Maghemite

Not to be confused with Magnesite or Magnetite.

Maghemite
Maghemite from Gancedo, Chaco Province, Argentina
General
Category	Oxide minerals
Formula (repeating unit)	γ-Fe2O3
Strunz classification	04.BB.15
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
Identification
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
Cleavage	None
Fracture	Subconchoidal
Mohs scale hardness	5
Luster	Dull
Streak	Brown
Diaphaneity	Opaque, transparent in thin fragments
Specific gravity	4.860 (calculated)
Optical properties	Isotropic
Other characteristics	Strongly magnetic
References

Maghemite (Fe₂O₃, γ-Fe₂O₃) is a member of the family of iron oxides. It has the same spinel ferrite structure as magnetite and is also ferrimagnetic.

Maghemite can be considered as an Fe(II)-deficient magnetite with formula $(\mathrm{Fe}^\mathrm{III}_8)_A[\mathrm{Fe}^{\mathrm{III}}_{40/3}\square_{8/3}]_B\mathrm{O}_{32}$ where $\square$ 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.

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.

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 also used in biomedicine, because they are biocompatible and non-toxic to humans, while their magnetism allows remote manipulation with external fields.