Fine-grained powder samples of Mg2SiO4 forsterite (&agr; phase) have been partially transformed to the beta phase in a 2000-ton uniaxial split-sphere apparatus (USSA--2000) at P=15 GPa and T=1000¿C in runs of 5 minutes duration. This technique allowed us to catch the first steps of the &agr;→&bgr; transformation in Mg2SiO4. A TEM study of the run products showed that a structurally disordered phase is an intermediate of the reaction. The oxygen sublattice of this intermediate phase is well ordered, but a very large density of planar faults on (010) planes destroys the long range order in the cation sublattices. A two-steps mechanism for the &agr;→&bgr; transformation is therefore probable, involving first the nucleation of the structurally disordered phase, then the ordering of this phase. These two steps may be associated with low energy activation barriers and could thus explain the fast kinetics of the &agr;→&bgr; transformation. ¿American Geophysical Union 1991