Topic summary
Spinel group
In minerology, the spinels are any of a class of minerals of general formulation AB2X4 which crystallise in the cubic (isometric) crystal system, with the X anions (typically chalcogens, like oxygen and sulfur) arranged in a close-packed cubic 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 (AB3+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.
A and B can also be the same metal with different valences, as is the case with magnetite, Fe3O4 (as FeFe3+2O2−4), which is the most abundant member of the spinel group. It is even possible for them to be alloys, as seen for example in LiNi0.5Mn1.5O4, a material used in some high energy density lithium ion batteries. Spinels are grouped in series by the B cation.
The group is named for spinel (MgAl2O4), which was once known as "spinel ruby". (Today the term ruby is used only for corundum.)