Peculiarities of diamonds formed in alkaline carbonate-carbon melts at pressures of 8–10 GPA: Scanning electron microscopy and cathodoluminescence data

Diamond crystallization with both spontaneous and seeded nucleation was realized in the system Na₂Mg(CO₃)₂-K₂Mg(CO₃)₂-C (graphite) at 8–10 GPa and 1700–1800°C. The crystallization products were transparent colorless diamond single crystals of octahedral habit up to 100–150 μm in size. Scanning electron microscopy (SEM) showed that the growing diamond material was precipitated on both octahedral {111} and cubic { 100 } faces of synthetic and natural diamond seed crystals by layers of octahedral orientation, much like the growth of the natural diamond. The physicochemical conditions for diamond crystallization are interpreted as a crystal growth from carbon solutions in alkaline-carbonate melts. Color cathodoluminescence scanning electron microscopy (CCL-SEM) and cathodoluminescence (CL) spectroscopy studies suggested specific peculiarities of the synthetic “carbonate-carbon”(CC) diamonds resembling natural crystals in comparison with diamonds produced by metal-carbon (MC) synthesis. The main feature of the CC product is the lack, for both spontaneous and seed stimulated diamonds, of surface color cathodoluminescence as in the case for natural diamonds with lower concentrations of nitrogen impurity (type II). The CL spectra of the CC diamonds showed the simultaneous luminescent three-band system - H3, 575 nm, and a weak blue A-band - the H3 band structure of which resembles that of natural diamonds of type IIa.