Formation process of diamond from supercritical H2O-CO2 fluid under high pressure and high temperature conditions

The formation process of diamond from supercritical H2O-CO2 fluid was studied using C-13-graphitic carbon and oxalic acid dihydrate, (COOH)(2). 2H(2)O, as starring materials under a diamond stable high pressure-high temperature (HP-HT) condition of 7.7 GPa and 1600 degreesC. The exchange reaction between C-13-graphitic carbon and (CO2)-C-12 in the supercritical H2O-CO2 fluid. which was first formed by the decomposition of oxalic acid dihydrate, occurred very rapidly and became nearly equilibrated after 6 h. At the same time, graphite was recrystallized and coexistent with the fluid until traces of diamond were first observed after 8 h. All graphite transformed into diamond after 17 h, showing that a considerably long induction time was present for the formation of diamond in this fluid system. (C) 2000 Elsevier Science B.V. All rights reserved.