed oxide lattice.
[8][10] Iron(II) oxide makes up approximately 9% of the Earth's mantle.