The spinels are any of a class of minerals of general formulation AB
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. Although the charges of A and B in the prototypical spinel structure are +2 and +3, respectively (A2+
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+
4), which is the most abundant member of the spinel group. Spinels are grouped in series by the B cation.
Spinel group members
Members of the spinel group include:
- Aluminium spinels:
- Iron spinels:
- Chromium spinels:
- Cobalt spinels:
- Vanadium spinels:
- 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
- Taaffeite: BeMgAl4O8 with an empirical formula as a spinel, but the chemical formula is four times as large.
- Musgravite: Be(Mg,Fe,Zn)2Al6O12 a type of "multi-spinel".
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.
The spinel structure
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. B ions occupy half the octahedral holes, while A ions occupy one-eighth of the tetrahedral holes. The mineral spinel MgAl2O4 has a normal spinel structure.
In a normal spinel structure, the ions are approximately in the following positions (where i, j, and k are arbitrary integers):
X: (1/4, 0, 0) + ((i+j)/2, (j+k)/2, (i+k)/2) (3/4, 0, 0) + ((i+j)/2, (j+k)/2, (i+k)/2) ( 0, 1/4, 0) + ((i+j)/2, (j+k)/2, (i+k)/2) (1/2, 1/4, 0) + ((i+j)/2, (j+k)/2, (i+k)/2) ( 0, 0, 1/4) + ((i+j)/2, (j+k)/2, (i+k)/2) (1/2, 0, 1/4) + ((i+j)/2, (j+k)/2, (i+k)/2) (1/4, 1/4, 1/4) + ((i+j)/2, (j+k)/2, (i+k)/2) (3/4, 1/4, 1/4) + ((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, 0, 0) + ((i+j)/2, (j+k)/2, (i+k)/2) (1/2, 1/4, 1/4) + ((i+j)/2, (j+k)/2, (i+k)/2) (3/4, 1/4, 0) + ((i+j)/2, (j+k)/2, (i+k)/2) (3/4, 0, 1/4) + ((i+j)/2, (j+k)/2, (i+k)/2)
The eight X positions differ in the orientation of their surroundings, as do the two A positions and the four B positions. A three-fold improper rotation 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 an improper three-fold rotation. 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.
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. 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, or intentional deposition of spinel coatings can be used to protect base metals from oxidation or corrosion. The presence of spinels may hereby serve as thin (few micrometer) hick 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.
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