The possibility of reaction between the liquid iron of the Earth's outer core and crystalline silicates ((Mg,Fe)SiO3) of the mantle, Mg0.9, Fe0.1SiO3(pv)+0.15Fe(&egr;)=0.9MgSiO3 ( pv)+0.2FeO(hpp)+0.05FeSi(&egr;)+0.05SiO2(st), was proposed by Knittle and Jeanloz (1989, 1991) on the basis of exploratory experiments. We calculate Gibbs free-energies for the reactants and the products of the above reaction in order to constrain the pressure-temperature range. Upon application of thermal expansion, equation of state and heat capacity data, we demonstrate that this reaction can occur at pressures as low as 30 GPa at 3500 K, and at temperatures as low as 900 K at the 130 GPa pressure of the present core-mantle boundary. Furthermore, we propose a modified equation of reaction at the core-mantle boundary: Mg0.9Fe0.1SiO3(pv)+0.3Fe(&egr;)= 0.9MgSiO3(pv)+0.3FeO(hpp)+0.1FeSi. Our results imply that similar reactions may occur during the early accretion history of the Earth. ¿ American Geophysical Union 1994