How do diamonds grow in metal melt together with silicate minerals? An experimental study of diamond morphology

Abstract

Abstract. The origin and evolution of metal melts in the Earth's mantle and their role in the formation of diamond are the subject of active discussion. It is widely accepted that portions of metal melts in the form of pockets can be a suitable medium for diamond growth. This raises questions about the role of silicate minerals that form the walls of these pockets and are present in the volume of the metal melt during the growth of diamonds. The aim of the present work was to study the crystallization of diamond in a complex heterogeneous system: metal-melt–basalt–carbon. The experiments were performed using a multianvil high-pressure apparatus of split-sphere type (BARS) at a pressure of 5.5 GPa and a temperature of 1500 ∘C. The results demonstrated crystallization of diamond in metal melt together with garnet and clinopyroxene, whose chemical compositions are similar to those of eclogitic inclusions in natural diamond. We show that the presence of silicates in the crystallization medium does not reduce the chemical ability of metal melts to catalyze the conversion of graphite into diamond, and, morphologically, diamond crystallizes mainly in the form of a cuboctahedron. When the content of the silicate material in the system exceeds 5 wt %, diamond forms parallel-growth aggregates, but 15 wt % of silicate phases block the crystallization chamber, preventing the penetration of metallic melt into them, thus interrupting the growth of diamond. We infer that the studied mechanism of diamond crystallization can occur at lower-mantle conditions but could also have taken place in the ancient continental mantle of the Earth, under reducing conditions that allowed the stability of Fe–Ni melts.

Growth rate and morphology study of tetrahydrofuran hydrate single crystals and the effect of salt

“…Formation of the parallel-growth aggregate is believed to be the result of the formation of several growth centers from the individual crystals. 39 During the single crystal growth experiments, there were other crystal shapes observed other than the regular octahedral crystals, such as distorted octahedral, dendritic growth, needles, and thin plates, especially when performed at much higher subcooling, similar to the observations from previous researchers. 18,29,40 The recorded videos are provided in ESI † for the experiments performed at the highest subcooling of 4.0 °C, where we observed severe defects formation to the crystal (ESI † SV2), and the instantaneous formation of octahedral single crystals when a seed crystal was dropped into the THF melt solution (ESI † SV3).…”

“…Formation of the parallel-growth aggregate is believed to be the result of the formation of several growth centers from the individual crystals. 39…”

Surface Porosity of Natural Diamond Crystals after the Catalytic Hydrogenation

“…A new, interesting example of a porous diamond is HPHT diamond synthesis in metal melt, obtained by varying the growing medium [29]. Our recently obtained results as well demonstrated the growth of HPHT diamonds with numerous caverns which were formed by the presence of solid components in the metal melt [30], as predicted earlier [31]. It is worth noting that several kinds of diamond materials are available today: natural crystals extracted from kimberlite deposits, several types of synthetic diamond produced by HPHT and Chemical Vapour Deposition (CVD) technologies, and synthetic detonation diamonds.…”

Characterization of the nitrogen state in HPHT diamonds grown in an Fe–C melt with a low sulfur addition

“…As has been already shown, mechanical impurities that are present in the metal melt can change the morphology of growing diamonds significantly. 47,48 In the present experiment, such objects could be small particles of sulfide or container materials which concentrate in the upper part of the reactor volume due to their significantly low density in comparison with that of metals. The effect of the influence of sulfur on the morphology change of growing diamonds should also be considered, 17 although in our experiment, the S content was much lower which, in our opinion, excludes the development of such a mechanism.…”