Crystallization of diamond from CO2 fluid was studied by using an equimolar
mixture of C-12 graphite and C-13 graphitic carbon as a carbon source and
PtO2 as an oxygen source under diamond-stable HP-HT condition at 7.7 GPa an
d 1500 degreesC. Before reaching the experimental condition, PtO2 was reduc
ed by carbon, and CO2 was formed. Flaky graphite crystals less than 1 mum i
n size and octahedral diamond crystals about 10 gm. in size were obtained i
n the experiments held for 8 and 24 h, respectively. Both were solid soluti
ons of C-12 and C-13, and the coexisting fluid was CO2 composed of (CO2)-C-
12 and (CO2)-C-13. This means that starting graphitic carbon atoms of C-12
and C-13 Were mixed by the dissolution into CO2 fluid, and solid solution o
f graphite was first crystallized followed by that of diamond after a consi
derably long time. (C) 2002 Elsevier Science BY. All rights reserved.