High‐pressure transformations in MgAl₂O₄

X ray diffraction and transmission electron microscopy on laser‐heated diamond cell samples show that with increasing pressure MgAl 2 O 4 spinel transforms first to Al 2 O 3 corundum + MgO periclase, then to the CaFe 2 O 4 ‐structured phase, and finally to a new phase having the CaTi 2 O 4 structure above ∼40 GPa. The CaFe 2 O 4 and the CaTi 2 O 4 structures are closely related and have almost the same densities and bulk moduli. Transformation from the CaFe 2 O 4 to the CaTi 2 O 4 phase would be expected to take place in oceanic crust that is subducted deep into the lower mantle.