PHASE FORMATION AND DIAMOND CRYSTALLIZATION IN CARBON-BEARING ULTRAPOTASSIC CARBONATE-SILICATE SYSTEMS

Abstract

Diamond crystallization and character of phase formation in thesystems K2CO3-C, K2CO3-SiO2-C, and K2CO3-Mg2SiO4-C were studied at 6.3 GPa, 1650 degreesC for 40 h using the multianvil split-sphere equipment. The SiO2/K2CO3 and Mg2SiO4/K2CO3 ratios,here chosen as variable parameters. The degree of graphite-to-diamond transformation and rate of diamond growth on seeds have been determined as a function of these ratios. Composition domains have been revealed in which spontaneous diamond nucleation and diamond growth on seeds proceed. As the concentrations of silica and forsterite in the systems K2CO3-SiO2-C and K2CO3-Mg2SiO4-C, respectively, increase to 10 wt.%, the diamond formation becomes more intense. Given a further increase in contents Of SiO2 or Mg2SiO4 this intensity gradually decreases until the complete termination of spontaneous nucleation and then, diamond growth on seeds. The conditions were created under which diamond was crystallized from a potassium carbonate-silicate melt, including the main components of deep-seated ultra-potassium fluxes, together with coesite in the system K2CO3-SiO2-C and with forsterite in the system K2CO3-Mg2SiO4-C.