Spinel

AB2O3

A = Mg (tetrahedral sites)
B = Al (octahedral sites)

Fd-3m (227) [cF56] 
8.083 8.083 8.083 90 90 90
Mg at 0,0,0
Al at 5/8,5/8,5/8
O at 0.387,0.387,0.387