Formation of diamond from supercritical H2O-CO2 fluid at high pressure andhigh temperature

Formation of diamond was studied from the starting graphite and anhydrous o xalic acid (COOH)(2) in a platinum-sealed capsule at 7.7 GPa and 1300-1500 degrees C. Small octahedral diamond crystals < 10 mu m in size were crystal lized and the fluid phase in coexistence with diamonds was found to be comp osed of H2O and CO2 by a mass spectrometer. Besides, when C-13 graphite was used as a starting material, (CO2)-C-13 was present with (CO2)-C-12 in the fluid, showing that a H2O-CO2 fluid was first formed by the decomposition of the oxalic acid, and then graphite dissolved into the fluid and precipit ated as diamond. (C) 2000 Elsevier Science B.V. All rights reserved.