The bulk moduli of silicate and oxide solid solutions depend very little on iron content, but are diagnostic of crystal structure. Comparison of laboratory with seismic data suggests that the upper mantle mineral assemblages as a function of depth are: I. olivine+pyroxene+garnet (200--400 km), II. &bgr;-spinel+pyroxene-garnet solid solution (400--500 km) III. (&bgr;+&ggr;) -spinel+pyx-ga s.s. (500--600) km, IV. &ggr;-spinel+SiO2 (stishovite) or mixed oxides (650--800 km) and V. mixed oxides or perovskite+MgO(>800 km). The 650 km discontinuity is primarily due to phase changes in the pyroxene-garnet, rather than the olivine, portion of the mantle.