Molten nickel saturated with carbon was cooled at slow rates from 1500 degr
eesC at 6GPa in the presence of diamond crystals. Graphite preferentially n
ucleated and grew in the nickel in spite of being less stable than diamond.
Diamond dendrites grew from the diamond already present while the graphite
was growing. The result suggests that under the conditions studied here, g
raphite has a lower barrier for nucleation than diamond, even if graphite i
s thermodynamically less stable than diamond.