Structural characterization of natural diamond shocked to 60 GPa; implications for Earth and planetary systems

Abstract

The possible presence of the high-density carbon polymorph with hexagonal symmetry known as "lonsdaleite" provides an important marker for shock impact events. It is typically considered to form as a metastable phase produced from graphite or other carbonaceous precursors. However, its existence has recently been called into question. Here we collected high-resolution synchrotron X-ray diffraction data for laboratory-shocked and natural impact diamonds that both show evidence for deviations from cubic symmetry, that would be consistent with the appearance of hexagonal stacking sequences. These results show that hexagonality can be achieved by shocking diamond as well as from graphite precursors. The diffraction results are analyzed in terms of a general model that describes intermediate stacking sequences between pure diamond (fully cubic) and "lonsdaleite" (fully hexagonal) phases, with provision made for ordered vs disordered stacking arrangements. This approach provides a "hexagonality index" that can be used to characterize and distinguish among samples that have experienced different degrees of shock or static high pressure-high temperature treatments. We have also examined the relative energetics of diamond and "lonsdaleite" structures using density functional theoretical (DFT) methods. The results set limits on the conditions under which a transformation between diamond and "lonsdaleite" structures can be achieved. Calculated Raman spectra provide an indicator for the presence of

Incipient plasticity of diamond during nanoindentation

“…The previous experimental and computational results demonstrated that the cubic diamond could transform into lonsdaleite phase under non-hydrostatic loading conditions. 18,[56][57][58] In fact, phase transition was rstly considered by Ruoff et al as a possible failure mode for a diamond at, which was pressed by a spherical diamond indenter. 59 Later on, by coupling the indentation loading with micro-Raman spectroscopy, Gogotsi et al directly observed the phase transitions of diamond during indenting process, including the transition of cubic diamond to lonsdaleite (hexagonal diamond) and amorphous diamond phase.…”

Transformations and Decomposition of MnCO3 at Earth's Lower Mantle Conditions

“…Carbon in the Earth may exist in various forms including carbides, diamond, graphite, lonsdaleite, hydrocarbons, CO 2 , and carbonates depending on the oxygen fugacity and P-T conditions (Dasgupta and Hirschmann, 2010;Hazen et al, 2013;Jones et al, 2016). Although it is generally thought that the conditions of the Earth's deep interior and of the subducting slab materials may be not compatible with the stability of carbonates or carbonate-rich liquid (Anzolini et al, 2016;Thomson et al, 2016), the observation of carbonate inclusions in diamonds potentially brought up to the Earth's surface from the deep mantle indicates that carbonates can exist at least locally in the mantle (e.g., Stachel et al, 1998Stachel et al, , 2000Leost et al, 2003;Bulanova et al, 2010).…”

Natural Graphite Cuboids

“…However, only controversial findings of lonsdaleite have been reported in eclogites from the Kola Peninsula [14], glaucophane-bearing eclogites of the Maksyutov complex [15], and garnet-biotite gneiss from the Kokchetav massif [16]. In contrast to lonsdaleite, which is always found as aggregates with other carbon polymorphs [17], diamond and graphite commonly occur separately, although in some localities, these carbon polymorphs can be recognized in an intimate association, as well (see [18] and the references therein). According to [19], most diamonds nucleated heterogeneously on mineral seeds (e.g., sulphide, native iron, wustite, monocrystalline graphite), which could lower the energy barrier to nucleation of monocrystals.…”