We studied the decomposition of CaCO3 by laser heated diamond anvil cell experiments at pressures between 9 and 21 GPa up to 4000 K. The quenched samples were characterized by micro-Raman spectroscopy. From the results we conclude that calcite decomposes into CaO + O2 + C across the whole pressure range investigated at temperatures around 3500 K, initially forming graphite nanoparticles with dimensions around 3–11 nm. The graphite particles may aggregate and transform into diamond with dimensions around 20 nm if the sample is annealed in the diamond stability field. We therefore conclude that diamond can be crystallized directly from carbonatitic melts by decomposition of CaCO3 at high pressures and temperatures, and that phase diagrams showing a decomposition into CaO + CO2 in this P, T-range need to be reevaluated.

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