Spinel

Spinel (MgAl2O4) At about 400 km, the olivine structure is no longer stable. It changes spontaneously from the olivine structure to a spinel-type structure, which takes up 8% less space; the composition remains the same (though we now call it beta-Mg2SiO4). This causes seismic discontinuity at about 300-400 km depth that is the boundary tranistion zone.

from: Mineralogy and Optical Mineralogy (2008) Dyar, M. and Gunther, M.; Mineralogical Society of America.