Maghemite (Fe₂O₃, γ-Fe₂O₃) is a member of the family of

iron oxide An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron ...

s. It has the same

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

, but the same spinel ferrite structure as

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

() and is also

ferrimagnetic 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 wh ...

. It is sometimes spelled as "maghaemite". ''Maghemite'' can be considered as an Fe(II)-deficient magnetite with formula

B\ce

where

\square

represents a vacancy, A indicates tetrahedral and B octahedral positioning.

Occurrence

Maghemite forms by weathering or low-temperature

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

of spinels containing iron(II) such as

or titanomagnetite. Maghemite can also form through dehydration and transformation of certain iron oxyhydroxide minerals, such as lepidocrocite and

ferrihydrite Ferrihydrite (Fh) is a widespread hydrous ferric oxyhydroxide mineral at the Earth's surface, and a likely constituent in extraterrestrial materials. It forms in several types of environments, from freshwater to marine systems, aquifers to hydro ...

. It occurs as widespread brown or yellow

pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...

in terrestrial

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

s and soils. It is associated with magnetite, ilmenite,

anatase Anatase is a metastable mineral form of titanium dioxide (TiO2) with a Tetragonal crystal system, tetragonal crystal structure. Although colorless or white when pure, anatase in nature is usually a black solid due to impurities. Three other Pol ...

, pyrite, marcasite, lepidocrocite and

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the α- polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

. It is known to also form in areas that have been subjected to bushfires (particularly in the Leonora area of Western Australia) magnetising iron minerals. Maghemite was named in 1927 for an occurrence at the Iron Mountain Mine, northwest of Redding, Shasta County, California. The name alludes to somewhat intermediate character between magnetite and hematite. It can appear blue with a grey shade, white, or brown. It has isometric crystals. Maghemite is formed by the topotactic oxidation of magnetite.

Cation distribution

There is experimental and theoretical evidence that Fe(III) cations and vacancies tend to be ordered in the octahedral sites, in a way that maximizes the homogeneity of the distribution and therefore minimizes the electrostatic energy of the

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

Electronic structure

Maghemite is a

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

with a

bandgap In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to the ...

of ca. 2 eV, although the precise value of the gap depends on the electron spin.

Applications

Maghemite exhibits

ordering with a high Néel temperature (~950 K), which together with its low cost and chemical stability led to its wide application as a magnetic pigment in electronic recording media since the 1940s. Maghemite nanoparticles are used in biomedicine, because they are biocompatible and non-toxic to humans, while their magnetism allows remote manipulation with external fields.

As pollutant

It was found in 2022 that high levels of maghemite particles small enough to enter the bloodstream if inhaled, some as small as five nanometres, were present in the

London Underground The London Underground (also known simply as the Underground or as the Tube) is a rapid transit system serving Greater London and some parts of the adjacent home counties of Buckinghamshire, Essex and Hertfordshire in England. The Undergro ...

transport system. The presence of the particles indicated that they are suspended for long periods due to poor ventilation, particularly on platforms. The health implications presented by the particles were not investigated.

References

{{reflist Iron(III) minerals Oxide minerals Ilmenite group Magnetic minerals Cubic minerals Minerals in space group 212 or 213