Evidence for a martensitic type mechanism for the olivine-spinel (α-&ggr;) transition in Fe2SiO4 has resulted from an investigation by rapid acquisition of diffraction data collected on Fayalite subjected to high temperature and pressure in a diamond anvil cell using an intense X ray beam from the Cornell High Energy Synchrotron Source (CHESS). An energy-dispersive technique employing a white X ray beam and a solid-state detector made it possible to collect a complete diffraction pattern every 2 min as pressure was increased across the phase transition at 520¿C. The series of patterns clearly showed that the &ggr; phase peaks which appeared first were those also compatible with a structure whose unit cell dimensions are half those of the spinel unit cell. After the time of three or four spectrum acquisitions, all of the peaks of the spinel structure were present. This same effect was observed in two other series. An additional series was run at 400¿C. As as soon as the &ggr; phase peaks appeared, the run was halted, and the sample was quenched and examined by the Debye-Scherrer method. The only &ggr; phase peaks observed were those also allowed for a structure with the smaller unit cell. These results are consistent with a restacking of the anions from the approximately hexagonal close-packed (hop) arrangement in the olivine structure to the nearly cubic close-packed (ccp) arrangement in the spinel structure, followed later by a reordering of the cations.