Revisiting claims of natural monocrystalline lonsdaleite: a re-assessment of published data

Abstract

This study re-evaluates the selected area electron diffraction (SAED) patterns and electron energy-loss spectrum (EELS) presented by Shumilova et al. (https://doi.org/10.1134/S1028334X11110201), who have reported that they have found natural hexagonal 2H diamond in samples from the Kumdykol (Kumdy-Kol) diamond deposit. A thorough re-evaluation of the original SAED data indicates that a diffraction pattern previously attributed to monocrystalline 2H diamond is, with a very high degree of certainty, not the claimed phase, since it exhibits a much stronger resemblance with the calculated pattern of a high-pressure phase of 2H graphite, and even more with the pattern of a cubic, high-pressure form of silicon carbide. Due to the absence of EDX data, the question regarding the precise composition of this crystalline species could not be conclusively resolved. Furthermore, a second SAED pattern, previously interpreted as a 3C–2H diamond intergrowth, was found compatible with a topotactic 2H graphite–3C mineral association, known as ‘diaphite’, or with sp³-bonded polytypes (3C–2nH, n = 2, 4). A carbon core-loss EEL spectrum, which was used in Shumilova et al. (Dokl Earth Sci 441:1552–1554, 2011) to confirm the presence of 2H diamond, was found to match with that of the 3C diamond structure. While these results do not rule out the natural occurrence of 2H diamonds in general, the re-assessment of the in Shumilova et al. (Dokl Earth Sci 441:1552–1554, 2011) published SAED and EELS data provides no concrete evidence for the presence of monocrystalline 2H diamond in the earlier examined specimens from the Kumdykol site. A correction of the in Shumilova et al. (Dokl Earth Sci 441:1552–1554, 2011) made claims is therefore of significance, to avoid further bias in the ongoing discussion on the nature of the mineral lonsdaleite.