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An increase in lattice constants and an order-disorder phase transition were observed in the magnesium aluminate spinel (MgAl(2)O(4)) powders after shock compression. Theoretical calculations on the basis of density functional theory confirm that the remarkable volume expansion in shocked MgAl(2)O(4) powders is closely related to the substantial site disorder in the MgAl(2)O(4) lattice. The calculations also show that the partially inverse MgAl(2)O(4) spinel with an inversion index of 0.7 represents the greatest disordered metastable phase and the most unstable structure. (C) 2010 American Institute of Physics. [doi:10.1063/1.3516345]