The adiabats of olivine, magnesium oxide, and quartz were measured up to 50 kbar and 1000 K. An end-loaded piston-cylinder apparatus with an in situ pressure gauge and a very fine thermocouple was used to measure (∂T/∂P)s, during adiabatic compression. A power law between (∂T/∂P)s and compression yields values of the power n=-∂ ln(∂T/∂P)s/∂ ln &rgr; that agree with previous results from salts, metals, and fluids. Assuming constant values for n, the adiabatic gradient for an olivine upper mantle and a magnesium oxide lower mantle was calculated. The results agree well with some previous theoretical estimates. The volume dependence of the Gr¿neisen parameter &ggr; was calculated from the thermodynamic equation ∂ln &ggr;/∂ ln &rgr;=∂B/∂P-n, where B is the isothermal bulk modulus, &ggr; is found to a good approximation to be proportional to volume.