Diamond and graphite crystallization from C–O–H fluids under high pressure and high temperature conditions

Abstract

Crystallization of diamond was studied in the CO 2–C, CO 2–H 2O–C, H 2O–C, and CH 4–H 2–C systems at 5.7 GPa and 1200–1420°C. Thermodynamic calculations show generation of CO 2, CO 2–H 2O, H 2O and CH 4–H 2 fluids in experiments with graphite and silver oxalate (Ag 2C 2O 4), oxalic acid dihydrate (H 2C 2O 4·2H 2O), water (H 2O), and anthracene (C 14H 10), respectively. Diamond nucleation and growth has been found in the CO 2–C, CO 2–H 2O–C, and H 2O–C systems at 1300–1420°C. At a temperature as low as 1200°C for 136 h there was spontaneous crystallization of diamond in the CO 2–H 2O–C system. For the CH 4–H 2–C system, at 1300–1420°C no diamond synthesis has been established, only insignificant growth on seeds was observed. Diamond octahedra form from the C–O–H fluids at all temperature ranges under investigation. Diamond formation from the fluids at 5.7 GPa and 1200–1420°C was accompanied with the active recrystallization of metastable graphite.