Maghemite from Gancedo, Chaco Province, Argentina
|Crystal symmetry||Isometric tetartoidal
H-M symbol: (2 3)
Space group: P 213
|Unit cell||a = 8.33 Å; Z = 8 or a = 8.35 Å c = 24.99 Å; Z = 32 for tetragonal supercell|
|Color||Brown, bluish black; brown to yellow in transmitted light; white to bluish gray in reflected light.|
|Crystal habit||Rarely as minute octahedral crystals, or acicular overgrowths; commonly as coatings on or replacements of magnetite; massive.|
|Crystal system||Cubic with a tetragonal supercell|
|Mohs scale hardness||5|
|Diaphaneity||Opaque, transparent in thin fragments|
|Specific gravity||4.860 (calculated)|
|Other characteristics||Strongly magnetic|
Maghemite can be considered as an Fe(II)-deficient magnetite with formula  where represents a vacancy, indicates tetrahedral positioning and octahedral.
Maghemite forms by weathering or low-temperature oxidation of spinels containing iron(II) such as magnetite or titanian magnetite. It occurs as widespread yellow pigment in terrestrial sediments and soils. It is associated with magnetite, ilmenite, anatase, pyrite, marcasite, lepidocrocite and goethite.
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 is 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.
Electronic structure 
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.
See also 
- Handbook of Mineralogy
- Maghemite on Mindat
- Maghemite on Webmineral
- R. M. Cornell and Udo Schwertmann: The iron oxides: structure, properties, reactions, occurrences, and uses, pp 32. Wiley-VCH, 2003
- Richard V. Gaines, H. Catherine W. Skinner, Eugene E. Foord, Brian Mason, and Abraham Rosenzweig: "Dana's new mineralogy", pp. 229-230. John Wiley & Sons, 1997
- C. Greaves, J. Solid State Chem. 49 325 (1983)
- R. Grau-Crespo, A. Y. Al-Baitai, I. Saaudoune, N.H. de Leeuw, "Vacancy ordering and electronic structure of γ -Fe2O3 (maghemite): a theoretical investigation" J. Phys. Condens. Matter 22, 255401 (2010) http://iopscience.iop.org/0953-8984/22/25/255401
- M. I. Litter and M. A. Blesa Can. J. Chem. 70, 2502 (1992)
- R. Dronskowski, "The little maghemite story: A classic functional material" Adv. Funct. Mater. 11, 27 (2001) http://onlinelibrary.wiley.com/doi/10.1002/chin.200125209/abstract
- Q. A. Pankhurst, J. Connolly, S. K. Jones and J. Dobson, "Applications of magnetic nanoparticles in biomedicine" J. Phys. D: Appl. Phys. 36, R167 (2003)