Iron

Iron is the fourth most abundant element in the earth's crust and the second most abundant metal, after aluminum. It makes up about 6.2% of the crust by weight. In addition, iron is thought to be the primary constituent of the earth's core as well as of siderite meteorites. Soil samples taken from the Moon indicate that about 0.5% of lunar soil consists of iron.

The primary ores of iron are hematite (Fe₂O₃), magnetite (Fe₃O₄), limonite (FeO(OH) • nH₂O), and siderite (FeCO₃). The element also occurs as the sulfide, iron pyrite (FeS), but this compound is not used commercially as a source of iron because of the difficulty in reducing the sulfide to the pure element. Iron pyrite has a beautiful golden appearance and is sometimes mistaken for elemental gold. This appearance explains its common name of fool's gold. Taconite is a low-grade ore of iron that contains no more than about 30% of the metal.

In nature, oxides, sulfides, and silicates of iron are often converted to other forms by the action of water. Iron(II) sulfate (FeSO₄) and iron(II) bicarbonate (Fe(HCO₃)₂) are the most commonly found of these.