Iron—(Anglo-Saxon, iron), Fe (L. ferrum); atomic weight 55.847; atomic number 26; melting point 1535°C; boiling point 2750°C; sp. gr. 7.874 (20°C); valence 2, 3, 4, or 6.
The use of iron is prehistoric. Genesis mentions that Tubal-Cain, seven generations from Adam, was "an instructor of every artificer in brass and iron." A remarkable iron pillar, dating to about A.D. 400, remains standing today in Delhi, India. This solid shaft of wrought iron is about 7 1/4 m high by 40 cm in diameter. Corrosion to the pillar has been minimal although it has been exposed to the weather since its erection.
Iron is a relatively abundant element in the universe. It is found in the sun and many types of stars in considerable quantity. Its nuclei are very stable. Iron is found native as a principal component of a class of meteorites known as siderites, and is a minor constituent of the other two classes. The core of the earth, 2150 mi in radius, is thought to be largely composed of iron, with about 10% occluded hydrogen.
The metal is the fourth most abundant element, by weight, making up the crust of the earth. The most common ore is hematite (Fe2O3), from which the metal is obtained by reduction with carbon. Iron is found in other widely distributed minerals such as magnetite, which is frequently seen as black sands along beaches and banks of streams. Taconite is becoming increasingly important as a commercial ore.
Common iron is a mixture of four isotopes. Six other isotopes are known to exist. Iron is a vital constituent of plant and animal life, and appears in hemoglobin. The pure metal is not often encountered in commerce, but is usually alloyed with carbon or other metals. The pure metal is very reactive chemically, and rapidly corrodes, especially in moist air or at elevated temperatures. It has four allotropic forms, or ferrites, known as α, β, γ, and δ, with transition points at 770, 928, and 1530°C. The α form is magnetic, but when transformed into the β form, the magnetism disappears although the lattice remains unchanged. The relations of these forms are peculiar. Pig iron is an alloy containing about 3% carbon with varying amounts of S, Si, Mn, and P. It is hard, brittle, fairly fusible, and is used to produce other alloys, including steel. Wrought iron contains only a few tenths of a percent of carbon, is tough, malleable, less fusible, and has usually a "fibrous" structure. Carbon steel is an alloy of iron with carbon, with small amounts of Mn, S, P, and Si. Alloy steels are carbon steels with other additives such as nickel, chromium, vanadium, etc.
Iron is the cheapest and most abundant, useful, and important of all metals.