Reduced silicon alloyed with Fe metal was shown to chemically react with FeCO3 siderite at pressures of 10–25 GPa and temperatures of 1700–1800 °C according to 2 FeCO3 siderite+3 Si in metal=2 Fe in metal+3 SiO2 stishovite+2 Cdiamond Since no carbon seeds were introduced, the only source of carbon for diamond formation was the carbonate phase. This observation provides a mechanism of diamond formation, possibly relevant to the early Earth. Thermodynamic modelling of the observed reactions shows that the stishovite/silicon oxygen fugacity buffer is far more reducing than the carbonate/diamond equilibrium, implying that, at Earth mantle conditions, no silicon-bearing metal can coexist with carbonates. Diamond formation by reactions between carbonates and highly reducing metal phases containing significant amounts of silicon might have occurred in the early Earth upon mixing of oxidized and reduced accretion components at pressures and temperature exceeding 10 GPa and 1700 °C.