The unavailability of seismic data from Venus severely limits what can be definitely known about the structure of the planet's mantle, but models of Earth's mantle have been modified to make predictions. It's expected that the uppermost mantle, from about to deep is mostly made of the mineral olivine. Descending through the mantle, the chemical composition remains largely the same but at somewhere between about and , the increasing pressure causes the crystal structure of olivine to change to the more densely packed structure of spinel. Another transition occurs between and deep, where the material takes on the progressively more compact crystal structures of ilmenite and perovskite, and gradually becomes more like perovskite until the core boundary is reached.