Diamond formation during sulfidation of metal–carbon melts

Abstract

Experimental studies modeling the mechanism of diamond crystallization during sulfidation of metal–carbon melts were performed on a multi-anvil high-pressure apparatus BARS at pressure of 6.3 GPa, temperature range of 1270–1470 °C, and run duration of 20 to 35 h. Sandwich experiments with layer filling were carried out using schemes FeS2-[Fe(Fe90Ni10) + C]-FeS2, FeS2-Ol-[Fe90Ni10 + C]-Ol-FeS2 and S-Ol-[Fe90Ni10 + C]-Ol-S. The experiments have revealed that crystallization of diamond and/or graphite occurs during sulfidation of a metal–carbon melt either due to mixing of metal–carbon and sulfide melts or due to influx of sulfur from external sources through a polycrystalline olivine layers. Transport of a sulfur fluid or a metal–sulfide–carbon melt through a silicate matrix occurs through the intergranular space and microcracks as well as by recrystallization of olivine. Sulfidation is accompanied by a decrease in the solubility of carbon and leads to supersaturation that is necessary for crystallization of diamond or graphite. The inhibitory effect of the sulfide component is the cause for changes in the diamond morphology and transition of carbon phase crystallization from diamond to metastable graphite. The synthesized diamond crystals are classified as low-nitrogen or nitrogen-free diamonds. The revealed regularities may be used to explain the genesis of diamonds with central inclusions and nitrogen-free CLIPPIR diamonds.