The orthorhombic to high-P monoclinic phase transition in (Mg,Fe)SiO3 pyroxene with a mantle-relevant composition (XFs=0.1) is expected to occur at ~300 km depth [Woodland and Angel, 1997>. However, the divariant nature of the phase transition in the Mg-Fe system leaves the question open as to whether this transition occurs over a narrow enough pressure interval to cause a seismic discontinuity. New experimental results with binary Mg-Fe pyroxenes constrain the divariant loop to be 0.2 GPa wide at the composition of XFs=0.4 and on the order of 0.15 GPa for a mantle-relevant composition. This implies that the phase transition will be complete over a depth interval of about 5--6 km in the mantle and it is concluded that the divariant loop of the orthorhombic to high-P monoclinic phase transition in (Mg,Fe)SiO3 pyroxene is indeed narrow enough to produce a jump in seismic velocities. The experimentally observed metastable behavior of orthopyroxene could further reduce the effective depth interval of this phase transition. The expected location of this phase transition coincides with a small magnitude seismic discontinuity, the X-discontinuity, occasionally observed in seismic profiles at ~300 km depth, and thus provides a viable petrologic explanation for the origin of this discontinuity, if it truly exists. ¿ 1998 American Geophysical Union