Spinel group

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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.

Contents

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.

The group is named for spinel (MgAl
2O
4), which was once known as "spinel ruby". [4] (Today the term ruby is used only for corundum.)

Spinel group members

Members of the spinel group include: [5]

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. [9]

The spinel structure

Crystal structure of spinel Spinel.GIF
Crystal structure of spinel

The space group for a spinel group mineral may be Fd3m (the same as for diamond), but in some cases (such as spinel itself, MgAl
2O
4, beyond 452.6 K [10] ) it is actually the tetrahedral F43m. [11] [12] [13] [14]

Normal spinel structures have oxygen ions closely approximating a cubic close-packed latice with eight tetrahedral and four octahedral sites per formula unit (but eight times as many per unit cell). The tetrahedral spaces are smaller than the octahedral spaces. B ions occupy half the octahedral holes, while A ions occupy one-eighth of the tetrahedral holes. [15] The mineral spinel MgAl2O4 has a normal spinel structure.

In a normal spinel structure, the ions are in the following positions, where i, j, and k are arbitrary integers and δ, ε, and ζ are small real numbers (note that the unit cell can be chosen differently, giving different coordinates): [16] 

X: (1/4-δ,   δ,     δ  ) + ((i+j)/2, (j+k)/2, (i+k)/2) ( δ,     1/4-δ,  δ  ) + ((i+j)/2, (j+k)/2, (i+k)/2) ( δ,      δ,   1/4-δ) + ((i+j)/2, (j+k)/2, (i+k)/2) (1/4-δ, 1/4-δ, 1/4-δ) + ((i+j)/2, (j+k)/2, (i+k)/2) (3/4+ε, 1/2-ε, 1/2-ε) + ((i+j)/2, (j+k)/2, (i+k)/2) (1-ε,   1/4+ε, 1/2-ε) + ((i+j)/2, (j+k)/2, (i+k)/2) (1-ε,   1/2-ε, 1/4+ε) + ((i+j)/2, (j+k)/2, (i+k)/2) (3/4+ε, 1/4+ε, 1/4+ε) + ((i+j)/2, (j+k)/2, (i+k)/2) A: (1/8, 1/8, 1/8) + ((i+j)/2, (j+k)/2, (i+k)/2) (7/8, 3/8, 3/8) + ((i+j)/2, (j+k)/2, (i+k)/2) B: (1/2+ζ,   ζ,     ζ  ) + ((i+j)/2, (j+k)/2, (i+k)/2) (1/2+ζ, 1/4-ζ, 1/4-ζ) + ((i+j)/2, (j+k)/2, (i+k)/2) (3/4-ζ, 1/4-ζ,   ζ  ) + ((i+j)/2, (j+k)/2, (i+k)/2) (3/4-ζ,   ζ,   1/4-ζ) + ((i+j)/2, (j+k)/2, (i+k)/2)

The first four X positions form a tetrahedron around the first A position, and the last four form one around the second A position. When the space group is Fd3m then δ=ε and ζ=0. In this case, a three-fold rotoinversion with axis in the 111 direction is centred on the point (0, 0, 0) (where there is no ion) and can also be centred on the B ion at (1/2, 1/2, 1/2), and in fact every B ion is the centre of a three-fold rotoinversion (point group D3d). Under this space group the two A positions are equivalent. If the space group is F43m then the three-fold rotoinversions become simple three-fold rotations (point group C3v) because the inversion disappears, and the two A positions are no longer equivalent.

Every ion is on at least three mirror planes and at least one three-fold rotation axis. The structure has tetrahedral symmetry around each A ion, and the A ions are arranged just like the carbon atoms in diamond. There are another eight tetrahedral sites per unit cell that are empty, each one surrounded by a tetrahedron of B as well as a tetrahedron of X ions.

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. [17] 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.

Common uses in industry and technology

Spinels commonly form in high temperature processes. Either native oxide scales of metals, [18] or intentional deposition of spinel coatings [19] can be used to protect base metals from oxidation or corrosion. The presence of spinels may hereby serve as thin (few micrometer thick) functional layers, that prevent the diffusion of oxygen (or other atmospheric) ions or specific metal ions such as chromium, which otherwise exhibits a fast diffusion process at high temperatures.

Further reading

Related Research Articles

<span class="mw-page-title-main">Spinel</span> Mineral or gemstone

Spinel is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula MgAl
2O
4 in the cubic crystal system. Its name comes from the Latin word spinella, a diminutive form of spine, in reference to its pointed crystals.

<span class="mw-page-title-main">Olivine</span> Magnesium iron silicate solid solution series mineral

The mineral olivine is a magnesium iron silicate with the chemical formula (Mg,Fe)2SiO4. It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickly on the surface. For this reason, olivine has been proposed as a good candidate for accelerated weathering to sequester carbon dioxide from the Earth's oceans and atmosphere, as part of climate change mitigation. Olivine also has many other historical uses, such as the gemstone peridot, as well as industrial applications like metalworking processes.

<span class="mw-page-title-main">Pyroxene</span> Group of inosilicate minerals with single chains of silica tetrahedra

The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula XY(Si,Al)2O6, where X represents calcium (Ca), sodium (Na), iron or magnesium (Mg) and more rarely zinc, manganese or lithium, and Y represents ions of smaller size, such as chromium (Cr), aluminium (Al), magnesium (Mg), cobalt (Co), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V) or even iron. Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes. They share a common structure consisting of single chains of silica tetrahedra. Pyroxenes that crystallize in the monoclinic system are known as clinopyroxenes and those that crystallize in the orthorhombic system are known as orthopyroxenes.

<span class="mw-page-title-main">Sekaninaite</span> Mg, Fe, Al cyclosilicate mineral

Sekaninaite ((Fe+2,Mg)2Al4Si5O18) is a silicate mineral, the iron-rich analogue of cordierite.

<span class="mw-page-title-main">Maghemite</span> Iron oxide with a spinel ferrite structure

Maghemite (Fe2O3, γ-Fe2O3) is a member of the family of iron oxides. It has the same formula as hematite, but the same spinel ferrite structure as magnetite (Fe3O4) and is also ferrimagnetic. It is sometimes spelled as "maghaemite".

<span class="mw-page-title-main">Hercynite</span>

Hercynite is a spinel mineral with the formula FeAl2O4.

In molecular physics, crystal field theory (CFT) describes the breaking of degeneracies of electron orbital states, usually d or f orbitals, due to a static electric field produced by a surrounding charge distribution. This theory has been used to describe various spectroscopies of transition metal coordination complexes, in particular optical spectra (colors). CFT successfully accounts for some magnetic properties, colors, hydration enthalpies, and spinel structures of transition metal complexes, but it does not attempt to describe bonding. CFT was developed by physicists Hans Bethe and John Hasbrouck van Vleck in the 1930s. CFT was subsequently combined with molecular orbital theory to form the more realistic and complex ligand field theory (LFT), which delivers insight into the process of chemical bonding in transition metal complexes. CFT can be complicated further by breaking assumptions made of relative metal and ligand orbital energies, requiring the use of inverted ligand field theory (ILFT) to better describe bonding.

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<span class="mw-page-title-main">Ferrite (magnet)</span> Ferrimagnetic ceramic material composed of iron(III) oxide and a divalent metallic element

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<span class="mw-page-title-main">Ceylonite</span>

Ceylonite and pleonaste or pleonast are dingy blue or grey to black varieties of spinel. Ceylonite, named for the island of Ceylon, is a ferroan spinel with Mg:Fe from 3:1 and 1:1, and little or no ferric iron. Pleonaste is named from the Greek for 'abundant,' for its many crystal forms, and is distinguished chemically by low Mg:Fe ratios of approximately 1:3. It is sometimes used as a gemstone.

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<span class="mw-page-title-main">Greigite</span> Iron sulfide mineral of spinel structure

Greigite is an iron sulfide mineral with the chemical formula Fe2+Fe3+2S4. It is the sulfur equivalent of the iron oxide magnetite (Fe3O4). It was first described in 1964 for an occurrence in San Bernardino County, California, and named after the mineralogist and physical chemist Joseph W. Greig (1895–1977).

<span class="mw-page-title-main">Ringwoodite</span> High-pressure phase of magnesium silicate

Ringwoodite is a high-pressure phase of Mg2SiO4 (magnesium silicate) formed at high temperatures and pressures of the Earth's mantle between 525 and 660 km (326 and 410 mi) depth. It may also contain iron and hydrogen. It is polymorphous with the olivine phase forsterite (a magnesium iron silicate).

<span class="mw-page-title-main">Carrollite</span> Mineral

Carrollite, CuCo2S4, is a sulfide of copper and cobalt, often with substantial substitution of nickel for the metal ions, and a member of the linnaeite group. It is named after the type locality in Carroll County, Maryland, US, at the Patapsco mine, Sykesville.

Zinc compounds are chemical compounds containing the element zinc which is a member of the group 12 of the periodic table. The oxidation state of zinc in most compounds is the group oxidation state of +2. Zinc may be classified as a post-transition main group element with zinc(II). Zinc compounds are noteworthy for their nondescript appearance and behavior: they are generally colorless, do not readily engage in redox reactions, and generally adopt symmetrical structures.

<span class="mw-page-title-main">Zemannite</span>

Zemannite is a very rare oxide mineral with the chemical formula Mg0.5ZnFe3+[TeO3]3·4.5H2O. It crystallizes in the hexagonal crystal system and forms small prismatic brown crystals. Because of the rarity and small crystal size, zemannite has no applications and serves as a collector's item.

Cuprospinel is a mineral. Cuprospinel is an inverse spinel with the chemical formula CuFe2O4, where copper substitutes some of the iron cations in the structure. Its structure is similar to that of magnetite, Fe3O4, yet with slightly different chemical and physical properties due to the presence of copper.

<span class="mw-page-title-main">Pimelite</span> Nickel-rich smectite deprecated as mineral species in 2006

Pimelite was discredited as a mineral species by the International Mineralogical Association (IMA) in 2006, in an article which suggests that "pimelite" specimens are probably willemseite, or kerolite. This was a mass discreditation, and not based on any re-examination of the type material. Nevertheless, a considerable number of papers have been written, verifying that pimelite is a nickel-dominant smectite. It is always possible to redefine a mineral wrongly discredited.

Antiperovskites is a type of crystal structure similar to the perovskite structure that is common in nature. The key difference is that the positions of the cation and anion constituents are reversed in the unit cell structure. In contrast to perovskite, antiperovskite compounds consist of two types of anions coordinated with one type of cation. Antiperovskite compounds are an important class of materials because they exhibit interesting and useful physical properties not found in perovskite materials, including as electrolytes in solid-state batteries.

Filipstadite is a very rare mineral of the spinel group, with the formula (Mn,Mg)(Sb5+0.5Fe3+0.5)O4. It is isometric, although it was previously thought to be orthorhombic. When compared to a typical spinel, both the octahedral and tetrahedral sites are split due to cation ordering. Filipstadite is chemically close to melanostibite. The mineral comes from Långban, Sweden, a manganese skarn deposit famous for many rare minerals.

References

  1. ↑ Robert J. Naumann: Introduction to the Physics and Chemistry of Materials CRC Press, 2008, ISBN   978-1-4200-6134-5. Retrieved 15 April 2018.
  2. ↑ 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.
  3. ↑ Ernst, W. G. (1969). Earth Materials (Print ed.). Englewood Cliffs, NJ: Prentice-Hall. p.  58.
  4. ↑ "ruby spinel". Encyclopædia Britannica . Retrieved 2022-11-25.
  5. ↑ Spinel group at Mindat
  6. ↑ Rawat, Pankaj Singh; Srivastava, R. C.; Dixit, Gagan; Joshi, G. C.; Asokan, K. (2019). "Facile synthesis and temperature dependent dielectric properties of MnFe2O4 nanoparticles". Dae Solid State Physics Symposium 2018. Vol. 2115. p. 030104. doi:10.1063/1.5112943. S2CID   199183122.
  7. ↑ Vestal, Christy R.; Zhang, Z. John (2003). "Effects of Surface Coordination Chemistry on the Magnetic Properties of MnFe2O4 Spinel Ferrite Nanoparticles". Journal of the American Chemical Society. 125 (32): 9828–9833. doi:10.1021/ja035474n. PMID   12904049.
  8. ↑ American Elements, Manganese Cobalt Oxide, Spinel Powder.
  9. ↑ Ernst, W. G. (1969). Earth Materials (Print ed.). Englewood Cliffs, NJ: Prentice-Hall. p.  59.
  10. ↑ Zhang, Liang; Ji, Guang-Fu; Zhao, Feng; Meng, Chuan-Min; Wei, Dong-Qing (February 2011). "The first-principle studies of the crystal phase transitions: Fd3m-MgAl2O4→F4-3m-MgAl2O4". Physica B: Condensed Matter. 406 (3): 335–338. Bibcode:2011PhyB..406..335Z. doi:10.1016/j.physb.2010.10.054.
  11. ↑ Robert John Lancashire. "Normal Spinels". CHEM2101 (C 21J) Inorganic Chemistry - Chemistry of Transition Metal Complexes. University of the West Indies. Archived from the original on 2023-08-08.
  12. ↑ 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. 386 (1791): 333–345. Bibcode:1983RSPSA.386..333G. doi:10.1098/rspa.1983.0039. JSTOR   2397417. S2CID   96560029.
  13. ↑ L. Hwang; et al. (Jul 1973). "On the space group of MgAl
    2    O
    4     spinel"    . Philosophical Magazine . doi:10.1080/14786437308217448.
  14. ↑ Assadi, M. Hussein N.; H., Katayama-Yoshida (2019). "Covalency a Pathway for Achieving High Magnetisation in TMFe2O4 Compounds". J. Phys. Soc. Jpn. 88 (4): 044706. arXiv: 2004.10948 . Bibcode:2019JPSJ...88d4706A. doi:10.7566/JPSJ.88.044706. S2CID   127456231.
  15. ↑ Sickafus, Kurt E.; Wills, John M.; Grimes, Norman W. (2004-12-21). "Structure of Spinel". Journal of the American Ceramic Society. 82 (12): 3279–3292. doi:10.1111/j.1151-2916.1999.tb02241.x.
  16. ↑ See Spinel Structure in the Encyclopedia of Crystallographic Prototypes, which gives coordinates for the Fd3m case.
  17. ↑ 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.
  18. ↑ Hyun Park, Joo (2007). "Formation Mechanism of Spinel-Type Inclusions in High-Alloyed Stainless Steel Melts". Metallurgical and Materials Transactions B. 38 (4): 657–663. Bibcode:2007MMTB...38..657P. doi:10.1007/s11663-007-9066-x. S2CID   135979316.
  19. ↑ Rose, L. (2011). On the degradation of porous stainless steel (Thesis). University of British Columbia. pp. 144–168. doi:10.14288/1.0071732.
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