 Changes in the spin state of electrons in iron in mantle minerals has been studied experimentally in ferropericlase.Samples are subject to the conditions of the lower mantle in a laser-heated diamond anvil cell and the spin-state is measured using synchrotron X-ray spectroscopy.Results indicate that the change from a high to low spin state in iron occurs with increasing depth over a range from 1000 km to 2200 km.