Spontaneous nucleation of diamond in the system MgCO3-CaCO3-C at 7.7 GPa

High-pressure and high-temperature experiments were carried out to determin e the minimum temperature required for spontaneous nucleation of diamond in the system comprising a carbonate mixture (60 mol% MgCO3 and 40 mol% CaCO3 ) and graphite at 7.7 GPa, for 1 h and longer reaction times up to 12.5 h, and also in the systems MgCO3-graphite and CaCO3-graphite for comparison. T he above carbonate mixture melted at a temperature between 1600 degrees C a nd 1700 degrees C. The minimum temperature for spontaneous nucleation of di amond was 1900 degrees C for a reaction time of 1 h, it was lowered to 1700 degrees C for 11 h, but no diamond was formed at 1600 degrees C for 12.5 h . These results indicate that the molten state of the solvent-catalyst and enough reaction time are necessary for diamond nucleation in the systems ex amined at 7.7 GPa. (C) 1999 Published by Elsevier Science S.A. All rights r eserved.