Sound velocities and elastic moduli of natural polycrystalline majorite (Mg0.78Fe0.21Ca0.01)SiO3, and ringwoodite (Mg0.75Fe0.25)2SiO4 from the Catherwood meteorite were measured by Brillouin spectroscopy. These are the first acoustic measurements for such natural high-pressure phases with Fe contents comparable to that of the Earth's mantle. The adiabatic bulk modulus of majorite is KS=164(4) GPa, and the shear modulus is &mgr;=87(2) GPa, which are typical values for garnet-structured silicates. Both the elastic moduli and the sound velocities of natural cubic majorite are slightly lower than values for aluminosilicate garnets with comparable Fe contents. Moreover, the bulk and shear moduli of natural majorite are indistinguishable from those of pure Mg majorite, and the dependence of these moduli on Fe content is small. In contrast, ringwoodite (KS=193(3), &mgr;=113(2)) exhibits a pronounced change in the aggregate elastic moduli and sound velocities with increasing Fe content. The addition of 10% Fe to &ggr;-phase changes the velocity by an amount comparable to the velocity change across the &bgr;→&ggr; transition in Mg2SiO4. It is therefore necessary to account for the effect of Fe in constructing mineralogical models of the transition zone. ¿ 1997 American Geophysical Union