Pyrolite  is a hypothetical mixture of distinct minerals which is widely believed to represent the composition of the Earth's mantle.The main pressure-induced phase transformations of the olivine component of pyrolite occur at about 13.5 GPa (alpha to beta) and 24 GPa (gamma to MgSiO3-rich perovskite + magnesiowustite) [2,3], which are thought to be responsible for the seismic discontinuities at 410 and 660 km depths in the mantle. Recent seismological studies, however, have demonstrated that the 410-km seismic discontinuity is sharper in some areas than that expected from the alpha to beta transformation in mantle olivine with a fixed composition [4-7]. Moreover, some mineral physics studies suggest that the seismic velocity jump at the 410-km discontinuity is inconsistent with that associated with the alpha to beta transformation in olivine [8,9]. Here we present a phase equilibria study of a material having pyrolite composition at pressures of 6-16 GPa. We found that the iron content in olivine changes significantly with increasing pressure, as a result of the formation of a relatively iron-rich majorite phase at these pressures. This variation in iron content can overcome, or at least reduce, both of the above difficulties encountered with the pyrolite model of mantle composition, by showing that the component mineral systems cannot be treated as separate.