Maghemite

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 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. 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.

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.

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.

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.

References

References

1. Warr, L.N.. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine.
2. [https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Maghemite Mineralienatlas]
3. (1997). "Handbook of Mineralogy". Mineralogical Society of America.
4. [http://www.mindat.org/min-2533.html Maghemite]. Mindat
5. [http://webmineral.com/data/Maghemite.shtml Maghemite]. Webmineral
6. Cornell, R. M. and Schwertmann, Udo (2003) ''The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses''. Wiley-VCH. p. 32. {{ISBN. 3527302743.
7. The name alludes to somewhat intermediate character between magnetite and hematite. It can appear blue with a grey shade, white, or brown.Gaines, Richard V.; [[H. Catherine W. Skinner. Skinner, H. Catherine W.]]; Foord, Eugene E.; Mason, Brian and Rosenzweig, Abraham (1997) ''Dana's new mineralogy'', John Wiley & Sons. pp. 229-230. {{ISBN. 0471193100.
8. Greaves, C.. (1983). "A powder neutron diffraction investigation of vacancy ordering and covalence in γ-Fe₂O₃". J. Solid State Chem..
9. (2010). "Vacancy ordering and electronic structure of γ-Fe₂O₃ (maghemite): a theoretical investigation". Journal of Physics: Condensed Matter.
10. (1992). "Photodissolution of iron oxides. IV. A comparative study on the photodissolution of hematite, magnetite, and maghemite in EDTA media". Can. J. Chem..
11. Dronskowski, R.. (2010). "The little maghemite story: A classic functional material". ChemInform.
12. (2003). "Applications of magnetic nanoparticles in biomedicine". Journal of Physics D: Applied Physics.
13. (15 December 2022). "Inhaled metal Tube dust can enter bloodstream, study finds".
14. (2022-12-15). "Magnetic and microscopic investigation of airborne iron oxide nanoparticles in the London Underground". Scientific Reports.