Reactions between carbon and a reduced C–O–H fluid under diamond-stable HP–HT condition

Abstract

Reactions between graphitic carbon and a reduced C–O–H fluid were investigated using a mixture of stearic acid C 18H 36O 2 and oxalic acid dihydrate C 2H 6O 6, as the fluid source at high pressure and temperature (HP–HT) of 7.7 GPa and 1500°C in a platinum sealed capsule. A reduced C–O–H fluid mainly composed of methane and water, was formed by the thermal decomposition of the fluid source before reaching the HP–HT condition. An exchange reaction between carbon and methane occurred and starting carbon was re-crystallized to flaky graphite crystals, but no diamond was formed for the duration up to 24 h. In the experiment for 48 h, octahedral diamond crystals of a few to a few tens of micrometers in size were observed along with the recrystallized graphite. These results show that reduced C–O–H fluid acts as a diamond forming catalyst, although a long incubation time is necessary for diamond formation.