The elastic properties of a solid solution consisting of 59 mole percent pyropic garnet and 41 percent Mg4Si4O12-majorite (a high-pressure form of pyroxene), have been measured by Brillouin spectroscopy under ambient conditions. Values of the adiabatic bulk modulus, KS=164¿15 GPa, and shear modulus, G=89.4¿3.7 GPa, are similar to those for low-pressure aluminosilicate garnets. The enstatite-majorite reaction is characterized by a large (49%) increase in KS and a much smaller (13%) increase in G. The properties of low-pressure garnets suggest that the contrast in shear moduli for this reaction is probably larger in Ca-rich systems. Taken at face value, the STP properties of these phases imply that large quantities of Ca-rich clinopyroxene are needed in order to satisfy seismologically determined shear modulus gradients in the transition zone. This suggestion is, however, subject to considerable uncertainties in the pressure and temperature derivatives of elastic properties for mantle minerals, and the velocity structure of the transition zone. ¿American Geophysical Union 1990