Formation of diamond from CaCO 3 in a reduced C–O–H fluid at HP–HT

Abstract

The formation of diamond from CaCO 3 was studied in a reduced C–O–H fluid mainly composed of CH 4 and H 2O under diamond-stable HP–HT conditions at 7.7 GPa and 1500 °C in order to examine the genesis of natural diamonds experimentally and to present a new synthesis process of diamond. As a result, CaCO 3 was reacted with reduced C–O–H fluid very rapidly, and most of the starting CaCO 3 was converted to Ca(OH) 2 and C within 1 h. The elemental carbon thus formed was graphite although the experiments were carried out deep in the diamond-stable HP–HT region, but it was transformed to diamond after 12 h. Graphite and diamond had the morphology characteristics of solution growth: the former was flaky and a few μm in size, and the latter was octahedral with sharp edges and flat surfaces and a few tens of μm in size.