Maghemite (Fe<sub>2</sub>O<sub>3</sub>, γ-Fe<sub>2</sub>O<sub>3</sub>) is a member of the family of iron oxides. It has the same formula as hematite, but the same spinel ferrite structure as magnetite () and is also ferrimagnetic. It is sometimes spelled as "maghaemite".
Maghemite can be considered as an Fe(II)-deficient magnetite with formula
<math chem>\left(\ce{Fe^{III}8}\right)_A\left[\ce{Fe^{III}_{40/3}\square_{8/3\right]_B\ce{O32}</math>
where
<math>\square</math> represents a vacancy, A indicates tetrahedral and B octahedral positioning.
Occurrence
Maghemite forms by weathering or low-temperature oxidation of spinels containing iron(II) such as magnetite or titanomagnetite. Maghemite can also form through dehydration and transformation of certain iron oxyhydroxide minerals, such as lepidocrocite and ferrihydrite. It occurs as widespread brown or yellow pigment in terrestrial sediments and soils. It is associated with magnetite, ilmenite, anatase, pyrite, marcasite, lepidocrocite and goethite. It has isometric crystals. 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.
Electronic structure
Maghemite is a semiconductor with a bandgap of ca. 2 eV, although the precise value of the gap depends on the electron spin.
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 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.