Sound velocities and elastic moduli have been measured on &ggr;-(Mg0.91Fe0.09)2SiO4 (ringwoodite) single-crystals to 16 GPa and to 923K by Brillouin spectroscopy. A third-order finite-strain fit to the data yields pressure derivatives of 4.1(2) and 1.3(1) for the adiabatic bulk modulus (Ks) and shear modulus (&mgr;) respectively. Mg-rich &agr;, &bgr;, and &ggr;(Mg,Fe)2SiO4 have identical values of ∂KS/∂P and of ∂&mgr;/∂P within the experimental uncertainties. ∂KS/∂T and ∂&mgr;/∂T for &ggr;-(Mg,Fe)2SiO4 are -0.021 and -0.016 GPa/K respectively. Our results indicate that the shear and compressional impedance contrasts associated with &bgr;- → &ggr;-(Mg,Fe)2SiO4 transition are sufficient to produce a visible discontinuity at 520 km even with a moderate (30--50%) amount of olivine. The generally low visibility of the 520 km discontinuity is likely due to its width and large amplitude topographic variations. ¿ 2001 American Geophysical Union