A form of the magnesium silicate mineral MgSiO₃ that is identical in composition to its more common perovskite (a group of minerals based on CaTiO₃) form, but much denser. In perovskite, the atoms form a three-dimensional bonding structure, and in post-perovskite they form stacks of two-dimensional sheets. The existence of post-perovskite was discovered in 2004 when a group at the Tokyo Institute of Technology led by Kei Hirose reproduced the high temperature and pressure found in the lowermost mantle, in the D-layer close to the boundary between the mantle and outer core. Post-perovskite was the predominant mineral present and its density accounts for the previously mysterious changes in speed of seismic waves passing through this region.