Maghemite
| Maghemite | |
|---|---|
|
Maghemite from Gancedo, Chaco Province, Argentina | |
| General | |
| Category | Oxide minerals |
| Formula (repeating unit) | γ-Fe2O3 |
| Strunz classification | 04.BB.15 |
| Crystal system | Cubic with a tetragonal supercell |
| 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 symmetry |
Isometric tetartoidal H-M symbol: (2 3) Space group: P 213 |
| 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 | [1][2][3] |
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}](../I/m/77070285986e98a0f745fe0b1e7782e9.png)
[4] where
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.[1]
Maghemite was named in 1927 for an occurrence at the Iron Mountain mine, northwest of Redding, Shasta County, California.[3] The name alludes to somewhat intermediate character between MAGnetite and HEMatite. It can appear blue with a grey shade, white, or brown.[5] It has isometric crystals.[2] Maghemite is formed by the topotactic oxidation of magnetite.
Cation distribution
There is experimental[6] and theoretical[7] 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,[8] although the precise value of the gap depends on the electron spin.[7]
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.[9]
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.[10]
References
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Wikimedia Commons has media related to Maghemite. |
- 1 2 Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W. and Nichols, Monte C., ed. (1997). "Maghemite". Handbook of Mineralogy (PDF). III (Halides, Hydroxides, Oxides). Chantilly, VA, US: Mineralogical Society of America. ISBN 0962209732.
- 1 2 Maghemite. Mindat
- 1 2 Maghemite. Webmineral
- ↑ Cornell, R. M. and Schwertmann, Udo (2003) The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses. Wiley-VCH. p. 32. ISBN 3527302743.
- ↑ 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. ISBN 0471193100.
- ↑ Greaves, C. (1983). "A powder neutron diffraction investigation of vacancy ordering and covalence in γ-Fe2O3". J. Solid State Chem. 49 (3): 325. doi:10.1016/S0022-4596(83)80010-3.
- 1 2 Grau-Crespo, Ricardo; Al-Baitai, Asmaa Y; Saadoune, Iman; De Leeuw, Nora H (2010). "Vacancy ordering and electronic structure of γ-Fe2O3 (maghemite): a theoretical investigation". Journal of Physics: Condensed Matter 22 (25): 255401. doi:10.1088/0953-8984/22/25/255401.
- ↑ Litter, M. I. and Blesa, M. A. (1992). "Photodissolution of iron oxides. IV. A comparative study on the photodissolution of hematite, magnetite, and maghemite in EDTA media". Can. J. Chem. 70 (9): 2502. doi:10.1139/v92-316.
- ↑ Dronskowski, R. (2010). "The little maghemite story: A classic functional material". ChemInform 32 (25): no. doi:10.1002/chin.200125209.
- ↑ Pankhurst, Q A; Connolly, J; Jones, S K; Dobson, J (2003). "Applications of magnetic nanoparticles in biomedicine". Journal of Physics D: Applied Physics 36 (13): R167. doi:10.1088/0022-3727/36/13/201.