The high-pressure transformation in MgSiO3 and those in the spinel phases of compositions from Mg2SiO4 to (Mg0.5Fe0.5)2SiO4 in the Mg2SiO4Fe2SiO4 system were investigated using a uniaxial split-sphere apparatus. The phase boundaries between ilmenite-perovskite in MgSiO3 and between Mg2SiO4 spinel and the assemblage of MgSiO3 perovskite and MgO periclase were determined to be P(GPa)=26.8--0.0025T(¿C) and P(GPa)=2.76--0.0028T(¿C), respectively, in the temperature range 1000--1600¿ C. The pseudobinary diagrams for the system Mg2SiO4-Fe2SiO4 were determined at temperatures of 1100¿ C and 1600¿ C. It was demonstrated that the magnesian spinel (with Fe/Mg+Fe<0.22 at 1100¿C and <0.26 at 1600¿ C) dissociates into perovskite and magnesiow¿stite within an extremely narrow pressure interval (<0.15 GPa at 1600¿ C). The dissociation pressure was found to be almost independent of iron content and to coincide to that at 670 km depth within exeprimental uncertainties. These experimental results indicate that the sharpness of the 7670-km discontinuity may indeed be due to this dissociation in a periotitic or pyrolitic mantle. The current status of our understanding of deep mantle mineralogy and chemistry is discussed based on recent high-pressure and high-temperature experiments. ¿ American Geophysical Union 1989