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This paper reports the results of high-pressure high-temperature (HPHT) diamonds growing in an Fe–C melt with introduction of 1 wt% sulfur.

The problem of diamond formation, despite the huge amount of accumulated information, has not been finally resolved. Currently, the most well-established hypothesis is that the diamond will be formed as a result of metasomatosis. According to this theory, the source of carbon were fluids of C-H-O-N-S composition. There are still questions concerning the environment for diamond crystallization. One of the most common inclusions in diamonds from kimberlite tubes are sulfides. They are also represented in diamondiferous xenoliths of peridotite and eclogite from diamondiferous tubes, but their quantity in diamonds is still higher in comparison with xenoliths. Modern scientific researches allow to assert that large diamonds, such as Kullinan (3106 carats), Koh-i-Noor, etc., were formed at great depths of about 360 – 750 km. Inclusions in these diamonds are, along with silicate minerals, iron-nickel alloy, iron-nickel carbide and sulfide (pyrrhotite). The present study is devoted to studying the model growth environment of a diamond in the Fe-C-S system with a sulfur content of 3 wt. % in relation to iron. The experiments of 0.5 hours duration were carried out at 6 GPa and 1450 С on a high-pressure apparatus of "cutting sphere" type. As a result, diamond synthesis was obtained. The following phases were recorded during the analysis of growth medium composition (metal-sulfide sintering): solid solution of carbon in iron, iron sulfide, iron carbide. Iron sulfide is represented by pyrrhotite. Thus, the phases established in solid products of the experiments fully correspond to the phases isolated from inclusions of natural diamonds.

Phase change and graphitization of diamonds from the Popigai impact crater (Krasnoyarsk Territory, Siberian platform, Russia) exposed to high-pressure high-temperature (HPHT) conditions of 5.5 GPa and 2000–2200 °C are studied by Raman spectroscopy and X-ray diffractometry (XRD). Light-color diamonds of type 1, free from inclusions, with 0 to 10 % lonsdaleite, are more resistant to HPHT effects than dark diamonds of type 2 rich in lonsdaleite and graphite. The lonsdaleite/diamond ratios in lonsdaleite-bearing impact diamonds become smaller upon annealing, possibly because lonsdaleite transforms to cubic diamond simultaneously with graphitization. Therefore, lonsdaleite is more likely a structure defect in diamond than a separate hexagonal phase.

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