Abstract We investigate the influence of short-range Mg–Fe cation ordering on the elastic and vibrational properties of olivine (Fe x x Mg 1- x ) 2 SiO 4 (0 < x < 1) and ferropericlase (Fe x x Mg 1- x )O (0 < x < 1) solid solutions using atomistic simulations based on empirical force fields. Supercells representing distinct local distributions of Mg and Fe cations around equiatomic composition (Fe:Mg 50:50, x 0.5) were generated and structurally relaxed. Short range order states include cation clustering, random cation ordering and preferred heteroatom paring. In both solid solution series, bulk physical properties, such as elastic moduli and seismic velocities, are primarily controlled by overall composition rather than the specific local cation arrangement. However, vibrational properties such as heat capacity and the phonon dispersion curves reveal a stronger sensitivity to short-range order. This sensitivity is enhanced in structurally simpler ferropericlase, while the increased structural complexity of olivine suppresses these differences.

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 sp 3 -bonded polytypes (3C–2 n H, 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.