The structure consists of octahedral layers of edge-sharing FeO 6 octahedra. These layers are stacked on top of each other so that each octahedron is forced to share a face with another. Note that the unit cell is hexagonal. Ilmenite Ilmenite is a common oxide phase in igneous rocks. The general spinel formula is AB 2 O 4 .
Spinels are very important for geothermometry and rock magnetism.