CHARACTERISTICS OF SYNTHETIC DIAMOND OVERGROWTHS ON CRUSHED NATURAL SEEDS IN METAL, SILICATE, AND CARBONATE FLUXES

As the first step towards the study of trace element incorporation in diamonds as a potential geothermobarometer, a study of diamond growth on natural seeds using a variety of growth media was conducted. Natural non-gem quality diamonds from unspecified mines in South Africa were crushed with a percussion mortar and pestle to a size of 150-500 μm. This material was used as a seed to nucleate diamond overgrowths. The crushed seeds were washed in ethanol, ultrasonically agitated for one minute, and rinsed with deionized water. The seeds were individually selected using a dissecting microscope to qualify that there was no surface contamination or obvious signs of inclusions, and stirred into the reagent mixes containing a flux and carbon black. A selection of the untreated crushed seeds were mounted on conductive carbon tape on a metal SEM stub and imaged on a JEOL-8900 electron microprobe at a current of 5 nA and an accelerating voltage of 15 kV. Three distinct surface morphologies were observed on the seeds: the individual crystal faces, chonchoidal fractures, and one or two directions of the cleavage along the {111} plane. Treatment was done at 1250-1670°C and 6.5-7.0 GPa in an 18/11 mm multi-anvil press using MgO and Ta crucibles with mixtures containing C black and Invar-MgO-SiO₂, Invar-forsterite, antigorite-CaCO₃-Cr-TiO₂-ZrO₂, and CaCO₃-SiO₂-Na₂HPO₄·7H₂O. The treated seeds were retrieved, mounted, and imaged in the same manner as the untreated seeds. Three distinct overgrowth textures were observed: a series of plates indicating near uniform overgrowth on a surface and small triangular hillocks indicating subhedral overgrowths as seen earlier by Litvin (2007, GSA Spec. Pap. 421). In addition, triangular hoppers formed by limited or no growth in the center with stepped sides of overgrowths were observed here. Further proof of the overgrowths was observed by the cathodoluminescent properties of the seeds. The untreated seeds exhibit a dull, uniform pale violet color throughout, while overgrowth textures exhibit significantly brighter cathodoluminescence and are bright white to light blue, with rare occurrences of yellow and green. Preliminary results from LA-ICP-MS and high-current microprobe analyses indicate a direct correlation of Mg, Si, Fe and Ni with temperature of growth.