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

Abstract

Abstract The formation process of diamond from supercritical H 2 O–CO 2 fluid was studied using 13 C-graphitic carbon and oxalic acid dihydrate, (COOH) 2 ·2H 2 O, as starting materials under a diamond stable high pressure–high temperature (HP–HT) condition of 7.7 GPa and 1600°C. The exchange reaction between 13 C-graphitic carbon and 12 CO 2 in the supercritical H 2 O–CO 2 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.