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We propose a new model of Earth's bulk composition based on enstatite chondrites (E-chondrites), the only chondrite group isotopically identical to the Earth. This model allows a quantitative study of accretion and differentiation processes in the early Earth. Conditions for core formation are evaluated using data on silica–iron equilibrium at high pressure and temperature and the exchange budget equation SiO₂+2Fe=Si+2FeO, which is the result of IW and Si–SiO₂ oxygen buffers' interaction and controls the evolution of mantle f_O2. Based on that equation, ranges for the compositions of the Bulk Silicate Earth, the lower mantle and the core are deduced from the compositions of E-chondrites and their constituents. For these ranges of compositions, we show that during core differentiation, the mantle f_O2 evolves naturally from ≈IW-3.2 to IW-1.4±0.1. The model compositions are tightened using geophysical constraints …