Spinel group: Difference between revisions
Eric Kvaalen (talk | contribs) The space group for spinel is usually given as Fd{{overline|3}}m, but it is more accurate to give it as F{{overline|4}}3m. |
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**[[Magnesiochromite]]: MgCr<sub>2</sub>O<sub>4</sub> |
**[[Magnesiochromite]]: MgCr<sub>2</sub>O<sub>4</sub> |
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**[[Zincochromite]]: ZnCr<sub>2</sub>O<sub>4</sub> |
**[[Zincochromite]]: ZnCr<sub>2</sub>O<sub>4</sub> |
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*Cobalt spinels: |
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**[[Manganesecobaltite]]: Mn<sub>1.5</sub>Co<sub>1.5</sub>O<sub>4</sub><ref>{{cite book |title=American Elements, Manganese Cobalt Oxide, Spinel Powder |url=https://www.americanelements.com/manganese-cobalt-oxide-spinel-powder}}</ref> |
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*Vanadium spinels: |
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**[[Coulsonite]]: FeV<sub>2</sub>O<sub>4</sub> |
**[[Coulsonite]]: FeV<sub>2</sub>O<sub>4</sub> |
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**[[Magnesiocoulsonite]]: MgV<sub>2</sub>O<sub>4</sub> |
**[[Magnesiocoulsonite]]: MgV<sub>2</sub>O<sub>4</sub> |
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**[[Ringwoodite]]: (Mg,Fe)<sub>2</sub>SiO<sub>4</sub>, an abundant [[olivine]] [[polymorphism (materials science)|polymorph]] within the [[Earth's mantle]] from about 520 to 660 km depth, and a rare mineral in meteorites |
**[[Ringwoodite]]: (Mg,Fe)<sub>2</sub>SiO<sub>4</sub>, an abundant [[olivine]] [[polymorphism (materials science)|polymorph]] within the [[Earth's mantle]] from about 520 to 660 km depth, and a rare mineral in meteorites |
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Revision as of 12:50, 2 December 2019
The spinels are any of a class of minerals of general formulation AB
2X
4 which crystallise in the cubic (isometric) crystal system, with the X anions (typically chalcogens, like oxygen and sulfur) arranged in a cubic close-packed lattice and the cations A and B occupying some or all of the octahedral and tetrahedral sites in the lattice.[1][2] Although the charges of A and B in the prototypical spinel structure are +2 and +3, respectively (A2+
B3+
2X2−
4), other combinations incorporating divalent, trivalent, or tetravalent cations, including magnesium, zinc, iron, manganese, aluminium, chromium, titanium, and silicon, are also possible. The anion is normally oxygen; when other chalcogenides constitute the anion sublattice the structure is referred to as a thiospinel.
A and B can also be the same metal with different valences, as is the case with magnetite, Fe3O4 (as Fe2+
Fe3+
2O2−
4), which is the most abundant member of the spinel group.[3] Spinels are grouped in series by the B cation.
Though spinels are often referred to as rubies, as in the Black Prince Ruby, the ruby is not a spinel.
Spinel group members
Members of the spinel group include:[4]
- Aluminium spinels:
- Iron spinels:
- Cuprospinel: CuFe2O4
- Franklinite: (Fe,Mn,Zn)(Fe,Mn)2O4
- Jacobsite: MnFe2O4
- Magnesioferrite: MgFe2O4
- Magnetite: FeFe2O4, where one Fe is +2 and two Fe's are +3, respectively.
- Trevorite: NiFe2O4
- Ulvöspinel: TiFe2O4
- Zinc ferrite: (Zn, Fe) Fe2O4
- Chromium spinels:
- Chromite: FeCr2O4
- Magnesiochromite: MgCr2O4
- Zincochromite: ZnCr2O4
- Cobalt spinels:
- Manganesecobaltite: Mn1.5Co1.5O4[5]
- Vanadium spinels:
- Coulsonite: FeV2O4
- Magnesiocoulsonite: MgV2O4
- Others with the spinel structure:
- Ringwoodite: (Mg,Fe)2SiO4, an abundant olivine polymorph within the Earth's mantle from about 520 to 660 km depth, and a rare mineral in meteorites
There are many more compounds with a spinel structure, e.g. the thiospinels and selenospinels, that can be synthesized in the lab or in some cases occur as minerals.
The heterogeneity of spinel group members varies based on composition with ferrous and magnesium based members varying greatly as in solid solution, which requires similarly sized cations. However, ferric and aluminium based spinels are almost entirely homogeneous due to their large size difference.[6]
The spinel structure
The space group for spinel is usually given as Fd3m, but it is more accurate to give it as F43m.[7][8]
Normal spinel structures are usually cubic close-packed oxides with eight tetrahedral and four octahedral sites per formula unit. The tetrahedral spaces are smaller than the octahedral spaces. B3+ ions occupy half the octahedral holes, while A2+ ions occupy one-eighth of the tetrahedral holes. Mineral spinel MgAl2O4 has a normal spinel structure.
Inverse spinel structures have a different cation distribution in that all of the A cations and half of the B cations occupy octahedral sites, while the other half of the B cations occupy tetrahedral sites. An example of an inverse spinel is Fe3O4, if the Fe2+ (A2+) ions are d6 high-spin and the Fe3+ (B3+) ions are d5 high-spin.
In addition, intermediate cases exist where the cation distribution can be described as (A1−xBx)[Ax⁄2B1−x⁄2]2O4, where parentheses () and brackets [] are used to denote tetrahedral and octahedral sites, respectively. The so-called inversion degree, x, adopts values between 0 (normal) and 1 (inverse), and is equal to 2⁄3 for a completely random cation distribution.
The cation distribution in spinel structures are related to the crystal field stabilization energies (CFSE) of the constituent transition metals. Some ions may have a distinct preference for the octahedral site depending on the d-electron count. If the A2+ ions have a strong preference for the octahedral site, they will displace half of the B3+ ions from the octahedral sites to tetrahedral sites. Similarly, if the B3+ ions have a low or zero octahedral site stabilization energy (OSSE), then they will occupy tetrahedral sites, leaving octahedral sites for the A2+ ions.
Burdett and co-workers proposed an alternative treatment of the problem of spinel inversion, using the relative sizes of the s and p atomic orbitals of the two types of atom to determine their site preferences.[9] This is because the dominant stabilizing interaction in the solids is not the crystal field stabilization energy generated by the interaction of the ligands with the d electrons, but the σ-type interactions between the metal cations and the oxide anions. This rationale can explain anomalies in the spinel structures that crystal-field theory cannot, such as the marked preference of Al3+ cations for octahedral sites or of Zn2+ for tetrahedral sites, which crystal field theory would predict neither has a site preference. Only in cases where this size-based approach indicates no preference for one structure over another do crystal field effects make any difference; in effect they are just a small perturbation that can sometimes affect the relative preferences, but which often do not.
See also
- Taaffeite, which has the same empirical formula as a spinel, but the chemical formula is four times as large.
- Musgravite, another and even rarer "multi-spinel".
Further reading
- Biagoni, C.; Pasero, M (2014). "The systematics of the spinel-type minerals: An overview". American Mineralogist. 99: 1254–1264.
References
- ^ Robert J. Naumann: Introduction to the Physics and Chemistry of Materials CRC Press, 2008, ISBN 978-1-4200-6134-5. Retrieved 15 April 2018.
- ^ H-J Meyer: Festkörperchemie in: H-J Meyer (ed.), Riedel Moderne Anorganische Chemie, Walter de Gruyter, 2012, ISBN 978-3-11-024900-2. Retrieved 15 April 2018.
- ^ Ernst, W. G. (1969). Earth Materials (Print ed.). Englewood Cliffs, NJ: Prentice-Hall. p. 58.
- ^ Spinel group at Mindat
- ^ American Elements, Manganese Cobalt Oxide, Spinel Powder.
- ^ Ernst, W. G. (1969). Earth Materials (Print ed.). Englewood Cliffs, NJ: Prentice-Hall. p. 59.
- ^ Robert John Lancashire. "Normal Spinels". CHEM2101 (C 21J) Inorganic Chemistry - Chemistry of Transition Metal Complexes. University of the West Indies.
- ^ N. W. Grimes; et al. (Apr 8, 1983). "New Symmetry and Structure for Spinel". Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences. doi:10.1098/rspa.1983.0039.
- ^ J.K. Burdett, G.L. Price and S.L. Price (1982). "Role of the crystal-field theory in determining the structures of spinels". J. Am. Chem. Soc. 104: 92–95. doi:10.1021/ja00365a019.