The formation of elemental carbon (diamond or graphite) via the carbonate-silicate interaction in MgCO3-SiO2, CaMg(CO3)2-SiO2, and MgCO3-(SiO2+Al2O3) Systems has been experimentally studied at 5.2–7.5 GPa and 1200 to 1800°C. A special high-pressure cell in which a source of hydrogen (TiH1.9) was placed outside the Pt-capsule containing a carbonate-silicate mixture, was used. As the result of the carbonate-silicate interaction, carbon of carbonates was converted into diamond in association with enstatite+coesite+magnesite, forsterite+enstatite+magnesite, coesite+diopsite+dolomite, and magnesite+coesite+pyrope. The main parameters governing the process of diamond formation via the carbonate-silicate interactions are the temperature, the pressure, and the oxygen fugacity. Depending on the temperature, diamond and graphite crystallized in either subsolidus fluid or melt. According to the gas chromatographic data, fluid composition during diamond crystallization varied from almost pure CO2 to aqueous. Under the experimental conditions corresponding to the field of thermodynamic stability of diamond, the increase in the temperature caused evolution of the processes of carbon formation, from crystallization of metastable graphite to nucleation of diamond.

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