Spinel group
A class of cubic minerals with diverse cation substitutions.
The spinel group comprises a class of minerals with the general formula AB2X4, crystallizing in the cubic (isometric) crystal system. They are defined by a close-packed cubic lattice of anions (typically oxygen or other chalcogens) with cations A and B occupying octahedral and tetrahedral sites. The group is named after spinel (MgAl2O4), historically called 'spinel ruby,' and includes numerous natural and synthetic compounds with diverse compositions and applications.
- field
- Mineralogy
- known_for
- Cubic crystal structure with general formula AB2X4; includes magnetite, chromite, and spinel itself
- composition
- Cations A and B can be divalent, trivalent, or tetravalent (e.g., Mg, Zn, Fe, Mn, Al, Cr, Ti, Si); anion is normally oxygen or other chalcogens (thiospinels)
Lore & Background
The spinel group is defined by its crystal structure: a cubic close-packed lattice of anions with cations occupying tetrahedral and octahedral sites. In the normal spinel structure, A ions fill one-eighth of the tetrahedral holes and B ions fill half of the octahedral holes. Members are grouped in series by the B cation, such as aluminium spinels (e.g., spinel, gahnite, hercynite), iron spinels (e.g., magnetite, franklinite, ulvöspinel), chromium spinels (e.g., chromite, magnesiochromite), and others like ringwoodite and musgravite.
Reader's Guide
The spinel group is significant in mineralogy and materials science due to its structural flexibility and wide range of compositions. The prototypical charge balance is A2+B3+2X2−4, but many combinations of divalent, trivalent, and tetravalent cations are possible, including alloys such as LiNi0.5Mn1.5O4 used in high-energy lithium-ion batteries. Magnetite (Fe3O4) is the most abundant member, with iron in two valence states. The group's heterogeneity varies: ferrous and magnesium members form solid solutions, while ferric and aluminium spinels are nearly homogeneous due to size differences. Natural spinels occur in Earth's mantle (e.g., ringwoodite) and in meteorites, and synthetic thiospinels and selenospinels expand the group. The structure's space group is usually Fd3m, but spinel itself shows a transition to F43m at high temperature.
Did You Know?
- The spinel group is named after spinel (MgAl2O4), once known as 'spinel ruby'; today 'ruby' refers only to corundum.
- Magnetite (Fe3O4) is the most abundant member of the spinel group and has iron in both +2 and +3 valence states.
- Thiospinels and selenospinels are spinel-structure compounds where the anion sublattice is made of chalcogens other than oxygen.
More in Minerals & Ore Deposits 1-24
Elsewhere in the Minerals & Ore Deposits universe
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record