Diamond syntheses in multicomponent carbonate–carbon melts of natural chemistry: elementary processes and properties

Abstract

It was found in experiments at pressures of 5.5–8.5 GPa that diamond effectively crystallizes in melt-solutions of the multicomponent carbonate–carbon system. By chemical mineralogy evidence, components of the system are among the compositions of the natural media for diamond formation. Carbon solutions oversaturated to diamond form in carbonate–carbon melts for two reasons: (1) the difference of solubilities of diamond and starting graphite (a thermodynamically instable phase at the diamond stability conditions) and (2) temperature gradient over the melts. The PT-boundary conditions for both spontaneous diamond crystallization and seeded diamond growth from, correspondingly, labile and metastable multicomponent carbonate–carbon melt-solutions oversaturated to diamond are experimentally determined. The processes of spontaneous re-crystallization of starting polycrystalline graphite into graphite single crystals (blocks, plates, spherules) in the metastable carbonate–carbon melt-solutions oversaturated to diamond is a phenomenal peculiarity of the diamond-forming systems. For the processes of diamond growth in the carbon solution-melts of natural chemistry, diffusion mass transfer mechanism reinforced by the convection one for the low-viscous carbon solution-melts is of main importance. Kinetics properties of carbon mass transfer as diamond crystallizes in the systems of natural chemistry depend on chemical composition of a solvent and carbon solubility therein, pressure and temperature of diamond growth processes. Growth rate is strongly variable within the range of several millimeters per minute to several micrometers per minute. In case of seeded diamond growth, this locates structurally the crystallization fronts and imposes morphological peculiarities of the layers overgrown: from polycentric and roughly blocked to smooth surfaced with nano-dimensional growth steps. For spontaneous crystallization, the estimated density of nucleation is not less than 3–5×10 2 (for individual octahedral crystals to 200 μm-sized, spinel-law and polysynthetic twins, aggregates) to 1×10 5 nuclea of diamond phase in 1 mm 3 (dense polycrystalline diamond aggregates of the natural diamondite type with microcrystals measuring from 10–20 nm to 50–100 μm).