Both silicate perovskites bridgmanite, MgSiO3, and davemaoite, CaSiO3 play important although very different roles in the deep mantle. Bridgmanite is the most abundant mineral in the lower mantle. It’s thermoelastic properties, heat capacity, thermal conductivity, its viscoelastic deformation control much of the bulk rock properties of the lower mantle. Davemaoite contributes only a few volume % of lower mantle rock but this mineral hosts almost all the minor and trace elements that are geochemically ‘incompatible’ with reference to the upper mantle such as the rare earth elements, U, Th, K. However, in the lower mantle, davemaoite is liquidus phase, thus it retains minor and trace elements in the residual during partial melting and in that sense most of the these elements that are incompataible with regard to the upper mantle are compatible in the lower mantle. Dry partial melting is not expected in the present lower mantle, but interaction with fluids is a possible means of mobilizing material at least in the upper part of the lower mantle. Exploring this possibility is not a purely experimental question, the answer also requires the identification and characterization of actual samples from that depth. Diamonds with inclusions of davemaoite and ice-VII are samples of this kind.