Spinel, MgAl structures

Spinel compounds have a common chemical formula, AB2X4. Structurally they have a cubic symmetry and are derived from magnesium alurmnate, MgAl O., a naturally occurring mineral. 

In general, the sequence of oxide-formation from a MgAl(C03) LDH proceeds as follows. Firstly, the LDH is converted to a mixed MgAl oxide with the MgO rock-salt type structure at approximately 400°C. The lattice parameters of the mixed oxide are generally lower, however, than those measured for pure MgO, indicating that the Al3+ ions are inserted into the structure, which also introduces lattice defects. At higher temperatures the mixed oxide decomposes into MgO and spinel, MgAl204 [160-164], A similar sequence has been observed for the thermal decomposition of a NiAl-LDH by Sato el al. [28],... 

In the normal structure of spinel one has the crystalline systems MgAl O (spinel), FeAl O (hercynite), FeCr O (cromite), with the reverse structure of spinel being represented by MgFe20, TiMg20, MgGap, etc. 

---