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Spinel group

The spinels are any of a class of minerals of general formulation which crystallise in the cubic (isometric) crystal system, with the oxide anions 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, 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, FeO (as ), which is the most abundant member of the spinel group. Spinels are grouped in series by the B cation.

Members of the spinel group include:

  • Aluminium spinels:
    • Spinel: MgAlO, after which this class of minerals is named
    • Chrysoberyl: BeAlO
    • Gahnite: ZnAlO
    • Hercynite: FeAlO
    • Galaxite: MnAlO
    • Pleonaste: (Mg,Fe)AlO
  • Iron spinels:
    • Cuprospinel: CuFeO
    • Franklinite: (Fe,Mn,Zn)(Fe,Mn)O
    • Jacobsite: MnFeO
    • Magnesioferrite: MgFeO
    • Magnetite: FeO
    • Trevorite: NiFeO
    • Ulvöspinel: TiFeO
    • Zinc ferrite: (Zn, Fe) FeO
  • Chromium spinels:
    • Chromite: FeCrO
    • Magnesiochromite: MgCrO
    • Zincochromite: ZnCrO
  • Others with the spinel structure:
    • Coulsonite: FeVO
    • Magnesiocoulsonite: MgVO
    • Ringwoodite: (Mg,Fe)SiO, 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.