The perovskite form of Mg0.981, Fe0.028Cr0.011Si0.979O3, quenched from 26 GPa and 2173 K in a uniaxial split-sphere-type high-pressure apparatus, was studied at the SUNY X3A beamline at the National Synchrotron Light Source. With a Si (111) double-crystal monochromator and a single crystal of perovskite measuring 29¿29¿43 &mgr;m, intensity of 127 independent reflections were collected with sin &thgr;/&lgr;3.5&sgr;I. This is the first-reported single-crystal diffraction study of a Mg-silicate perovskite containing a significant amount of Fe or other cations. The unit cell parameters obtained through the refinement of 30 reflections are: a=4.7823(3), b=4.9331(3), c=6.9026(8) ¿, V=162.84(2) ¿3. The result of structural refinement in space group Pbnm indicates that Fe and a small amount of Cr substitute for Mg rather than for Si, contrary to the result of Jackson et al. (1987) who reported Fe in the octahedral site of a perovskite synthesized at 50 GPa and 2000 K in a diamond-anvil cell with laser heating. ¿ American Geophysical Union 1990