Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe 2 O is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare, and iron(II,III) oxide (Fe 3 O 4), which also occurs naturally as the mineral the mineral known as hematite, Fe 2 O 3 is the main source of iron for the steel industry.