The elastic properties of candidate mantle phases are used to test the viability of olivine-rich (pyrolitic) and CaO+Al2O2-rich (eclogitic) assemblages for the mantle. High temperature adiabats for each phase of interest are constructed and compared to mantle seismic properties. Both pyrolitic and eclogitic assemblages satisfy the seismic properties between ~200 and 400 km. Between 400 and 670 km depth on eclogitic assemblances yields a superior match to velociti4s and velocity gradients. The 400 km seismic discontinuity may represent a chemical boundary between pyrolite and picritic eclogite (''piclogite'') or phase transformations in the olivine +orthopyroxene components of a piclogitic assemblage containing about 16% olivine. High velocity gradients in the transition zone may be explained by the transformation of Ca-rich cpx to majorite garnet. Seismic properties at the top of the lower mantle are consistent with pyrolite, piclogie or perovskite, implying that the 670 km discontinuity may be a chemical boundary.