Post-perovskite

Post-perovskite is a high-pressure phase of magnesium silicate (MgSiO3) that is believed to exist in the Earth's lower mantle, approximately 660 to 2,900 kilometers (about 410 to 1,800 miles) below the Earth's surface. It has a distinct crystal structure that is different from the more common perovskite structure, which is typically stable at shallower mantle depths.