Maghemite

Template:Short description Template:Distinguish Template:Infobox mineral Maghemite (Fe₂O₃, γ-Fe₂O₃) is a member of the family of iron oxides. It has the same formula as hematite, but the same spinel ferrite structure as magnetite (Template:Chem2) 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> <ref>Cornell, R. M. and Schwertmann, Udo (2003) The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses. Wiley-VCH. p. 32. Template:ISBN.</ref> 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.<ref name=Handbook/> 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.<ref name=Webmin/> The name alludes to somewhat intermediate character between magnetite and hematite. It can appear blue with a grey shade, white, or brown.<ref>Gaines, Richard V.; Skinner, H. Catherine W.; Foord, Eugene E.; Mason, Brian and Rosenzweig, Abraham (1997) Dana's new mineralogy, John Wiley & Sons. pp. 229-230. Template:ISBN.</ref> It has isometric crystals.<ref name=Mindat/> Maghemite is formed by the topotactic oxidation of magnetite.

Cation distribution

There is experimental<ref>Template:Cite journal</ref> and theoretical<ref name=jpcm2011>Template:Cite journal</ref> 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,<ref>Template:Cite journal</ref> although the precise value of the gap depends on the electron spin.<ref name=jpcm2011/>

Applications

Maghemite exhibits ferrimagnetic 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.<ref>Template:Cite journal</ref>

Maghemite nanoparticles are used in biomedicine, because they are biocompatible and non-toxic to humans, while their magnetism allows remote manipulation with external fields.<ref>Template:Cite journal</ref>

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref>