San Carlos olivine and its synthetic ringwoodite polymorph have been transformed to (Mg,Fe)SiO3-perovskite and magnesiow¿stite at a pressure of 26 GPa in a 2000-t uniaxial split-sphere apparatus (USSA-2000) for temperatures ranging from 700 ¿C to 1600 ¿C and run durations at peak temperatures of 0 min to 19 hours. The recovered samples were studied by analytical transmission electron microscopy to determine the evolution of the microstructures and the crystallographical relationships and iron partitioning between the coexisting phases in these assemblages. At 700 ¿C, metastable olivine remained untransformed even after 19 hours. In runs performed at 1000 ¿C and 1200 ¿C, ringwoodite, in a topotactic relation with olivine, was identified even though olivine was used as starting material. Our results indicate that ringwoodite is an intermediate phase in the olivine→(Mg,Fe)SiO3-perovskite+magnesiow¿stite transformation in this temperature range. At or above 1300 ¿C the transformation of olivine or ringwoodite (used as the starting material) into (Mg,Fe)SiO3-perovskite+magnesiow¿stite was complete in less than 10 min. The first microstructures that appear are eutectoid-like as already described by previous authors. For longer run durations the microstructure consisted mostly of cylindrical magnesiow¿stite crystals embedded within large, twinned (Mg,Fe)SiO3-perovskite crystals. These observations suggest that magnesiow¿stite grains are very unlikely to be interconnected for a wide range of possible bulk mantle compositions; magnesiow¿stite will therefore play a relatively minor role in determining the transport properties of Earth's lower mantle. Analyses of (Mg,Fe)SiO3-perovskite and magnesiow¿stite crystals formed in the first steps of the transformation show that most of the iron-magnesium partitioning is completed within the first minutes of the reaction and that subsequently only isochemical grain growth of the two phases occurs. The iron-magnesium distribution between (Mg,Fe)SiO3-perovskite (pv) and magnesiow¿stite (mw), characterized by Kd- Fe=(Fe/Mg)mw/(Fe/Mg)pv was precisely measured by analytical transmission electron microscopy in equilibrated runs and found to be Kd-Fe=3.8(3) at 1300 ¿C and Kd-Fe=4.3(4) at 1600 ¿C.¿ 1997 American Geophysical Union