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

Crystallization of diamond was studied in the CO2-C, CO2-H2O-C, H2O-C, and CH4-H-2-C systems at 5.7 GPa and 1200-1420 degreesC. Thermodynamic calculations show generation of CO2, CO2-H2O, H2O and CH4-H-2 fluids in experiments with graphite and silver oxalate (Ag2C2O4), oxalic acid dihydrate (H2C2O4. 2H(2)O), water (H2O), and anthracene (C14H10), respectively. Diamond nucleation and growth has been found in the CO2-C, CO2-H2O-C, and H2O-C systems at 1300-1420 degreesC. At a temperature as low as 1200 degreesC for 136 h there was spontaneous crystallization of diamond in the CO2-H2O-C system. For the CH4-H-2-C system, at 1300-1420 degreesC 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 degreesC was accompanied with the active recrystallization of metastable graphite. (C) 2001 Elsevier Science B.V. All rights reserved.