ScAlO3 has been synthesized at 1000¿C and 120 kbar as an orthorhombic perovskite with unit cell dimensions a=4.933¿0.003 ¿, b=5.226¿0.003 ¿, and c=7.193¿0.005 ¿. Ions as small as 0.73 ¿ in radius can thus occupy the A site of this ABO3 structure type. Refinement of the structure from X ray power diffraction data shows that the oxygen atom coordination is fourfold and fivefold, the average Al-O distance is 1.93 ¿, and Sc3+ has six near-neighbor oxygen atoms at an average distance of 2.12 ¿ and two more at 2.64 ¿ to give an average eight-coordinate distance of 2.25 ¿. It is inferred that both Mg2+ and Fe2+, with radii of 0.72 and 0.77 ¿, respectively, as well as Mn2+ (0.82 ¿) Ca2+, could occupy the A sites of silicate perovskites to give mantle assemblages denser than those available to any other known structural type. These densities are deduced from the systematics of perovskite unit cell volumes, and by comparison with Hugoniot data for shocked high-pressure assemblages they are consistent with estatite, bronzitite, fayalite, and dunite compositions forming perovskite or perovskite assemblages at very high pressures.