A group of minerals and inorganic compounds made up of iron that is in +2 (ferrous) and +3 (ferric) valence states and oxygen in the -2 valence state, such as ferrous oxide, FeO, and ferric oxide, Fe₂O₃. Fe₃O₄ is a mixture of ferric oxide and ferrous oxide that commonly occurs in a fine-grained, magnetic crystalline form. Hematite, Fe₂O₃, the most common iron oxide, exists in several crystalline forms. Other forms of hematite are too abrasive to use as weighting material in drilling fluids.