Iron is the most common element on earth by weight. It is the main component of the earth's core (the other being nickel). Iron is usually found in the earth's crust as iron oxide minerals such as hematite (Fe2O3), magnetite (Fe3O4), and wüstite (FeO). Iron and iron oxides are also found in meteorites, usually as the primary component. Iron forms compounds in both ferrous (II), and ferric (III) valences. Iron reacts with air in the presence of water to form iron(III)oxide (Fe2O3), commonly known as rust. It also will form mixed valence compounds like magnetite (Fe3O4), that contain both ferrous and ferric state iron in their structure. Iron reacts with the halogens to form its halide salts. It will also react with sulfur at elevated temperatures to form iron sulfide (FeS), known in mineral form as pyrite , or fool's gold.
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