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Iron is an abundant element in the universe; it is found in many stars, including the sun. Iron is the fourth most abundant element in the earth's crust, of which it constitutes about 5% by weight, and is believed to be the major component of the earth's core. Iron is found distributed in the soil in low concentrations and is found dissolved in groundwaters and the ocean to a limited extent. It is rarely found uncombined in nature except in meteorites, but iron ores and minerals are abundant and widely distributed.
The principal ores of iron are hematite (ferric oxide, Fe2O3) and limonite (ferric oxide trihydrate, Fe2O3·3H2O). Other ores include siderite (ferrous carbonate, FeCO3), taconite (an iron silicate), and magnetite (ferrous-ferric oxide, Fe3O4), which often occurs as a white sand. Iron pyrite (iron disulfide, FeS2) is a crystalline gold-colored mineral known as fool's gold. Chromite is a chromium ore that contains iron. Lodestone is a form of magnetite that exhibits natural magnetic properties.
Production and Refining
See W. H. Dennis, Metallurgy of the Ferrous Metals (1963) and Foundations of Iron and Steel Metallurgy (1967).
Iron(religion, spiritualism, and occult)
Iron is regarded as near-magical and frequently credited with supernatural powers. The first iron implements appeared about the third millennium BCE, in Mesopotamia. The Aztecs called iron "the gift from heaven" because it fell in the form of meteors. The ancient Egyptians called it "the metal from the sky." Talismans of meteoric iron were placed in the tomb of Tutankhamen. The Roman historian Pliny claimed that a house would be protected from evil spirits if iron coffinnails were placed above the entranceway.
Dr. G. Storms points out that iron manifestly takes its power from the fact that the material was "better and scarcer than wood or stone for making tools, and from the mysterious way in which it was originally found: in meteoric stones." Specialists were needed to obtain the iron from the ore and harden it. These specialists— blacksmiths—became looked upon as magicians. The best-known smith was probably Wayland Smith, the smith of the Norse gods, depicted on the seventh-century Northumbrian Franks Casket.
Whitlock suggests that when the bronze-using British were invaded by ironusing newcomers from Europe in the first millennium BCE, they retreated, feeling that they had been defeated by magic rather than superior strength. Iron has since remained a magical metal, used to fight evil, keep fairies and demons at bay, and even ward off witches. An iron horseshoe nail would be laid in a baby's cradle to prevent the fairies from stealing the baby and leaving a changeling. An iron horseshoe would be nailed up over the entrance to a house to protect the house from fire and to keep evil at bay. Early pins were made of iron, so to stick a wax or clay figure with pins was to work powerful magic (see image magic).
(Latin Fermm), Fe, a chemical element in Group VIII of the Mendeleev periodic system. Atomic number, 26; atomic weight, 55.847. A shiny silver-white metal. The element in the natural state consists of four stable isotopes: 54Fe (5.84 percent), 56Fe (91.68 percent), 57Fe (2.17 percent), and 58Fe(0.31 percent).
Historical information. Iron was known even in prehistoric times, but it came into extensive use much later, since it is very rarely found naturally in the free state, and its preparation from ores became possible only at a certain level of technical development. It is probable that man first became acquainted with meteoric iron; this is indicated by its names in the languages of ancient peoples. The ancient Egyptian be-ni-pet means “celestial iron,” and the ancient Greek sideros is associated with the Latin sidus (genitive sideris), meaning “star” or “heavenly body.” Hittite texts of the 14th century B.C. mention iron as a metal that has fallen from heaven. The Romance languages preserve the root of the name given it by the Romans (for example, French fer and Italian ferro).
A method of producing iron from ores was invented in western Asia in the second millennium B.C.; the use of iron subsequently spread to Babylon, Egypt, and Greece. The Bronze Age was followed by the Iron Age. In the 23rd book of the Iliad, Homer recounts that Achilles awarded a discus made of iron bloom to the victor in a discus-throwing competition. For many centuries in Europe and Old Russia iron was made by blooming. Iron ore was reduced with wood charcoal in a furnace made in a pit; air was blown onto the hearth by means of bellows, and the product of reduction (the bloom) was separated from the slag by hammering, and various articles were forged from it. As the methods of blowing were perfected and the furnace height was increased, the temperature of the process rose, and part of the iron was carburized—that is, pig iron was produced. This comparatively brittle product was regarded as scrap; hence its name. Later it was found that when the furnace was charged with pig iron rather than iron ore, a low-carbon bloom also resulted; such a two-stage process proved more favorable than blooming. The refinery method was used extensively as early as the 12th and 13th centuries. In the 14th century cast iron began to be smelted not only as an intermediate for further processing but also as a material for casting various articles. The redesigning of the hearth into a shaft furnace and then into a blast furnace took place during that period. In Europe in the mid-18th century the crucible process began to be used to make steel. This process was known in Syria even in the early Middle Ages but was later lost. In this method steel was made in small vessels (crucibles) of highly refractory material by melting metal charges. The puddling process for converting pig iron to iron on the hearth of a reverberatory furnace began to develop in the last quarter of the 18th century. The Industrial Revolution of the 18th and early 19th century, the invention of the steam engine, and the building of railroads, large bridges, and a steam-powered fleet gave rise to a great demand for iron and its alloys. However, the existing methods of making iron could not satisfy the needs of the market. Large-scale production of iron began only in the mid-19th century when the Bessemer, Thomas, and open-hearth processes were developed. In the 20th century the electric smelting process, which yielded high quality steel, was discovered and became widespread.
Occurrence in nature. Iron is the second most common metal in the lithosphere (4.65 percent by weight; aluminum is in first place). It migrates vigorously in the earth’s crust, forming about 300 minerals (oxides, sulfides, silicates, carbonates, titanates, phosphates, and others). Iron participates actively in magmatic, hydro thermal, and supergene processes, with which the formation of various kinds of its deposits are associated. Iron is a metal of the earth’s depths; it accumulates at the early stages of crystallization of magma, in ultrabasic rock (9.85 percent) and basic rock (8.56 percent). (In granites the total amount is 2.7 percent.) In the biosphere, iron accumulates in many marine and continental sediments, forming sedimentary ores.
Oxidation-reduction reactions—the passage of divalent iron to trivalent and vice versa—play an important part in the geochemistry of iron. In the biosphere, in the presence of organic substances, Fe3+ is reduced to Fe2+, migrates readily, and, upon encountering atmospheric oxygen, is oxidized, yielding an accumulation of trivalent iron hydroxides. Wide-spread trivalent iron compounds are red, yellow, and brown; this determines the color of many sedimentary rocks and their name, “red formation” (red and brown loams and clays, yellow sands, and so on).
Physical and chemical properties. The significance of iron in contemporary technology is determined not only by its extensive natural distribution but also by a combination of very valuable properties. It is plastic, easily forged whether cold or hot, and can be drawn, stamped, and rolled. Its capacity for dissolving carbon and other elements is the basis for making various iron alloys.
Iron can exist in the form of two crystal lattices: a body-centered cubic lattice and a face-centered cubic lattice. Below 910°C, αFe with a body-centered cubic lattice is stable (a = 2.86645 angstroms [Å] at 20°C). Between 910°C and 1400°C, the γ-modification with a face-centered cubic lattice is stable (a = 3.64 Å). Above 1400°C the body-centered cubic lattice of δ-Fe (a = 2.94 Å) forms and is stable up to its melting point (1539°C). Up to 769°C (the Curie point), α-Fe is ferromagnetic. The γ- and δ-Fe modifications are paramagnetic.
The polymorphous transformations of iron and steel upon heating and cooling were discovered in 1868 by D. K. Chernov. Carbon forms solid interstitial solutions with iron, in which carbon atoms of small atomic radius (0.77 Å) are located in the interstices of the crystal lattice of the metal, which consists of larger atoms (the atomic radius of Fe is 1.26 Å). The solid solution of carbon in γ-Fe is called austenite; the solution in α-Fe is called ferrite. The saturated solid solution of carbon in γ-Fe contains 2.0 percent carbon by weight at 1130°C; α-Fe dissolves only 0.02–0.04 percent C at 723°C and less than 0.01 percent at room temperature. Therefore, hardening of austenite yields martensite, a supersaturated solid solution of carbon in α-Fe, which is very hard and brittle. A combination of hardening and tempering (heating to relatively low temperatures to reduce internal stresses) makes it possible to impart to steel the required combination of hardness and plasticity.
The physical properties of iron depend on its purity. In industrial ferrous materials, iron is usually accompanied by admixtures of carbon, nitrogen, oxygen, hydrogen, sulfur, and phosphorus. Even at very low concentrations these impurities can greatly alter the properties of the metal. Thus, sulfur causes so-called red shortness, phosphorus (even 10−2 percent P) causes cold brittleness, carbon and nitrogen reduce plasticity, and hydrogen increases brittleness (so-called hydrogen brittleness). A reduction in impurities to 10−7−10−9 percent leads to an essential change in the properties of the metal, particularly an increase in plasticity. The physical properties of iron, mainly those for metal with a total impurity content of less than 0.01 percent by weight, are given in Table 1.
The configuration of the external electron shell of the Fe atom is 3d64s2. The valence of iron is variable (divalent and trivalent iron compounds are the most stable). With oxygen, iron yields ferrous oxide, FeO; ferric oxide, Fe2O3; and ferrosoferric oxide, Fe3O4 (a compound of FeO with Fe2O3, having
|Table 1. Main properties of iron|
|Ionic radii..................||0.80Å (Fe2+)|
0.67 Å (Fe3+)
|Density (20°C)..................||7.874 g/cm3|
|Boiling point..................||c. 3200°C|
|Coefficient of linear expansion (20°C)..................||11.7 × 10-6|
|Thermal conductivity (25°C)..................||74.04 W/(m°K), or 0.177 cal/(cm.sec.deg)|
|Specific heat..................||depends on structure and varies complexly with temperature: mean specific heat (0°-1000°C), 640.57 J/(kg°K), or 0.153 cal/(g.deg)|
|Specific electric resistance (20°C)..................||9.7 × 10-8 ohms.m, or 9.7 × 10-6 ohms.cm|
|Temperature coefficient of electric resistance (0°-100°C)..................||6.51×10-3|
|Young’s modulus..................||190-210 × 103 MN/m2, or 19-21 × 103 kgf/mm2|
|Temperature coefficient of Young’s modulus..................||4 × 10-8|
|Shear modulus..................||84.0 × 103 MN/m2, or 8.4×103 kgf/mm2|
|Short-term tensile strength..................||170-210 MN/m2, or 17-21 kgf/mm2|
|Brinell hardness..................||350-450 MN/m2, or 35-45 kgf/mm2|
|Yield point..................||100 MN/m2, or 10 kgf/mm2|
|Impact strength..................||300 MN/m2, or 30 kgf/mm2|
a spinel structure). In moist air at ordinary temperatures iron becomes covered with a friable rust (Fe2O3 . nH2O). Because of its porosity, the rust does not prevent access of oxygen and moisture to the metal and therefore does not protect it from further oxidation. Millions of tons of iron are lost annually through various forms of corrosion. When iron is heated in dry air above 200°C, it becomes covered with a very thin oxide film, which protects the metal from corrosion at ordinary temperatures; this is the basis of the technical method of protecting carburized or low-alloyed steel and pig iron by bluing. Upon heating in water vapor, iron oxidizes, with the formation of Fe3O4 (below 570°C) or FeO (above 570°C) and the liberation of hydrogen.
The hydroxide Fe(OH)2 forms as a white precipitate upon the action of caustic alkalies or ammonia on aqueous solutions of Fe2+ salts in a hydrogen or nitrogen atmosphere. Upon contact with air, Fe(OH)2 first turns green, then darkens and finally quickly becomes a reddish-brown hydroxide Fe(OH)3. Ferrous oxide, FeO, exhibits basic properties. The oxide Fe2O3 is amphoteric and has a weak acidic function; it reacts with more basic reagents (such as MgO) to yield fer-rites, compounds of the type Fe2O3.MeO, which have ferromagnetic properties and are widely used in radio electronics. Acidic properties are also exhibited by hexavalent iron, which exists in the form of ferrates, such as K2FeO4, and salts of ferric acid, which has not been produced in the free state.
Iron reacts readily with halogens and halogen hydrides to give salts, such as the chlorides FeCl2 and FeCl3. Heating iron with sulfur gives the sulfides FeS and FeS2. The carbides of iron are Fe3C (cementite) and Fe2C (∊-carbide), which precipitate from solid solutions of carbon in iron upon cooling. The compound Fe3C also separates from solutions of carbon in liquid iron at high carbon concentrations. Like carbon, nitrogen forms solid interstitial solutions with iron; the nitrides Fe4N and Fe2N separate from them. With hydrogen, iron gives only unstable hydrides whose composition has not been accurately determined. Silicon and phosphorus react vigorously with iron upon heating, forming silicides (for example, Fe3Si) and phosphides (Fe3P).
The compounds of iron with many elements (such as oxygen and sulfur) have a crystalline structure and variable composition; thus, in the monosulfide the sulfur content can vary from 50 to 53.3 atomic percent. This is caused by defects in the crystal structure. Thus, in ferrous oxide, some of the Fe2+ ions at the lattice sites are replaced by Fe3+ ions; to preserve electrical neutrality, some lattice sites that belonged to Fe2+ ions remain empty, and under ordinary conditions the phase (wustite) has the formula Fe0.947O.
For the reaction Fe ⇆ Fe2+ + 2e, the normal electrode potential of iron in aqueous solutions of its salts is –0.44 volts (V); for the reaction Fe ⇆ Fe3+ + 3e, it is –0.036 volts. Thus, iron is to the left of hydrogen in the activity series. It dis-solves readily in dilute acids, with the liberation of H2 and the formation of Fe2+ ions.
The reaction of iron with nitric acid is unique. Concentrated nitric acid (density 1.45 g/cm3) renders iron passive through the formation of a protective oxide film. More dilute nitric acid dissolves iron to give Fe2+ or Fe3+ ions, reducing to NH3or N2O and N2.
Solutions of the salts of divalent iron are unstable in air; Fe2+ gradually oxidizes to Fe3+. Aqueous solutions of iron salts are acidic as a result of hydrolysis. The addition of thiocyanate ions SCN– to solutions of ferric salts produces a bright blood-red color as a result of the formation of Fe(SCN)3, making possible the detection of 1 part Fe3+ in about 106 parts water. The formation of complex compounds is characteristic of iron.
Preparation and use. Pure iron is produced in relatively small quantities by electrolysis of aqueous solutions of its salts or by the reduction of iron oxides by hydrogen. A method of producing iron directly from its ores by electrolysis of melts is being developed. The production of iron of sufficient purity by direct reduction of ore concentrates with hydrogen, natural gas, or coal at relatively low temperatures is increasing.
Iron is the most important metal in modern technology. There is virtually no use for pure iron because of its low strength, although in everyday life articles made of steel or cast iron are often called iron articles. Most iron is used in the form of alloys of very diverse compositions and properties. About 95 percent of the entire output of metals consists of iron alloys. High-carbon alloys (2 percent or more by weight)—pig irons—are smelted from enriched iron ores in blast furnaces. Various grades of steel (with less than 2 percent carbon by weight) are made from pig iron in open hearth and electric furnaces by the oxidation of excess carbon, removal of harmful impurities (mainly sulfur, phosphorus, and oxygen), and the addition of alloying elements. High-alloyed steels (with a high content of nickel, chromium, tungsten, and other elements) are smelted in electric arc and induction furnaces. New processes, including vacuum and electroslag re-melting and plasma and electron-beam melting, are used in making steel and iron alloys for particularly critical applications. Methods are being developed for smelting steel in continuous plants, which provide high metal quality and a high degree of automation of the process.
Iron is used as the basis for creating materials able to with-stand high and low temperatures, vacuum, high pressures, aggressive mediums, large variable stresses, nuclear radiation, and so on. The production of iron and its alloys is constantly increasing. In 1971, 89.3 million tons of pig iron and 121 million tons of steel were smelted in the USSR.
L. A. SHVARTSMAN and L. V. VANIUKOVA
Iron as an artistic material. Iron was used as an artistic material from ancient times in Egypt (head support from the tomb of Tutankhamen near Thebes, mid-14th century B.C., Ashmolean Museum, Oxford), Mesopotamia (daggers found near Carchemish, 500 B.C., British Museum, London), and India (iron column in Delhi, A.D. 415). Numerous highly artistic iron objects from the Middle Ages—forged railings, door hinges, wall brackets, weathervanes, bindings of chests, and mountings for torches—have been preserved in European countries (Britain, France, Italy, and Russia). One-piece forged articles made from rods, as well as articles of cut sheet iron (often with a mica backing), are characterized by their plane shapes and sharp linear and graphic outline and look well against a background of light and air. Today, iron is used to make gratings, fences, openwork interior partitions, candle-sticks, and monuments.
Iron in the organism. Iron is present in all animals and plants (on the average, about 0.02 percent); it is mainly necessary for oxygen exchange and oxidation processes. Some organisms (so-called concentrators) are able to accumulate large amounts of iron (for example, iron bacteria, up to 17–20 percent). Almost all the iron in animal and plant organisms is bound to proteins. Iron deficiency causes retarded growth and chlorosis of plants, which are associated with reduced chlorophyll formation. Excess iron also has a bad effect on plant growth, causing flower sterility in rice, as well as chlorosis. In alkaline soils, iron compounds that are not available for assimilation by plant roots are formed, and the plants do not get enough iron; in acidic soils, iron passes into soluble compounds in excess quantities. When insufficient or excess assimilable iron compounds are present in soils, sickness of plants may occur over considerable areas.
In humans and animals, iron is ingested with food (richest in iron are liver, meat, eggs, legumes, grains, groats, spinach, and beets). Humans ordinarily ingest 60–110 mg of iron in their diet, which greatly exceeds the daily requirement. Iron ingested with the food is absorbed in the upper part of the small intestine, from which it passes into the blood in a form bound to protein and is conveyed in the blood to various organs and tissues, where it is deposited as an iron-protein complex, ferritin. The main repository of iron in the body is in the liver and spleen. The synthesis of all iron compounds in the body takes place because of ferritin; the divalent iron pigment hemoglobin is synthesized in bone marrow, myoglobin is synthesized in the muscles, and cytochromes are synthesized in various tissues. Iron is excreted from the body mainly through the walls of the colon (in humans, about 6–10 mg in 24 hours), and to an insignificant extent through the kidneys.
The body’s iron requirements change with growth and physical condition. Children need 0.6 mg per kg of weight per day; adults, 0.1 mg; and pregnant women, 0.3 mg. For animals the approximate iron requirement is not less than 50 mg per kg (dry weight) of feed for milch cows, 30–50 mg for young stock, up to 200 mg for young pigs, and 60 mg for pregnant sows.
V. V. KOVAL’SKII
Iron in medicine. Medicinal preparations made from iron (reduced iron, ferrous lactate, ferric glycerophosphate, ferrous sulfate, Blaud’s pills, iron malate solution, feramid, and gemostimulin) are used to treat the disorders accompanying iron insufficiency in the body (iron-deficiency anemia) and as general tonics (during recovery from infectious diseases). Iron isotopes (52Fe, 55Fe, and 59Fe) are used as tracers in medical and biological research and the diagnosis of blood diseases (anemia, leucosis, and polycythemia).
REFERENCESObshchaia metallurgiia. Moscow, 1967.
Nekrasov, B. V. Osnovy obshchei khimii, vol. 3. Moscow, 1970.
Remy, H. Kurs neorganicheskoi khimii, vol. .2. Moscow, 1966. (Translated from German.)
Kratkaia khimicheskaia entsiklopediia, vol. 2. Moscow, 1963.
Levinson, N. R. [“Izdeliia iz tsvetnogo i chernogo metalla.”] In Russkoe dekorativnoe iskusstvo, vols. 1–3. Moscow, 1962–65.
Vernadskii, V.I. Biogeokhimicheskie ocherki: 1922–1932. Moscow-Leningrad, 1940.
Granik, S. “Obmen zheleza u zhivotnykh i rastenii.” In the collection Mikroelementy. Moscow, 1962. (Translated from English.)
Dixon, M., and E. Webb. Fermenty. Moscow, 1966. (Translated from English.)
Neogi, P. Iron in Ancient India. Calcutta, 1914.
Friend, J. N. Iron in Antiquity. London, 1926.
Frank, E. B. Old French Ironwork. Cambridge, Mass., 1950.
Lister, R. Decorative Wrought Ironwork in Great Britain. London, 1960.
What does it mean when you dream about iron?
Iron is associated with strength and willpower (an “iron will”), which may play into the meaning of a dream in which iron is explicitly a part.
See also dinosaur.