Iron () is a
chemical element
A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...
with
symbol Fe (from la,
ferrum) and
atomic number
The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...
26. It is a
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
that belongs to the
first transition series and
group 8 of the
periodic table. It is,
by mass, the most common element on
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
, right in front of
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
(32.1% and 30.1%, respectively), forming much of Earth's
outer and
inner core
Earth's inner core is the innermost geologic layer of planet Earth. It is primarily a solid ball with a radius of about , which is about 20% of Earth's radius or 70% of the Moon's radius.
There are no samples of Earth's core accessible for d ...
. It is the fourth most common
element in the Earth's crust.
In its metallic state, iron is rare in the
Earth's crust, limited mainly to deposition by
meteorites.
Iron ores, by contrast, are among the most abundant in the Earth's crust, although extracting usable metal from them requires
kiln
A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes. Kilns have been used for millennia to turn objects made from clay int ...
s or
furnaces capable of reaching or higher, about higher than that required to
smelt copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
. Humans started to master that process in
Eurasia
Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelago ...
during the
2nd millennium BCE and the use of iron
tool
A tool is an object that can extend an individual's ability to modify features of the surrounding environment or help them accomplish a particular task. Although many animals use simple tools, only human beings, whose use of stone tools dates ba ...
s and
weapon
A weapon, arm or armament is any implement or device that can be used to deter, threaten, inflict physical damage, harm, or kill. Weapons are used to increase the efficacy and efficiency of activities such as hunting, crime, law enforcement, ...
s began to displace
copper alloys
Copper alloys are metal alloys that have copper as their principal component. They have high resistance against corrosion. The best known traditional types are bronze, where tin is a significant addition, and brass, using zinc instead. Both of ...
, in some regions, only around 1200 BCE. That event is considered the transition from the
Bronze Age
The Bronze Age is a historic period, lasting approximately from 3300 BC to 1200 BC, characterized by the use of bronze, the presence of writing in some areas, and other early features of urban civilization. The Bronze Age is the second prin ...
to the
Iron Age
The Iron Age is the final epoch of the three-age division of the prehistory and protohistory of humanity. It was preceded by the Stone Age ( Paleolithic, Mesolithic, Neolithic) and the Bronze Age ( Chalcolithic). The concept has been mostl ...
. In the
modern world, iron alloys, such as
steel,
stainless steel,
cast iron
Cast iron is a class of iron– carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impur ...
and
special steels, are by far the most common industrial metals, because of their mechanical properties and low cost. The
iron and steel industry is thus very important economically, and iron is the cheapest metal, with a price of a few dollars per kilogram or per pound (see
Metal#uses).
Pristine and smooth pure iron surfaces are mirror-like silvery-gray. However, iron reacts readily with
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
and
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
to give brown to black
hydrated
iron oxides, commonly known as
rust
Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...
. Unlike the oxides of some other metals that form
passivating layers, rust occupies more volume than the metal and thus flakes off, exposing more fresh surfaces for corrosion. Although iron readily reacts, high purity iron, called
electrolytic iron, has better corrosion resistance.
The body of an adult human contains about 4 grams (0.005% body weight) of iron, mostly in
hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyt ...
and
myoglobin. These two
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s play essential roles in
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...
, respectively
oxygen transport by
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
and oxygen storage in
muscles. To maintain the necessary levels,
human iron metabolism
Human iron metabolism is the set of chemical reactions that maintain human homeostasis of iron at the systemic and cellular level. Iron is both necessary to the body and potentially toxic. Controlling iron levels in the body is a critically imp ...
requires a minimum of iron in the diet. Iron is also the metal at the active site of many important
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
enzymes dealing with
cellular respiration
Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be des ...
and
oxidation and reduction in plants and animals.
Chemically, the most common oxidation states of iron are
iron(II) and
iron(III). Iron shares many properties of other
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...
s, including the other
group 8 element
Group 8 is a group (column) of chemical elements in the periodic table. It consists of iron (Fe), ruthenium (Ru), osmium (Os) and hassium (Hs).Leigh, G. J. ''Nomenclature of Inorganic Chemistry: Recommendations 1990''. Blackwell Science, 1990. ...
s,
ruthenium
Ruthenium is a chemical element with the symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most other chemical ...
and
osmium. Iron forms compounds in a wide range of
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
s, −2 to +7. Iron also forms many
coordination compounds; some of them, such as
ferrocene,
ferrioxalate, and
Prussian blue
Prussian blue (also known as Berlin blue, Brandenburg blue or, in painting, Parisian or Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula Fe CN)">Cyanide.html" ;"title="e(Cyani ...
, have substantial industrial, medical, or research applications.
Characteristics
Allotropes
![Iron-alpha-pV](https://upload.wikimedia.org/wikipedia/commons/8/88/Iron-alpha-pV.svg)
At least four allotropes of iron (differing atom arrangements in the solid) are known, conventionally denoted
α,
γ,
δ, and
ε.
![Pure iron phase diagram (EN)](https://upload.wikimedia.org/wikipedia/commons/5/59/Pure_iron_phase_diagram_%28EN%29.png)
The first three forms are observed at ordinary pressures. As molten iron cools past its freezing point of 1538 °C, it crystallizes into its δ allotrope, which has a
body-centered cubic
In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals.
There are three main varieties of ...
(bcc)
crystal structure
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns ...
. As it cools further to 1394 °C, it changes to its γ-iron allotrope, a
face-centered cubic
In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals.
There are three main varieties of ...
(fcc) crystal structure, or
austenite
Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K ...
. At 912 °C and below, the crystal structure again becomes the bcc α-iron allotrope.
The physical properties of iron at very high pressures and temperatures have also been studied extensively,
because of their relevance to theories about the cores of the Earth and other planets. Above approximately 10 GPa and temperatures of a few hundred kelvin or less, α-iron changes into another
hexagonal close-packed
In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occu ...
(hcp) structure, which is also known as
ε-iron. The higher-temperature γ-phase also changes into ε-iron, but does so at higher pressure.
Some controversial experimental evidence exists for a stable
β phase at pressures above 50 GPa and temperatures of at least 1500 K. It is supposed to have an
orthorhombic or a double hcp structure.
(Confusingly, the term "β-iron" is sometimes also used to refer to α-iron above its Curie point, when it changes from being ferromagnetic to paramagnetic, even though its crystal structure has not changed.
)
The
inner core
Earth's inner core is the innermost geologic layer of planet Earth. It is primarily a solid ball with a radius of about , which is about 20% of Earth's radius or 70% of the Moon's radius.
There are no samples of Earth's core accessible for d ...
of the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
is generally presumed to consist of an iron-
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductilit ...
with ε (or β) structure.
Melting and boiling points
The melting and boiling points of iron, along with its
enthalpy of atomization
In chemistry, the enthalpy of atomization (also atomisation in British English) is the enthalpy change that accompanies the total separation of all atoms in a chemical substance (either an element or a compound). This is often represented by th ...
, are lower than those of the earlier 3d elements from
scandium to
chromium, showing the lessened contribution of the 3d electrons to metallic bonding as they are attracted more and more into the inert core by the nucleus;
[Greenwood and Earnshaw, p. 1116] however, they are higher than the values for the previous element
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
because that element has a half-filled 3d sub-shell and consequently its d-electrons are not easily delocalized. This same trend appears for ruthenium but not osmium.
The melting point of iron is experimentally well defined for pressures less than 50 GPa. For greater pressures, published data (as of 2007) still varies by tens of gigapascals and over a thousand kelvin.
Magnetic properties
![Magnetization curves](https://upload.wikimedia.org/wikipedia/commons/a/a7/Magnetization_curves.svg)
Below its
Curie point
In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...
of , α-iron changes from
paramagnetic
Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...
to
ferromagnetic: the
spins of the two unpaired electrons in each atom generally align with the spins of its neighbors, creating an overall
magnetic field.
This happens because the orbitals of those two electrons (d
''z''2 and d
''x''2 − ''y''2) do not point toward neighboring atoms in the lattice, and therefore are not involved in metallic bonding.
In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into
magnetic domain
A magnetic domain is a region within a magnetic material in which the magnetization is in a uniform direction. This means that the individual magnetic moments of the atoms are aligned with one another and they point in the same direction. When c ...
s, about 10 micrometers across,
such that the atoms in each domain have parallel spins, but some domains have other orientations. Thus a macroscopic piece of iron will have a nearly zero overall magnetic field.
Application of an external magnetic field causes the domains that are magnetized in the same general direction to grow at the expense of adjacent ones that point in other directions, reinforcing the external field. This effect is exploited in devices that need to channel magnetic fields to fulfill design function, such as
electrical transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer's c ...
s,
magnetic recording
Magnetic storage or magnetic recording is the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetisation in a magnetizable material to store data and is a form of non-volatile memory. The information is ac ...
heads, and
electric motor
An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate for ...
s. Impurities,
lattice defects, or grain and particle boundaries can "pin" the domains in the new positions, so that the effect persists even after the external field is removed – thus turning the iron object into a (permanent)
magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nicke ...
.
Similar behavior is exhibited by some iron compounds, such as the
ferrites including the mineral
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
, a crystalline form of the mixed iron(II,III) oxide (although the atomic-scale mechanism,
ferrimagnetism
A ferrimagnetic material is a material that has populations of atoms with opposing magnetic moments, as in antiferromagnetism, but these moments are unequal in magnitude so a spontaneous magnetization remains. This can for example occur when t ...
, is somewhat different). Pieces of magnetite with natural permanent magnetization (
lodestone
Lodestones are naturally magnetized pieces of the mineral magnetite. They are naturally occurring magnets, which can attract iron. The property of magnetism was first discovered in antiquity through lodestones. Pieces of lodestone, suspen ...
s) provided the earliest
compass
A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself wit ...
es for navigation. Particles of magnetite were extensively used in magnetic recording media such as
core memories,
magnetic tapes,
floppies, and
disks, until they were replaced by
cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, p ...
-based materials.
Isotopes
Iron has four stable
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numb ...
s:
54Fe (5.845% of natural iron),
56Fe (91.754%),
57Fe (2.119%) and
58Fe (0.282%). 20-30 artificial isotopes have also been created. Of these stable isotopes, only
57Fe has a
nuclear spin (−). The
nuclide 54Fe theoretically can undergo
double electron capture
Double electron capture is a decay mode of an atomic nucleus. For a nuclide (''A'', ''Z'') with a number of nucleons ''A'' and atomic number ''Z'', double electron capture is only possible if the mass of the nuclide (''A'', ''Z''−2) is lower.
...
to
54Cr, but the process has never been observed and only a lower limit on the half-life of 3.1×10
22 years has been established.
60Fe is an
extinct radionuclide
An extinct radionuclide is a radionuclide that was formed by nucleosynthesis before the formation of the Solar System, about 4.6 billion years ago, but has since decayed to virtually zero abundance and is no longer detectable as a primordial nuc ...
of long
half-life
Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...
(2.6 million years).
It is not found on Earth, but its ultimate decay product is its granddaughter, the stable nuclide
60Ni.
Much of the past work on isotopic composition of iron has focused on the
nucleosynthesis
Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons (protons and neutrons) and nuclei. According to current theories, the first nuclei were formed a few minutes after the Big Bang, through nuclear reactions in ...
of
60Fe through studies of
meteorites and ore formation. In the last decade, advances in
mass spectrometry have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the
stable isotope
The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...
s of iron. Much of this work is driven by the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
and
planetary science communities, although applications to biological and industrial systems are emerging.
In phases of the meteorites ''Semarkona'' and ''Chervony Kut,'' a correlation between the concentration of
60Ni, the
granddaughter
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideal ...
of
60Fe, and the abundance of the stable iron isotopes provided evidence for the existence of
60Fe at the time of
formation of the Solar System. Possibly the energy released by the decay of
60Fe, along with that released by
26Al, contributed to the remelting and
differentiation of
asteroids after their formation 4.6 billion years ago. The abundance of
60Ni present in
extraterrestrial material may bring further insight into the origin and early history of the
Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
.
The most abundant iron isotope
56Fe is of particular interest to nuclear scientists because it represents the most common endpoint of
nucleosynthesis
Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons (protons and neutrons) and nuclei. According to current theories, the first nuclei were formed a few minutes after the Big Bang, through nuclear reactions in ...
. Since
56Ni (14
alpha particle
Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be pr ...
s) is easily produced from lighter nuclei in the
alpha process
The alpha process, also known as the alpha ladder, is one of two classes of nuclear fusion reactions by which stars convert helium into heavier elements, the other being the triple-alpha process.
The triple-alpha process consumes only helium, a ...
in
nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a transformatio ...
s in supernovae (see
silicon burning process), it is the endpoint of fusion chains inside
extremely massive stars, since addition of another alpha particle, resulting in
60Zn, requires a great deal more energy. This
56Ni, which has a half-life of about 6 days, is created in quantity in these stars, but soon decays by two successive positron emissions within supernova decay products in the
supernova remnant
A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...
gas cloud, first to radioactive
56Co, and then to stable
56Fe. As such, iron is the most abundant element in the core of
red giants, and is the most abundant metal in
iron meteorite
Iron meteorites, also known as siderites, or ferrous meteorites, are a type of meteorite that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that usually consists of two mineral phases: kamacite and taenite. Most iron ...
s and in the dense metal
cores of planets such as
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
.
[Greenwood and Earnshaw, p. 12] It is also very common in the universe, relative to other stable
metals
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typical ...
of approximately the same
atomic weight
Relative atomic mass (symbol: ''A''; sometimes abbreviated RAM or r.a.m.), also known by the deprecated synonym atomic weight, is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a giv ...
.
Iron is the sixth most
abundant element in the
universe
The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. ...
, and the most common
refractory
In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...
element.
Although a further tiny energy gain could be extracted by synthesizing
62Ni, which has a marginally higher binding energy than
56Fe, conditions in stars are unsuitable for this process. Element production in supernovas and distribution on Earth greatly favor iron over nickel, and in any case,
56Fe still has a lower mass per nucleon than
62Ni due to its higher fraction of lighter protons. Hence, elements heavier than iron require a
supernova for their formation, involving
rapid neutron capture by starting
56Fe nuclei.
In the
far future
While the future cannot be predicted with certainty, present understanding in various scientific fields allows for the prediction of some far-future events, if only in the broadest outline. These fields include astrophysics, which studies how ...
of the universe, assuming that
proton decay
In particle physics, proton decay is a hypothetical form of particle decay in which the proton decays into lighter subatomic particles, such as a neutral pion and a positron. The proton decay hypothesis was first formulated by Andrei Sakharov ...
does not occur, cold
fusion
Fusion, or synthesis, is the process of combining two or more distinct entities into a new whole.
Fusion may also refer to:
Science and technology Physics
*Nuclear fusion, multiple atomic nuclei combining to form one or more different atomic nucl ...
occurring via
quantum tunnelling would cause the light nuclei in ordinary matter to fuse into
56Fe nuclei. Fission and
alpha-particle emission would then make heavy nuclei decay into iron, converting all stellar-mass objects to cold spheres of pure iron.
Origin and occurrence in nature
Cosmogenesis
Iron's abundance in
rocky planets like Earth is due to its abundant production during the runaway fusion and explosion of type
Ia supernovae, which scatters the iron into space.
Metallic iron
![Widmanstatten hand](https://upload.wikimedia.org/wikipedia/commons/3/36/Widmanstatten_hand.jpg)
Metallic or
native iron is rarely found on the surface of the Earth because it tends to oxidize. However, both the Earth's
inner and
outer core
Earth's outer core is a fluid layer about thick, composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. The outer core begins approximately beneath Earth's surface at the core-mantle boundary and e ...
, that account for 35% of the mass of the whole Earth, are believed to consist largely of an iron alloy, possibly with
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
. Electric currents in the liquid outer core are believed to be the origin of the
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic ...
. The other
terrestrial planet
A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, ...
s (
Mercury,
Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...
, and
Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
) as well as the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
are believed to have a metallic core consisting mostly of iron. The
M-type asteroid
M-type (aka M-class) asteroids are a spectral class of asteroids which appear to contain higher concentrations of metal phases (e.g. iron-nickel) than other asteroid classes, and are widely thought to be the source of iron meteorites.
Definition ...
s are also believed to be partly or mostly made of metallic iron alloy.
The rare
iron meteorite
Iron meteorites, also known as siderites, or ferrous meteorites, are a type of meteorite that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that usually consists of two mineral phases: kamacite and taenite. Most iron ...
s are the main form of natural metallic iron on the Earth's surface. Items made of
cold-worked meteoritic iron have been found in various archaeological sites dating from a time when iron smelting had not yet been developed; and the
Inuit
Inuit (; iu, ᐃᓄᐃᑦ 'the people', singular: Inuk, , dual: Inuuk, ) are a group of culturally similar indigenous peoples inhabiting the Arctic and subarctic regions of Greenland, Labrador, Quebec, Nunavut, the Northwest Territories ...
in
Greenland
Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland i ...
have been reported to use iron from the
Cape York meteorite
The Cape York meteorite, also known as the Innaanganeq meteorite, is one of the largest known iron meteorites, classified as a medium octahedrite in chemical group IIIAB. In addition to many small fragments, at least eight large fragments with a ...
for tools and hunting weapons. About 1 in 20
meteorites consist of the unique iron-nickel minerals
taenite (35–80% iron) and
kamacite
Kamacite is an alloy of iron and nickel, which is found on Earth only in meteorites. According to the International Mineralogical Association (IMA) it is considered a proper nickel-rich variety of the mineral native iron. The proportion iron: ...
(90–95% iron). Native iron is also rarely found in basalts that have formed from magmas that have come into contact with carbon-rich sedimentary rocks, which have reduced the oxygen
fugacity
In chemical thermodynamics, the fugacity of a real gas is an effective partial pressure which replaces the mechanical partial pressure in an accurate computation of the chemical equilibrium constant. It is equal to the pressure of an ideal gas whic ...
sufficiently for iron to crystallize. This is known as
Telluric iron
Telluric iron, also called native iron, is iron that originated on Earth, and is found in a metallic form rather than as an ore. Telluric iron is extremely rare, with only one known major deposit in the world, located in Greenland.
Introduction
W ...
and is described from a few localities, such as
Disko Island
Disko Island ( kl, Qeqertarsuaq, da, Diskoøen) is a large island in Baffin Bay, off the west coast of Greenland. It has an area of ,[Yakutia
Sakha, officially the Republic of Sakha (Yakutia),, is the largest republic of Russia, located in the Russian Far East, along the Arctic Ocean, with a population of roughly 1 million. Sakha comprises half of the area of its governing Far E ...](_blank ...<br></span></div> in West Greenland, <div class=)
in Russia and Bühl in Germany.
Mantle minerals
Ferropericlase , a solid solution of
periclase
Periclase is a magnesium mineral that occurs naturally in contact metamorphic rocks and is a major component of most basic refractory bricks. It is a cubic form of magnesium oxide ( Mg O). In nature it usually forms a solid solution with wüstit ...
(MgO) and
wüstite
Wüstite ( Fe O) is a mineral form of iron(II) oxide found with meteorites and native iron. It has a grey colour with a greenish tint in reflected light. Wüstite crystallizes in the isometric-hexoctahedral crystal system in opaque to translu ...
(FeO), makes up about 20% of the volume of the
lower mantle of the Earth, which makes it the second most abundant mineral phase in that region after
silicate perovskite ; it also is the major host for iron in the lower mantle. At the bottom of the
transition zone of the mantle, the reaction γ- transforms
γ-olivine into a mixture of silicate perovskite and ferropericlase and vice versa. In the literature, this mineral phase of the lower mantle is also often called magnesiowüstite.
[Ferropericlase](_blank)
Mindat.org Silicate perovskite may form up to 93% of the lower mantle,
and the magnesium iron form, , is considered to be the most abundant
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
in the Earth, making up 38% of its volume.
Earth's crust
![Roussillon_sentier_des_ocres2](https://upload.wikimedia.org/wikipedia/commons/c/cc/Roussillon_sentier_des_ocres2.JPG)
While iron is the most abundant element on Earth, most of this iron is concentrated in the
inner and
outer cores. The fraction of iron that is in
Earth's crust only amounts to about 5% of the overall mass of the crust and is thus only the fourth most abundant element in that layer (after
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
,
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ta ...
, and
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
).
Most of the iron in the crust is combined with various other elements to form many
iron minerals
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in fro ...
. An important class is the
iron oxide minerals such as hematite (Fe
2O
3),
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
(Fe
3O
4), and siderite (FeCO
3), which are the major iron ore, ores of iron. Many igneous rocks also contain the sulfide minerals pyrrhotite and pentlandite.
[Klein, Cornelis and Cornelius S. Hurlbut, Jr. (1985) ''Manual of Mineralogy,'' Wiley, 20th ed, pp. 278–79 ] During weathering, iron tends to leach from sulfide deposits as the sulfate and from silicate deposits as the bicarbonate. Both of these are oxidized in aqueous solution and precipitate in even mildly elevated pH as iron(III) oxide.
[Greenwood and Earnshaw, p. 1071]
![Outcropping banded iron formation - panoramio](https://upload.wikimedia.org/wikipedia/commons/1/15/Outcropping_banded_iron_formation_-_panoramio.jpg)
Large deposits of iron are banded iron formations, a type of rock consisting of repeated thin layers of iron oxides alternating with bands of iron-poor shale and chert. The banded iron formations were laid down in the time between and .
Materials containing finely ground iron(III) oxides or oxide-hydroxides, such as ochre, have been used as yellow, red, and brown pigments since pre-historical times. They contribute as well to the color of various rocks and clays, including entire geological formations like the Painted Hills in Oregon and the Buntsandstein ("colored sandstone", British Bunter (geology), Bunter). Through ''Eisensandstein'' (a Brown Jurassic, jurassic 'iron sandstone', e.g. from Donzdorf in Germany) and Bath stone in the UK, iron compounds are responsible for the yellowish color of many historical buildings and sculptures. The proverbial Mars surface color, red color of the surface of Mars is derived from an iron oxide-rich regolith.
Significant amounts of iron occur in the iron sulfide mineral pyrite (FeS
2), but it is difficult to extract iron from it and it is therefore not exploited. In fact, iron is so common that production generally focuses only on ores with very high quantities of it.
According to the International Resource Panel's Metal Stocks in Society report, the global stock of iron in use in society is 2,200 kg per capita. More-developed countries differ in this respect from less-developed countries (7,000–14,000 vs 2,000 kg per capita).
Oceans
Ocean science demonstrated the role of the iron in the ancient seas in both marine biota and climate.
Chemistry and compounds
Iron shows the characteristic chemical properties of the
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...
s, namely the ability to form variable oxidation states differing by steps of one and a very large coordination and organometallic chemistry: indeed, it was the discovery of an iron compound,
ferrocene, that revolutionalized the latter field in the 1950s.
[Greenwood and Earnshaw, p. 905] Iron is sometimes considered as a prototype for the entire block of transition metals, due to its abundance and the immense role it has played in the technological progress of humanity.
Its 26 electrons are arranged in the electron configuration, configuration [Ar]3d
64s
2, of which the 3d and 4s electrons are relatively close in energy, and thus it can lose a variable number of electrons and there is no clear point where further ionization becomes unprofitable.
Iron forms compounds mainly in the
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
s +2 (
iron(II), "ferrous") and +3 (
iron(III), "ferric"). Iron also occurs in high-valent iron, higher oxidation states, e.g. the purple potassium ferrate (K
2FeO
4), which contains iron in its +6 oxidation state. Although iron(VIII) oxide (FeO
4) has been claimed, the report could not be reproduced and such a species from the removal of all electrons of the element beyond the preceding inert gas configuration (at least with iron in its +8 oxidation state) has been found to be improbable computationally. However, one form of anionic [FeO
4]
– with iron in its +7 oxidation state, along with an iron(V)-peroxo isomer, has been detected by infrared spectroscopy at 4 K after cocondensation of laser-ablated Fe atoms with a mixture of O
2/Ar. Iron(IV) is a common intermediate in many biochemical oxidation reactions.
Numerous organoiron chemistry, organoiron compounds contain formal oxidation states of +1, 0, −1, or even −2. The oxidation states and other bonding properties are often assessed using the technique of Mössbauer spectroscopy. Many mixed valence compounds contain both iron(II) and iron(III) centers, such as
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
and
Prussian blue
Prussian blue (also known as Berlin blue, Brandenburg blue or, in painting, Parisian or Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula Fe CN)">Cyanide.html" ;"title="e(Cyani ...
().
The latter is used as the traditional "blue" in blueprints.
Iron is the first of the transition metals that cannot reach its group oxidation state of +8, although its heavier congeners ruthenium and osmium can, with ruthenium having more difficulty than osmium.
Ruthenium exhibits an aqueous cationic chemistry in its low oxidation states similar to that of iron, but osmium does not, favoring high oxidation states in which it forms anionic complexes.
In the second half of the 3d transition series, vertical similarities down the groups compete with the horizontal similarities of iron with its neighbors
cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, p ...
and
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
in the periodic table, which are also ferromagnetic at room temperature and share similar chemistry. As such, iron, cobalt, and nickel are sometimes grouped together as the iron triad.
[Greenwood and Earnshaw, p. 1070]
Unlike many other metals, iron does not form amalgams with mercury (element), mercury. As a result, mercury is traded in standardized 76 pound flasks (34 kg) made of iron.
Iron is by far the most reactive element in its group; it is pyrophoricity, pyrophoric when finely divided and dissolves easily in dilute acids, giving Fe
2+. However, it does not react with concentrated nitric acid and other oxidizing acids due to the formation of an impervious oxide layer, which can nevertheless react with hydrochloric acid.
High purity iron, called
electrolytic iron, is considered to be resistant to rust, due to its oxide layer.
Binary compounds
Oxides and hydroxides
Iron forms various iron oxide, oxide and hydroxide compounds; the most common are iron(II,III) oxide (Fe
3O
4), and iron(III) oxide (Fe
2O
3). Iron(II) oxide also exists, though it is unstable at room temperature. Despite their names, they are actually all non-stoichiometric compounds whose compositions may vary.
These oxides are the principal ores for the production of iron (see bloomery and blast furnace). They are also used in the production of
ferrites, useful magnetic storage media in computers, and pigments. The best known sulfide is iron pyrite (FeS
2), also known as fool's gold owing to its golden luster.
It is not an iron(IV) compound, but is actually an iron(II) polysulfide containing Fe
2+ and ions in a distorted sodium chloride structure.
[Greenwood and Earnshaw, p. 1079]
Halides
![Iron(III) chloride hexahydrate](https://upload.wikimedia.org/wikipedia/commons/e/ec/Iron%28III%29_chloride_hexahydrate.jpg)
The binary ferrous and ferric halides are well-known. The ferrous halides typically arise from treating iron metal with the corresponding hydrohalic acid to give the corresponding hydrated salts.
:Fe + 2 HX → FeX
2 + H
2 (X = F, Cl, Br, I)
Iron reacts with fluorine, chlorine, and bromine to give the corresponding ferric halides, ferric chloride being the most common.
:2 Fe + 3 X
2 → 2 FeX
3 (X = F, Cl, Br)
Ferric iodide is an exception, being thermodynamically unstable due to the oxidizing power of Fe
3+ and the high reducing power of I
−:
:2 I
− + 2 Fe
3+ → I
2 + 2 Fe
2+ (E
0 = +0.23 V)
Ferric iodide, a black solid, is not stable in ordinary conditions, but can be prepared through the reaction of iron pentacarbonyl with iodine and carbon monoxide in the presence of hexane and light at the temperature of −20 °C, with oxygen and water excluded.
Complexes of ferric iodide with some soft bases are known to be stable compounds.
Solution chemistry
![Ferrate_and_permanganate_solution](https://upload.wikimedia.org/wikipedia/commons/5/56/Ferrate_and_permanganate_solution.jpg)
The standard reduction potentials in acidic aqueous solution for some common iron ions are given below:
[Greenwood and Earnshaw, pp. 1075–79]
The red-purple tetrahedral ferrate(VI) anion is such a strong oxidizing agent that it oxidizes nitrogen and ammonia at room temperature, and even water itself in acidic or neutral solutions:
[Greenwood and Earnshaw, pp. 1082–84]
:4 + 10 → 4 + 20 + 3 O
2
The Fe
3+ ion has a large simple cationic chemistry, although the pale-violet hexaquo ion is very readily hydrolyzed when pH increases above 0 as follows:
[Greenwood and Earnshaw, pp. 1088–91]
![Iron(II)-sulfate-heptahydrate-sample](https://upload.wikimedia.org/wikipedia/commons/a/ab/Iron%28II%29-sulfate-heptahydrate-sample.jpg)
As pH rises above 0 the above yellow hydrolyzed species form and as it rises above 2–3, reddish-brown hydrous iron(III) oxide precipitates out of solution. Although Fe
3+ has a d
5 configuration, its absorption spectrum is not like that of Mn
2+ with its weak, spin-forbidden d–d bands, because Fe
3+ has higher positive charge and is more polarizing, lowering the energy of its ligand-to-metal charge-transfer complex, charge transfer absorptions. Thus, all the above complexes are rather strongly colored, with the single exception of the hexaquo ion – and even that has a spectrum dominated by charge transfer in the near ultraviolet region.
On the other hand, the pale green iron(II) hexaquo ion does not undergo appreciable hydrolysis. Carbon dioxide is not evolved when carbonate anions are added, which instead results in white iron(II) carbonate being precipitated out. In excess carbon dioxide this forms the slightly soluble bicarbonate, which occurs commonly in groundwater, but it oxidises quickly in air to form iron(III) oxide that accounts for the brown deposits present in a sizeable number of streams.
[Greenwood and Earnshaw, pp. 1091–97]
Coordination compounds
Due to its electronic structure, iron has a very large coordination and organometallic chemistry.
![2-isomers-of-ferrioxalate](https://upload.wikimedia.org/wikipedia/commons/0/06/2-isomers-of-ferrioxalate.svg)
Many coordination compounds of iron are known. A typical six-coordinate anion is hexachloroferrate(III), [FeCl
6]
3−, found in the mixed salt (chemistry), salt tetrakis(methylammonium) hexachloroferrate(III) chloride. Complexes with multiple bidentate ligands have geometric isomers. For example, the ''trans''-chlorohydridobis(bis-1,2-(diphenylphosphino)ethane)iron(II) complex is used as a starting material for compounds with the moiety (chemistry), moiety. The ferrioxalate ion with three oxalate ligands (shown at right) displays helical chirality with its two non-superposable geometries labelled ''Λ'' (lambda) for the left-handed screw axis and ''Δ'' (delta) for the right-handed screw axis, in line with IUPAC conventions.
Potassium ferrioxalate is used in chemical actinometry and along with its sodium ferrioxalate, sodium salt undergoes photoreduction applied in old-style photographic processes. The hydrate, dihydrate of iron(II) oxalate has a polymeric structure with co-planar oxalate ions bridging between iron centres with the water of crystallisation located forming the caps of each octahedron, as illustrated below.
![Pentaaqua(thiocyanato)iron(III) chloride](https://upload.wikimedia.org/wikipedia/commons/1/1e/Pentaaqua%28thiocyanato%29iron%28III%29_chloride.jpg)
Iron(III) complexes are quite similar to those of
chromium(III) with the exception of iron(III)'s preference for ''O''-donor instead of ''N''-donor ligands. The latter tend to be rather more unstable than iron(II) complexes and often dissociate in water. Many Fe–O complexes show intense colors and are used as tests for phenols or enols. For example, in the ferric chloride test, used to determine the presence of phenols, iron(III) chloride reacts with a phenol to form a deep violet complex:
:3 ArOH + FeCl
3 → Fe(OAr)
3 + 3 HCl (Ar = aryl)
Among the halide and pseudohalide complexes, fluoro complexes of iron(III) are the most stable, with the colorless [FeF
5(H
2O)]
2− being the most stable in aqueous solution. Chloro complexes are less stable and favor tetrahedral coordination as in [FeCl
4]
−; [FeBr
4]
− and [FeI
4]
− are reduced easily to iron(II). Thiocyanate is a common test for the presence of iron(III) as it forms the blood-red [Fe(SCN)(H
2O)
5]
2+. Like manganese(II), most iron(III) complexes are high-spin, the exceptions being those with ligands that are high in the spectrochemical series such as cyanide. An example of a low-spin iron(III) complex is [Fe(CN)
6]
3−. The cyanide ligands may easily be detached in [Fe(CN)
6]
3−, and hence this complex is poisonous, unlike the iron(II) complex [Fe(CN)
6]
4− found in Prussian blue,
which does not release hydrogen cyanide except when dilute acids are added.
Iron shows a great variety of electronic spin states (d electrons), spin states, including every possible spin quantum number value for a d-block element from 0 (diamagnetic) to (5 unpaired electrons). This value is always half the number of unpaired electrons. Complexes with zero to two unpaired electrons are considered low-spin and those with four or five are considered high-spin.
Iron(II) complexes are less stable than iron(III) complexes but the preference for ''O''-donor ligands is less marked, so that for example is known while is not. They have a tendency to be oxidized to iron(III) but this can be moderated by low pH and the specific ligands used.
Organometallic compounds
![Sample of iron pentacarbonyl](https://upload.wikimedia.org/wikipedia/commons/d/df/Sample_of_iron_pentacarbonyl.jpg)
Organoiron chemistry is the study of organometallic compounds of iron, where carbon atoms are covalently bound to the metal atom. They are many and varied, including cyanometallate, cyanide complexes, carbonyl complexes, sandwich compound, sandwich and half-sandwich compounds.
Prussian blue
Prussian blue (also known as Berlin blue, Brandenburg blue or, in painting, Parisian or Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula Fe CN)">Cyanide.html" ;"title="e(Cyani ...
or "ferric ferrocyanide", Fe
4[Fe(CN)
6]
3, is an old and well-known iron-cyanide complex, extensively used as pigment and in several other applications. Its formation can be used as a simple wet chemistry test to distinguish between aqueous solutions of Fe
2+ and Fe
3+ as they react (respectively) with potassium ferricyanide and potassium ferrocyanide to form Prussian blue.
Another old example of an organoiron compound is iron pentacarbonyl, Fe(CO)
5, in which a neutral iron atom is bound to the carbon atoms of five carbon monoxide molecules. The compound can be used to make carbonyl iron powder, a highly reactive form of metallic iron. Thermal decomposition, Thermolysis of iron pentacarbonyl gives triiron dodecacarbonyl, , a complex with a cluster of three iron atoms at its core. Collman's reagent, disodium tetracarbonylferrate, is a useful reagent for organic chemistry; it contains iron in the −2 oxidation state. Cyclopentadienyliron dicarbonyl dimer contains iron in the rare +1 oxidation state.
A landmark in this field was the discovery in 1951 of the remarkably stable sandwich compound
ferrocene , by Pauson and Kealy and independently by Miller and colleagues,
whose surprising molecular structure was determined only a year later by Robert Burns Woodward, Woodward and Geoffrey Wilkinson, Wilkinson and Ernst Otto Fischer, Fischer.
Ferrocene is still one of the most important tools and models in this class.
[Greenwood and Earnshaw, p. 1104]
Iron-centered organometallic species are used as catalysts. The Knölker complex, for example, is a transfer hydrogenation catalyst for ketones.
Industrial uses
The iron compounds produced on the largest scale in industry are iron(II) sulfate (FeSO
4·7Water of crystallization, H
2O) and iron(III) chloride (FeCl
3). The former is one of the most readily available sources of iron(II), but is less stable to aerial oxidation than Mohr's salt (). Iron(II) compounds tend to be oxidized to iron(III) compounds in the air.
History
Development of iron metallurgy
Iron is one of the elements undoubtedly known to the ancient world. It has been worked, or wrought iron, wrought, for millennia. However, iron artefacts of great age are much rarer than objects made of gold or silver due to the ease with which iron corrodes. The technology developed slowly, and even after the discovery of smelting it took many centuries for iron to replace bronze as the metal of choice for tools and weapons.
Meteoritic iron
![Meteorite iron harpoon](https://upload.wikimedia.org/wikipedia/commons/d/d5/Meteorite_iron_harpoon.jpg)
Beads made from meteoric iron in 3500 BC or earlier were found in Gerzeh culture, Gerzeh, Egypt by G.A. Wainwright. The beads contain 7.5% nickel, which is a signature of meteoric origin since iron found in the Earth's crust generally has only minuscule nickel impurities.
Meteoric iron was highly regarded due to its origin in the heavens and was often used to forge weapons and tools. For example, a Tutankhamun's iron dagger blade, dagger made of meteoric iron was found in the tomb of Tutankhamun, containing similar proportions of iron, cobalt, and nickel to a meteorite discovered in the area, deposited by an ancient meteor shower.
Items that were likely made of iron by Egyptians date from 3000 to 2500 BC.
Meteoritic iron is comparably soft and ductile and easily cold forging, cold forged but may get brittle when heated because of the
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
content.
Wrought iron
![Mars symbol (fixed width)](https://upload.wikimedia.org/wikipedia/commons/7/7a/Mars_symbol_%28fixed_width%29.svg)
The first iron production started in the Middle Bronze Age, but it took several centuries before iron displaced bronze. Samples of smelting, smelted iron from Asmar, Mesopotamia and Tall Chagar Bazaar in northern Syria were made sometime between 3000 and 2700 BC. The Hittites established an empire in north-central Anatolia around 1600 BC. They appear to be the first to understand the production of iron from its ores and regard it highly in their society. The Hittites began to smelt iron between 1500 and 1200 BC and the practice spread to the rest of the Near East after their empire fell in 1180 BC. The subsequent period is called the
Iron Age
The Iron Age is the final epoch of the three-age division of the prehistory and protohistory of humanity. It was preceded by the Stone Age ( Paleolithic, Mesolithic, Neolithic) and the Bronze Age ( Chalcolithic). The concept has been mostl ...
.
Artifacts of smelted iron are found in History of metallurgy in the Indian subcontinent, India dating from 1800 to 1200 BC,
and in the Levant from about 1500 BC (suggesting smelting in Anatolia or the Caucasus). Alleged references (compare history of metallurgy in South Asia) to iron in the Indian Vedas have been used for claims of a very early usage of iron in India respectively to date the texts as such. The rigveda term ''ayas'' (metal) refers to copper, while iron which is called as ''śyāma ayas'', literally "black copper", first is mentioned in the post-rigvedic Atharvaveda.
Some archaeological evidence suggests iron was smelted in Zimbabwe and southeast Africa as early as the eighth century BC. Iron working was introduced to Ancient Greece, Greece in the late 11th century BC, from which it spread quickly throughout Europe.
The spread of ironworking in Central and Western Europe is associated with Celts, Celtic expansion. According to Pliny the Elder, iron use was common in the Ancient Rome, Roman era. In the lands of what is now considered China, iron appears approximately 700–500 BC. Iron smelting may have been introduced into China through Central Asia.
[Pigott, Vincent C. (1999). ''The Archaeometallurgy of the Asian Old World''. Philadelphia: University of Pennsylvania Museum of Archaeology and Anthropology. , p. 8.] The earliest evidence of the use of a blast furnace in China dates to the 1st century AD,
and cupola furnaces were used as early as the Warring States period (403–221 BC).
[Pigott, Vincent C. (1999). ''The Archaeometallurgy of the Asian Old World''. Philadelphia: University of Pennsylvania Museum of Archaeology and Anthropology. , p. 191.] Usage of the blast and cupola furnace remained widespread during the Tang dynasty, Tang and Song dynasty, Song dynasties.
During the Industrial Revolution in Britain, Henry Cort began refining iron from pig iron to wrought iron (or bar iron) using innovative production systems. In 1783 he patented the Puddling (metallurgy), puddling process for refining iron ore. It was later improved by others, including Joseph Hall (metallurgist), Joseph Hall.
Cast iron
Cast iron was first produced in China during 5th century BC, but was hardly in Europe until the medieval period.
The earliest cast iron artifacts were discovered by archaeologists in what is now modern Luhe County, Jiangsu in China. Cast iron was used in ancient China for warfare, agriculture, and architecture.
During the medieval period, means were found in Europe of producing wrought iron from cast iron (in this context known as pig iron) using finery forges. For all these processes, charcoal was required as fuel.
![Philipp Jakob Loutherbourg d](https://upload.wikimedia.org/wikipedia/commons/4/42/Philipp_Jakob_Loutherbourg_d._J._002.jpg)
Medieval blast furnaces were about tall and made of fireproof brick; forced air was usually provided by hand-operated bellows.
Modern blast furnaces have grown much bigger, with hearths fourteen meters in diameter that allow them to produce thousands of tons of iron each day, but essentially operate in much the same way as they did during medieval times.
In 1709, Abraham Darby I established a Coke (fuel), coke-fired blast furnace to produce cast iron, replacing charcoal, although continuing to use blast furnaces. The ensuing availability of inexpensive iron was one of the factors leading to the Industrial Revolution. Toward the end of the 18th century, cast iron began to replace wrought iron for certain purposes, because it was cheaper. Carbon content in iron was not implicated as the reason for the differences in properties of wrought iron, cast iron, and steel until the 18th century.
Since iron was becoming cheaper and more plentiful, it also became a major structural material following the building of the innovative The Iron Bridge, first iron bridge in 1778. This bridge still stands today as a monument to the role iron played in the Industrial Revolution. Following this, iron was used in rails, boats, ships, aqueducts, and buildings, as well as in iron cylinders in steam engines.
[Greenwood and Earnshaw, p. 1072] Railways have been central to the formation of modernity and ideas of progress and various languages (e.g. French, Spanish, Italian and German) refer to railways as ''iron road''.
Steel
Steel (with smaller carbon content than pig iron but more than wrought iron) was first produced in antiquity by using a bloomery. Blacksmiths in Luristan in western Persia were making good steel by 1000 BC. Then improved versions, Wootz steel by India and Damascus steel were developed around 300 BC and AD 500 respectively. These methods were specialized, and so steel did not become a major commodity until the 1850s.
New methods of producing it by carburizing bars of iron in the cementation process were devised in the 17th century. In the Industrial Revolution, new methods of producing bar iron without charcoal were devised and these were later applied to produce steel. In the late 1850s, Henry Bessemer invented a new steelmaking process, involving blowing air through molten pig iron, to produce mild steel. This made steel much more economical, thereby leading to wrought iron no longer being produced in large quantities.
Foundations of modern chemistry
In 1774, Antoine Lavoisier used the reaction of water steam with metallic iron inside an incandescent iron tube to produce hydrogen in his experiments leading to the demonstration of the conservation of mass, which was instrumental in changing chemistry from a qualitative science to a quantitative one.
Symbolic role
![Eisernes Kreuz (1914)](https://upload.wikimedia.org/wikipedia/commons/1/1f/Eisernes_Kreuz_%281914%29.jpg)
Iron plays a certain role in mythology and has found various usage Iron (metaphor), as a metaphor and in Iron in folklore, folklore. The Greeks, Greek poet Hesiod's ''Works and Days'' (lines 109–201) lists different ages of man named after metals like gold, silver, bronze and iron to account for successive ages of humanity. The Iron Age was closely related with Rome, and in Ovid's ''Metamorphoses''
An example of the importance of iron's symbolic role may be found in the German Campaign of 1813. Frederick William III commissioned then the first Iron Cross as military decoration. Berlin iron jewellery reached its peak production between 1813 and 1815, when the Prussian royal family urged citizens to donate gold and silver jewellery for military funding. The inscription ''Gold gab ich für Eisen'' (I gave gold for iron) was used as well in later war efforts.
Laboratory routes
For a few limited purposes when it is needed, pure iron is produced in the laboratory in small quantities by reducing the pure oxide or hydroxide with hydrogen, or forming iron pentacarbonyl and heating it to 250 °C so that it decomposes to form pure iron powder.
Another method is electrolysis of ferrous chloride onto an iron cathode.
Main industrial route
Nowadays, the industrial production of iron or steel consists of two main stages. In the first stage, iron ore is redox, reduced with coke (fuel), coke in a blast furnace, and the molten metal is separated from gross impurities such as silicate minerals. This stage yields an alloy—pig iron—that contains relatively large amounts of carbon. In the second stage, the amount of carbon in the pig iron is lowered by oxidation to yield wrought iron, steel, or cast iron.
[Greenwood and Earnshaw, p. 1073] Other metals can be added at this stage to form alloy steels.
Blast furnace processing
The blast furnace is loaded with iron ores, usually hematite or
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
, along with coke (coal that has been separately baked to remove volatile components) and Flux (metallurgy), flux (limestone or Dolomites, dolomite). "Blasts" of air pre-heated to 900 °C (sometimes with oxygen enrichment) is blown through the mixture, in sufficient amount to turn the carbon into carbon monoxide:
:
2 C + O2 -> 2 CO
This reaction raises the temperature to about 2000 °C. The carbon monoxide reduces the iron ore to metallic iron
:
Fe2O3 + 3 CO -> 2 Fe + 3 CO2
Some iron in the high-temperature lower region of the furnace reacts directly with the coke:
:
2Fe2O3 + 3C -> 4Fe + 3CO2
The flux removes silicaceous minerals in the ore, which would otherwise clog the furnace: The heat of the furnace decomposes the carbonates to calcium oxide, which reacts with any excess silica to form a slag composed of calcium silicate or other products. At the furnace's temperature, the metal and the slag are both molten. They collect at the bottom as two immiscible liquid layers (with the slag on top), that are then easily separated.
The slag can be used as a material in road construction or to improve mineral-poor soils for agriculture.
Steelmaking thus remains one of the largest industrial contributors of CO
2 emissions in the world.
Steelmaking
![Melted raw-iron](https://upload.wikimedia.org/wikipedia/commons/d/d8/Melted_raw-iron.jpg)
The pig iron produced by the blast furnace process contains up to 4–5% carbon (by mass), with small amounts of other impurities like sulfur, magnesium, phosphorus, and manganese. This high level of carbon makes it relatively weak and brittle. Reducing the amount of carbon to 0.002–2.1% produces
steel, which may be up to 1000 times harder than pure iron. A great variety of steel articles can then be made by cold working, hot rolling, forging, machining, etc. Removing the impurities from pig iron, but leaving 2–4% carbon, results in
cast iron
Cast iron is a class of iron– carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impur ...
, which is cast by foundry, foundries into articles such as stoves, pipes, radiators, lamp-posts, and rails.
Steel products often undergo various heat treatments after they are forged to shape. annealing (metallurgy), Annealing consists of heating them to 700–800 °C for several hours and then gradual cooling. It makes the steel softer and more workable.
[Verhoeven, J.D. (1975) ''Fundamentals of Physical Metallurgy'', Wiley, New York, p. 326]
Direct iron reduction
Owing to environmental concerns, alternative methods of processing iron have been developed. "Direct reduced iron, Direct iron reduction" Sponge iron reaction, reduces iron ore to a ferrous lump called sponge iron, "sponge" iron or "direct" iron that is suitable for steelmaking.
Two main reactions comprise the direct reduction process:
Natural gas is partially oxidized (with heat and a catalyst):
:
2 CH4 + O2 -> 2 CO + 4 H2
Iron ore is then treated with these gases in a furnace, producing solid sponge iron:
:
Fe2O3 + CO + 2 H2 -> 2 Fe + CO2 + 2 H2O
Silica is removed by adding a limestone flux as described above.
Thermite process
Ignition of a mixture of aluminium powder and iron oxide yields metallic iron via the thermite reaction:
:
Fe2O3 + 2 Al -> 2 Fe + Al2O3
Alternatively pig iron may be made into steel (with up to about 2% carbon) or wrought iron (commercially pure iron). Various processes have been used for this, including finery forges, Puddling (metallurgy), puddling furnaces, Bessemer converters, open hearth furnaces, basic oxygen furnaces, and electric arc furnaces. In all cases, the objective is to oxidize some or all of the carbon, together with other impurities. On the other hand, other metals may be added to make alloy steels.
Applications
As structural material
Iron is the most widely used of all the metals, accounting for over 90% of worldwide metal production. Its low cost and high strength often make it the material of choice material to withstand stress or transmit forces, such as the construction of machinery and machine tools, track (rail transport), rails, automobiles, hull (ship), ship hulls, rebar, concrete reinforcing bars, and the load-carrying framework of buildings. Since pure iron is quite soft, it is most commonly combined with alloying elements to make steel.
[Greenwood and Earnshaw, pp. 1070–71]
Mechanical properties
The mechanical properties of iron and its alloys are extremely relevant to their structural applications. Those properties can be evaluated in various ways, including the Brinell scale, Brinell test, the Rockwell scale, Rockwell test and the Vickers hardness test.
The properties of pure iron are often used to calibrate measurements or to compare tests.
However, the mechanical properties of iron are significantly affected by the sample's purity: pure, single crystals of iron are actually softer than aluminium,
and the purest industrially produced iron (99.99%) has a hardness of 20–30 Brinell. The pure iron (99.9%~99.999%), especially called
electrolytic iron, is industrially produced by Electrolytic process, electrolytic refining.
An increase in the carbon content will cause a significant increase in the hardness and tensile strength of iron. Maximum hardness of Rockwell scale, 65 R
c is achieved with a 0.6% carbon content, although the alloy has low tensile strength. Because of the softness of iron, it is much easier to work with than its heavier Congener (chemistry), congeners
ruthenium
Ruthenium is a chemical element with the symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most other chemical ...
and
osmium.
[Greenwood and Earnshaw, pp. 1074–75]
Types of steels and alloys
α-Iron is a fairly soft metal that can dissolve only a small concentration of carbon (no more than 0.021% by mass at 910 °C). Austenite (γ-iron) is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04% by mass at 1146 °C). This form of iron is used in the type of
stainless steel used for making cutlery, and hospital and food-service equipment.
Commercially available iron is classified based on purity and the abundance of additives. Pig iron has 3.5–4.5% carbon
and contains varying amounts of contaminants such as sulfur, silicon and phosphorus. Pig iron is not a saleable product, but rather an intermediate step in the production of cast iron and steel. The reduction of contaminants in pig iron that negatively affect material properties, such as sulfur and phosphorus, yields cast iron containing 2–4% carbon, 1–6% silicon, and small amounts of
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
.
Pig iron has a melting point in the range of 1420–1470 K, which is lower than either of its two main components, and makes it the first product to be melted when carbon and iron are heated together.
Its mechanical properties vary greatly and depend on the form the carbon takes in the alloy.
"White" cast irons contain their carbon in the form of cementite, or iron carbide (Fe
3C).
This hard, brittle compound dominates the mechanical properties of white cast irons, rendering them hard, but unresistant to shock. The broken surface of a white cast iron is full of fine facets of the broken iron carbide, a very pale, silvery, shiny material, hence the appellation. Cooling a mixture of iron with 0.8% carbon slowly below 723 °C to room temperature results in separate, alternating layers of cementite and α-iron, which is soft and malleable and is called pearlite for its appearance. Rapid cooling, on the other hand, does not allow time for this separation and creates hard and brittle martensite. The steel can then be tempered by reheating to a temperature in between, changing the proportions of pearlite and martensite. The end product below 0.8% carbon content is a pearlite-αFe mixture, and that above 0.8% carbon content is a pearlite-cementite mixture.
In gray iron the carbon exists as separate, fine flakes of graphite, and also renders the material brittle due to the sharp edged flakes of graphite that produce stress concentration sites within the material.
A newer variant of gray iron, referred to as ductile iron, is specially treated with trace amounts of magnesium to alter the shape of graphite to spheroids, or nodules, reducing the stress concentrations and vastly increasing the toughness and strength of the material.
Wrought iron contains less than 0.25% carbon but large amounts of slag that give it a fibrous characteristic.
It is a tough, malleable product, but not as fusible as pig iron. If honed to an edge, it loses it quickly. Wrought iron is characterized by the presence of fine fibers of slag entrapped within the metal. Wrought iron is more corrosion resistant than steel. It has been almost completely replaced by mild steel for traditional "wrought iron" products and blacksmithing.
Mild steel corrodes more readily than wrought iron, but is cheaper and more widely available. Carbon steel contains 2.0% carbon or less,
with small amounts of
manganese
Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
, sulfur, phosphorus, and silicon. Alloy steels contain varying amounts of carbon as well as other metals, such as
chromium, vanadium, molybdenum, nickel, tungsten, etc. Their alloy content raises their cost, and so they are usually only employed for specialist uses. One common alloy steel, though, is
stainless steel. Recent developments in ferrous metallurgy have produced a growing range of microalloyed steels, also termed 'HSLA steel, HSLA' or high-strength, low alloy steels, containing tiny additions to produce high strengths and often spectacular toughness at minimal cost.
![Ironattenuation](https://upload.wikimedia.org/wikipedia/commons/6/69/Ironattenuation.PNG)
Alloys with high purity elemental makeups (such as alloys of
electrolytic iron) have specifically enhanced properties such as ductility, tensile strength, toughness, fatigue limit, fatigue strength, heat resistance, and corrosion resistance.
Apart from traditional applications, iron is also used for protection from ionizing radiation. Although it is lighter than another traditional protection material, lead, it is much stronger mechanically. The attenuation of radiation as a function of energy is shown in the graph.
The main disadvantage of iron and steel is that pure iron, and most of its alloys, suffer badly from
rust
Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...
if not protected in some way, a cost amounting to over 1% of the world's economy.
Painting, galvanization, Passivation (chemistry), passivation, plastic coating and bluing (steel), bluing are all used to protect iron from rust by excluding
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
and oxygen or by cathodic protection. The mechanism of the rusting of iron is as follows:
[Greenwood and Earnshaw, p. 1076]
:Cathode: 3 O
2 + 6 H
2O + 12 e
− → 12 OH
−
:Anode: 4 Fe → 4 Fe
2+ + 8 e
−; 4 Fe
2+ → 4 Fe
3+ + 4 e
−
:Overall: 4 Fe + 3 O
2 + 6 H
2O → 4 Fe
3+ + 12 OH
− → 4 Fe(OH)
3 or 4 FeO(OH) + 4 H
2O
The electrolyte is usually iron(II) sulfate in urban areas (formed when atmospheric sulfur dioxide attacks iron), and salt particles in the atmosphere in seaside areas.
Iron compounds
Although the dominant use of iron is in metallurgy, iron compounds are also pervasive in industry. Iron catalysts are traditionally used in the Haber–Bosch process for the production of ammonia and the Fischer–Tropsch process for conversion of carbon monoxide to hydrocarbons for fuels and lubricants. Powdered iron in an acidic solvent was used in the Bechamp reduction the reduction of nitrobenzene to aniline. Iron based catalysts play a crucial role in converting biobased raw materials into valuable bulk- and fine chemicals, in fuel cells as well as in removal of hazardous chemicals.
Iron(III) oxide mixed with
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
powder can be ignited to create a thermite, thermite reaction, used in welding large iron parts (like railroad, rails) and purifying ores. Iron(III) oxide and Iron(III) oxyhydroxide, oxyhydroxide are used as reddish and ocher pigments.
Iron(III) chloride finds use in water purification and sewage treatment, in the dyeing of cloth, as a coloring agent in paints, as an additive in animal feed, and as an industrial etching, etchant for
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
in the manufacture of printed circuit boards.
It can also be dissolved in alcohol to form tincture of iron, which is used as a medicine to stop bleeding in Domestic canary, canaries.
Iron(II) sulfate is used as a precursor to other iron compounds. It is also used to redox, reduce chromate in cement. It is used to fortify foods and treat iron deficiency anemia. Iron(III) sulfate is used in settling minute sewage particles in tank water. Iron(II) chloride is used as a reducing flocculating agent, in the formation of iron complexes and magnetic iron oxides, and as a reducing agent in organic synthesis.
Biological and pathological role
Iron is required for life.
The iron–sulfur clusters are pervasive and include nitrogenase, the enzymes responsible for biological nitrogen fixation. Iron-containing proteins participate in transport, storage and used of oxygen.
Iron proteins are involved in electron transfer.
![Heme b](https://upload.wikimedia.org/wikipedia/commons/b/be/Heme_b.svg)
Examples of iron-containing proteins in higher organisms include hemoglobin, cytochrome (see high-valent iron), and catalase.
The average adult human contains about 0.005% body weight of iron, or about four grams, of which three quarters is in hemoglobin – a level that remains constant despite only about one milligram of iron being absorbed each day,
because the human body recycles its hemoglobin for the iron content.
Microbial growth may be assisted by oxidation of iron(II) or by reduction of iron (III).
Biochemistry
Iron acquisition poses a problem for aerobic organisms because ferric iron is poorly soluble near neutral pH. Thus, these organisms have developed means to absorb iron as complexes, sometimes taking up ferrous iron before oxidising it back to ferric iron.
In particular, bacteria have evolved very high-affinity wikt:sequester, sequestering agents called siderophores.
After uptake in human cell (biology), cells, iron storage is precisely regulated.
A major component of this regulation is the protein transferrin, which binds iron ions absorbed from the duodenum and carries it in the bloodstream, blood to cells.
Transferrin contains Fe
3+ in the middle of a distorted octahedron, bonded to one nitrogen, three oxygens and a chelating carbonate anion that traps the Fe
3+ ion: it has such a high Stability constants of complexes, stability constant that it is very effective at taking up Fe
3+ ions even from the most stable complexes. At the bone marrow, transferrin is reduced from Fe
3+ and Fe
2+ and stored as ferritin to be incorporated into hemoglobin.
The most commonly known and studied bioinorganic chemistry, bioinorganic iron compounds (biological iron molecules) are the heme proteins: examples are
hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyt ...
,
myoglobin, and cytochrome P450.
These compounds participate in transporting gases, building
enzymes, and transferring electrons.
[Greenwood and Earnshaw, pp. 1098–104] Metalloproteins are a group of proteins with metal ion cofactor (biochemistry), cofactors. Some examples of iron metalloproteins are ferritin and rubredoxin.
Many enzymes vital to life contain iron, such as catalase,
lipoxygenases,
and IRE-BP.
Hemoglobin is an oxygen carrier that occurs in red blood cells and contributes their color, transporting oxygen in the arteries from the lungs to the muscles where it is transferred to
myoglobin, which stores it until it is needed for the metabolic oxidation of glucose, generating energy.
Here the hemoglobin binds to carbon dioxide, produced when glucose is oxidized, which is transported through the veins by hemoglobin (predominantly as bicarbonate anions) back to the lungs where it is exhaled.
In hemoglobin, the iron is in one of four heme groups and has six possible coordination sites; four are occupied by nitrogen atoms in a porphyrin ring, the fifth by an imidazole nitrogen in a histidine residue of one of the protein chains attached to the heme group, and the sixth is reserved for the oxygen molecule it can reversibly bind to.
When hemoglobin is not attached to oxygen (and is then called deoxyhemoglobin), the Fe
2+ ion at the center of the heme group (in the hydrophobic protein interior) is in a Spin states (d electrons)#High-spin and low-spin systems, high-spin configuration. It is thus too large to fit inside the porphyrin ring, which bends instead into a dome with the Fe
2+ ion about 55 picometers above it. In this configuration, the sixth coordination site reserved for the oxygen is blocked by another histidine residue.
When deoxyhemoglobin picks up an oxygen molecule, this histidine residue moves away and returns once the oxygen is securely attached to form a hydrogen bond with it. This results in the Fe
2+ ion switching to a low-spin configuration, resulting in a 20% decrease in ionic radius so that now it can fit into the porphyrin ring, which becomes planar.
(Additionally, this hydrogen bonding results in the tilting of the oxygen molecule, resulting in a Fe–O–O bond angle of around 120° that avoids the formation of Fe–O–Fe or Fe–O
2–Fe bridges that would lead to electron transfer, the oxidation of Fe
2+ to Fe
3+, and the destruction of hemoglobin.) This results in a movement of all the protein chains that leads to the other subunits of hemoglobin changing shape to a form with larger oxygen affinity. Thus, when deoxyhemoglobin takes up oxygen, its affinity for more oxygen increases, and vice versa.
Myoglobin, on the other hand, contains only one heme group and hence this cooperative effect cannot occur. Thus, while hemoglobin is almost saturated with oxygen in the high partial pressures of oxygen found in the lungs, its affinity for oxygen is much lower than that of myoglobin, which oxygenates even at low partial pressures of oxygen found in muscle tissue.
As described by the Bohr effect (named after Christian Bohr, the father of Niels Bohr), the oxygen affinity of hemoglobin diminishes in the presence of carbon dioxide.
![Carboxyhemoglobin from 1AJ9](https://upload.wikimedia.org/wikipedia/commons/3/36/Carboxyhemoglobin_from_1AJ9.png)
Carbon monoxide and phosphorus trifluoride are poisonous to humans because they bind to hemoglobin similarly to oxygen, but with much more strength, so that oxygen can no longer be transported throughout the body. Hemoglobin bound to carbon monoxide is known as carboxyhemoglobin. This effect also plays a minor role in the toxicity of cyanide, but there the major effect is by far its interference with the proper functioning of the electron transport protein cytochrome a.
The cytochrome proteins also involve heme groups and are involved in the metabolic oxidation of glucose by oxygen. The sixth coordination site is then occupied by either another imidazole nitrogen or a methionine sulfur, so that these proteins are largely inert to oxygen – with the exception of cytochrome a, which bonds directly to oxygen and thus is very easily poisoned by cyanide.
Here, the electron transfer takes place as the iron remains in low spin but changes between the +2 and +3 oxidation states. Since the reduction potential of each step is slightly greater than the previous one, the energy is released step-by-step and can thus be stored in adenosine triphosphate. Cytochrome a is slightly distinct, as it occurs at the mitochondrial membrane, binds directly to oxygen, and transports protons as well as electrons, as follows:
:4 Cytc
2+ + O
2 + 8H → 4 Cytc
3+ + 2 H
2O + 4H
Although the heme proteins are the most important class of iron-containing proteins, the iron–sulfur proteins are also very important, being involved in electron transfer, which is possible since iron can exist stably in either the +2 or +3 oxidation states. These have one, two, four, or eight iron atoms that are each approximately tetrahedrally coordinated to four sulfur atoms; because of this tetrahedral coordination, they always have high-spin iron. The simplest of such compounds is rubredoxin, which has only one iron atom coordinated to four sulfur atoms from cysteine residues in the surrounding peptide chains. Another important class of iron–sulfur proteins is the ferredoxins, which have multiple iron atoms. Transferrin does not belong to either of these classes.
The ability of sea mussels to maintain their grip on rocks in the ocean is facilitated by their use of organometallic chemistry, organometallic iron-based bonds in their protein-rich cuticles. Based on synthetic replicas, the presence of iron in these structures increased elastic modulus 770 times, tensile strength 58 times, and toughness 92 times. The amount of stress required to permanently damage them increased 76 times.
Nutrition
Diet
Iron is pervasive, but particularly rich sources of dietary iron include red meat, oysters, lentils, beans, poultry, fish, leaf vegetables, watercress, tofu, chickpeas, black-eyed peas, and blackstrap molasses.
Bread and breakfast cereals are sometimes specifically fortified with iron.
Iron provided by dietary supplements is often found as iron(II) fumarate, although iron(II) sulfate is cheaper and is absorbed equally well.
Elemental iron, or reduced iron, despite being absorbed at only one-third to two-thirds the efficiency (relative to iron sulfate), is often added to foods such as breakfast cereals or enriched wheat flour. Iron is most available to the body when Chelation, chelated to amino acids
and is also available for use as a common iron supplement. Glycine, the least expensive amino acid, is most often used to produce iron glycinate supplements.
Dietary recommendations
The U.S. Institute of Medicine (IOM) updated Estimated Average Requirements (EARs) and Recommended Dietary Allowances (RDAs) for iron in 2001.
The current EAR for iron for women ages 14–18 is 7.9 mg/day, 8.1 for ages 19–50 and 5.0 thereafter (post menopause). For men the EAR is 6.0 mg/day for ages 19 and up. The RDA is 15.0 mg/day for women ages 15–18, 18.0 for 19–50 and 8.0 thereafter. For men, 8.0 mg/day for ages 19 and up. RDAs are higher than EARs so as to identify amounts that will cover people with higher than average requirements. RDA for pregnancy is 27 mg/day and, for lactation, 9 mg/day.
For children ages 1–3 years 7 mg/day, 10 for ages 4–8 and 8 for ages 9–13. As for safety, the IOM also sets Tolerable upper intake levels (ULs) for vitamins and minerals when evidence is sufficient. In the case of iron the UL is set at 45 mg/day. Collectively the EARs, RDAs and ULs are referred to as Dietary Reference Intakes.
The European Food Safety Authority (EFSA) refers to the collective set of information as Dietary Reference Values, with Population Reference Intake (PRI) instead of RDA, and Average Requirement instead of EAR. AI and UL defined the same as in United States. For women the PRI is 13 mg/day ages 15–17 years, 16 mg/day for women ages 18 and up who are premenopausal and 11 mg/day postmenopausal. For pregnancy and lactation, 16 mg/day. For men the PRI is 11 mg/day ages 15 and older. For children ages 1 to 14 the PRI increases from 7 to 11 mg/day. The PRIs are higher than the U.S. RDAs, with the exception of pregnancy. The EFSA reviewed the same safety question did not establish a UL.
Infants may require iron supplements if they are bottle-fed cow's milk. Frequent Blood donation, blood donors are at risk of low iron levels and are often advised to supplement their iron intake.
For U.S. food and dietary supplement labeling purposes the amount in a serving is expressed as a percent of Daily Value (%DV). For iron labeling purposes 100% of the Daily Value was 18 mg, and remained unchanged at 18 mg.
A table of the old and new adult daily values is provided at Reference Daily Intake.
Deficiency
Iron deficiency is the most common nutritional deficiency in the world.
When loss of iron is not adequately compensated by adequate dietary iron intake, a state of latent iron deficiency occurs, which over time leads to iron-deficiency anemia if left untreated, which is characterised by an insufficient number of red blood cells and an insufficient amount of hemoglobin. Children, pre-menopausal women (women of child-bearing age), and people with poor diet are most susceptible to the disease. Most cases of iron-deficiency anemia are mild, but if not treated can cause problems like fast or irregular heartbeat, complications during pregnancy, and delayed growth in infants and children.
Excess
Human iron metabolism, Iron uptake is tightly regulated by the human body, which has no regulated physiological means of excreting iron. Only small amounts of iron are lost daily due to mucosal and skin epithelial cell sloughing, so control of iron levels is primarily accomplished by regulating uptake. Regulation of iron uptake is impaired in some people as a result of a Genetic disorder, genetic defect that maps to the HLA-H gene region on chromosome 6 and leads to abnormally low levels of hepcidin, a key regulator of the entry of iron into the circulatory system in mammals.
In these people, excessive iron intake can result in iron overload disorders, known medically as hemochromatosis.
Many people have an undiagnosed genetic susceptibility to iron overload, and are not aware of a family history of the problem. For this reason, people should not take iron supplements unless they suffer from iron deficiency (medicine), iron deficiency and have consulted a doctor. Hemochromatosis is estimated to be the cause of 0.3 to 0.8% of all metabolic diseases of Caucasians.
Overdoses of ingested iron can cause excessive levels of free iron in the blood. High blood levels of free ferrous iron react with peroxides to produce highly reactive free radicals that can damage DNA, proteins, lipids, and other cellular components. Iron toxicity occurs when the cell contains free iron, which generally occurs when iron levels exceed the availability of transferrin to bind the iron. Damage to the cells of the Human gastrointestinal tract, gastrointestinal tract can also prevent them from regulating iron absorption, leading to further increases in blood levels. Iron typically damages cells in the heart, liver and elsewhere, causing adverse effects that include coma, metabolic acidosis, Shock (circulatory), shock, liver failure, coagulopathy, long-term organ damage, and even death.
Humans experience iron toxicity when the iron exceeds 20 milligrams for every kilogram of body mass; 60 milligrams per kilogram is considered a lethal dose.
Overconsumption of iron, often the result of children eating large quantities of ferrous sulfate tablets intended for adult consumption, is one of the most common toxicological causes of death in children under six.
The Dietary Reference Intake (DRI) sets the Tolerable Upper Intake Level (UL) for adults at 45 mg/day. For children under fourteen years old the UL is 40 mg/day.
The medical management of iron toxicity is complicated, and can include use of a specific chelation, chelating agent called deferoxamine to bind and expel excess iron from the body.
ADHD
Some research has suggested that low Thalamus, thalamic iron levels may play a role in the pathophysiology of Attention deficit hyperactivity disorder, ADHD. Some researchers have found that iron supplementation can be effective especially in the Attention deficit hyperactivity disorder predominantly inattentive, inattentive subtype of the disorder. One study also showed that iron may be able to decrease the risk of cardiovascular events during treatment with ADHD drugs.
Some researchers in the 2000s suggested a link between low levels of iron in the blood and ADHD. A 2012 study found no such correlation.
Cancer
The role of iron in cancer defense can be described as a "double-edged sword" because of its pervasive presence in non-pathological processes. People having chemotherapy may develop iron deficiency and anemia, for which Intravenous iron infusion, intravenous iron therapy is used to restore iron levels.
Iron overload, which may occur from high consumption of red meat,
may initiate tumor growth and increase susceptibility to cancer onset,
particularly for colorectal cancer.
Marine systems
Iron plays an essential role in marine systems and can act as a limiting nutrient for planktonic activity. Because of this, too much of a decrease in iron may lead to a decrease in growth rates in phytoplanktonic organisms such as diatoms. Iron can also be oxidized by marine microbes under conditions that are high in iron and low in oxygen.
Iron can enter marine systems through adjoining rivers and directly from the atmosphere. Once iron enters the ocean, it can be distributed throughout the water column through ocean mixing and through recycling on the cellular level. In the arctic, sea ice plays a major role in the store and distribution of iron in the ocean, depleting oceanic iron as it freezes in the winter and releasing it back into the water when thawing occurs in the summer. The iron cycle can fluctuate the forms of iron from aqueous to particle forms altering the availability of iron to primary producers. Increased light and warmth increases the amount of iron that is in forms that are usable by primary producers.
[Tagliabue, A., Bopp, L., Aumont,O., & Arrigo, K.R. (2009). Influence of light and temperature on the marine iron cycle: From theoretical to global modeling. Global Biogeochemical Cycles, 23.
]
See also
* El Mutún in Bolivia, where 10% of the world's accessible iron ore is located
* Iron and steel industry
* Iron cycle
* Iron nanoparticle
* Iron–platinum nanoparticle
* Iron fertilization – proposed fertilization of oceans to stimulate phytoplankton growth
* Iron-oxidizing bacteria
* List of countries by iron production
* Pelletizing, Pelletising – process of creation of iron ore pellets
* Rustproof iron
* Steel
References
Bibliography
*
*
Further reading
* H.R. Schubert, ''History of the British Iron and Steel Industry ... to 1775 AD'' (Routledge, London, 1957)
* R.F. Tylecote, ''History of Metallurgy'' (Institute of Materials, London 1992).
* R.F. Tylecote, "Iron in the Industrial Revolution" in J. Day and R.F. Tylecote, ''The Industrial Revolution in Metals'' (Institute of Materials 1991), 200–60.
External links
It's Elemental – Ironat ''The Periodic Table of Videos'' (University of Nottingham)
Metallurgy for the non-Metallurgistby J.B. Calvert
{{good article
Iron,
Chemical elements
Transition metals
Ferromagnetic materials
Dietary minerals
Cubic minerals
Minerals in space group 225
Minerals in space group 229
Building materials
Pyrotechnic fuels
Chemical elements with body-centered cubic structure
Native element minerals