X ray diffraction and transmission electron microscopy on laser-heated diamond cell samples show that with increasing pressure MgAl2O4 spinel transforms first to Al2O3 corundum+MgO periclase, then to the CaFe2O4-structured phase, and finally to a new phase having the CaTi2O4 structure above ~40 GPa. The CaFe2O4 and the CaTi2O4 structures are closely related and have almost the same densities and bulk moduli. Transformation from the CaFe2O4 to the CaTi2O4 phase would be expected to take place in oceanic crust that is subducted deep into the lower mantle. ¿ 1998 American Geophysical Union