iron - Element, Occurrence, & Compounds - Britannica
Iron is also found combined with other elements in hundreds of minerals; of greatest importance as iron ore are hematite (ferric oxide, Fe 2 O 3), magnetite (triiron tetroxide, Fe 3 O 4), limonite (hydrated ferric oxide hydroxide, FeO(OH)∙nH 2 O), and siderite (ferrous carbonate, FeCO 3) Igneous rocks average about 5 percent iron content.