Crystallization of diamond from CO 2 fluid at high pressure and high temperature

Abstract

Crystallization of diamond from CO 2 fluid was studied by using an equimolar mixture of 12C graphite and 13C graphitic carbon as a carbon source and PtO 2 as an oxygen source under diamond-stable HP–HT condition at 7.7 GPa and 1500°C. Before reaching the experimental condition, PtO 2 was reduced by carbon, and CO 2 was formed. Flaky graphite crystals less than 1 μm in size and octahedral diamond crystals about 10 μm in size were obtained in the experiments held for 8 and 24 h, respectively. Both were solid solutions of 12C and 13C, and the coexisting fluid was CO 2 composed of 12CO 2 and 13CO 2. This means that starting graphitic carbon atoms of 12C and 13C were mixed by the dissolution into CO 2 fluid, and solid solution of graphite was first crystallized followed by that of diamond after a considerably long time.