The gaseous phase in experiments of diamond crystallisation in the carbonate-carbon systems Li2CO3-C, Na2CO3-C, K2CO3-C, CS2CO3-C, CaCO3-C, CaMg(CO3)2-C at P = 7 GPa and T =1 700–1750°C (using the “split-sphere” high-pressure device) was studied by means of gas chromatography. Unlike in traditional metal-carbon systems, in which diamond forms under highly reducing conditions in the presence of a methane-hydrogen fluid, crystallisation of diamond in carbonate-carbon systems occurs in the presence of a H2O-CO2-rich fluid. The results provide experimental confirmation of the possibility for diamond to crystallise in nature in a wide range of redox conditions.

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