Quantifying hexagonal stacking in diamond

Mara Murri,Christoph G Salzmann,Sergey A Vishnevsky,Maria C Domeneghetti,Kit Mccoll,Matteo Alvaro,Furio Corà,Paul F Mcmillan,Adrian P Jones,Martin Hart,Nikolay V Sobolev,Fabrizio Nestola,Alla M Logvinova,Rachael L Smith,Péter Németh

doi:10.1038/s41598-019-46556-3

Abstract

Diamond is a material of immense technological importance and an ancient signifier for wealth and societal status. In geology, diamond forms as part of the deep carbon cycle and typically displays a highly ordered cubic crystal structure. Impact diamonds, however, often exhibit structural disorder in the form of complex combinations of cubic and hexagonal stacking motifs. The structural characterization of such diamonds remains a challenge. Here, impact diamonds from the Popigai crater were characterized with a range of techniques. Using the MCDIFFaX approach for analysing X-ray diffraction data, hexagonality indices up to 40% were found. The effects of increasing amounts of hexagonal stacking on the Raman spectra of diamond were investigated computationally and found to be in excellent agreement with trends in the experimental spectra. Electron microscopy revealed nanoscale twinning within the cubic diamond structure. Our analyses lead us to propose a systematic protocol for assigning specific hexagonality attributes to the mineral designated as lonsdaleite among natural and synthetic samples.

Highlights

Impact cratering is one of the most common geological processes resulting in accretion and remodelling of planetary surfaces, and contributing to the development of their atmosphere and even biological evolution
A form of diamond exhibiting hexagonal features in its X-ray diffraction pattern was described by Bundy and Kasper for a phase produced by high-pressure high-temperature (HPHT) treatment of graphite[4]
That description was based on the observation of a doubled feature in the X-ray diffraction pattern that could well be interpreted as two peaks of coexisting cubic diamond structures formed within different strain regimes that can occur during shock processes

Summary

Introduction

Impact cratering is one of the most common geological processes resulting in accretion and remodelling of planetary surfaces, and contributing to the development of their atmosphere and even biological evolution. The extreme P-T conditions generated can cause melting and even vaporization of refractory phases, and result in structural and phase transformations among minerals of the impacted rocks One such mineralogical marker is the presence of features indicating hexagonal symmetry in the X-ray diffraction patterns of diamonds recovered from the impact site[2,3]. The hexagonal diamond form was associated with the 2H stacking polytype of elemental carbon by analogy with wurtzite vs sphalerite structures of tetrahedrally bonded compounds such as SiC and ZnS It was assigned the mineral name lonsdaleite in recognition of the contributions of Kathleen Lonsdale to crystallography[2]. That description was based on the observation of a doubled feature in the X-ray diffraction pattern that could well be interpreted as two peaks of coexisting cubic diamond structures formed within different strain regimes that can occur during shock processes. The reflections attributed to hexagonal diamond are consistent with nanotwinned cubic diamond reported by Németh et al.[11]

Methods

Results

Discussion

Conclusion