spinelstructure

The spinel structure is a common crystal structure adopted by minerals of the spinel group and other related compounds. It is based on a cubic close-packed arrangement of oxygen anions, with cations occupying specific interstitial sites within this framework. There are two types of interstitial sites available: tetrahedral sites, which are surrounded by four oxygen atoms, and octahedral sites, which are surrounded by six oxygen atoms. In a normal spinel structure, divalent cations such as Mg2+ occupy the tetrahedral sites, while trivalent cations such as Al3+ occupy the octahedral sites. The general formula for a normal spinel is AB2O4, where A represents the divalent cation and B represents the trivalent cation.

In contrast, inverse spinels have the divalent cations occupying octahedral sites and half of the trivalent cations occupying tetrahedral sites, with the other half of the trivalent cations occupying octahedral sites. Some spinels exhibit a random distribution of cations between the tetrahedral and octahedral sites, which is referred to as a random spinel. The cation distribution significantly influences the physical and chemical properties of spinel compounds, including their magnetic and optical characteristics. The spinel structure is found in a wide variety of industrially important materials, including magnetic ferrites, gemstones like ruby and sapphire (which are actually corundum, an aluminum oxide with a related but distinct structure, though the term "spinel ruby" is sometimes used for red spinel gemstones), and refractory materials.