The high-pressure mineral assemblage produced from a natural peridotite sample (xMg=0.897) has been synthesized in the laser-heated diamond cell and identified by x-ray diffraction at 54 GPa. Orthorhombic perovskite, (Mg, Fe)SiO3, magnesiow¿stite, (Mg, Fe)O, and cubic perovskite, CaSiO3, are observed in estimated molar ratios of 1.00 : 0.46 : 0.08 (1.00 : 0.21 : 0.09 by volume) respectively, with no evidence of other phases being present. The lattice parameters measured at 54 GPa yield a density of 4.97(¿0.03) Mg/m3 for the high pressure assemblage at 300 k, in good agreement with prior calculations of density for candidate lower mantle assemblages. ¿ American Geophysical Union 1990