The spinels are any of a class of minerals of general formulation which crystallise in the cubic crystal system, with the X anions arranged in a cubic close-packedlattice 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, 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, which is the most abundant member of the spinel group. 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.
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.
The spinel structure
The space group for a spinel group mineral may be Fdm, but in some cases it is actually the tetrahedral F3m. Normal spinel structures are usually cubicclose-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 in the following positions : X: + /2, /2, + /2, /2, + /2, /2, + /2, /2, + /2, /2, + /2, /2, + /2, /2, + /2, /2, A: + /2, /2, + /2, /2, B: + /2, /2, + /2, /2, + /2, /2, + /2, /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 Fdm then δ=ε and ζ=0. In this case, a three-fold improper rotation with axis in the 111 direction is centred on the point and can also be centred on the B ion at, and in fact every B ion is the centre of an improper three-fold rotation. Under this space group the two A positions are equivalent. If the space group is F3m then the improper three-fold rotations become proper three-fold rotations 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. 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+ ions are d6 high-spin and the Fe3+ ions are d5 high-spin. In addition, intermediate cases exist where the cation distribution can be described as 2O4, where parentheses and brackets are used to denote tetrahedral and octahedral sites, respectively. The so-called inversion degree, x, adopts values between 0 and 1, and is equal to for a completely random cation distribution. The cation distribution in spinel structures are related to the crystal field stabilization energies 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, 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 functional layers, that prevent the diffusion of oxygen ions or specific metal ions such as chromium, which otherwise exhibits a fast diffusion process at high temperatures.
