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

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 extended hexagonal stacking sequences within natural and laboratory-prepared samples. Our results show that comparable crystallographic structures may be developed by impact-generated shockwaves starting from ambient conditions using either of the two different allotropes of carbon (diamond, graphite). This broadens the scope for its occurrence in terrestrial and planetary systems. (C) 2016 Published by Elsevier B.V.