Structural peculiarities, mineral inclusions and point defects in yakutites -- a variety of impact-related diamond

TL;DR

This study characterizes yakutites, impact-related diamonds from the Popigai crater, revealing their nanocrystalline structure, embedded single crystals, and mineral inclusions, advancing understanding of impact diamond formation and properties.

Contribution

It provides the first detailed analysis of yakutite's nanocrystalline structure, mineral inclusions, and formation mechanisms using multiple advanced characterization techniques.

Findings

Yakutites contain nanocrystalline diamond matrix with embedded single crystal grains.

Spectroscopic data reveal features of two-phonon infra-red absorption in nanodiamond.

Presence of ZrO2 inclusions indicates baddeleyite in the impact target rocks.

Abstract

An unusual variety of impact-related diamond from the Popigai impact structure - yakutites - is characterized by complementary methods including optical microscopy, X-ray diffraction, radiography and tomography, infra-red, Raman and luminescence spectroscopy providing structural information at widely different scales. It is shown that relatively large graphite aggregates may be transformed to diamond with preservation of many morphological features. Spectroscopic and X-ray diffraction data indicate that the yakutite matrix represents bulk nanocrystalline diamond. For the first time, features of two-phonon infra-red absorption spectra of bulk nanocrystalline diamond are interpreted in the framework of phonon dispersion curves. Luminescence spectra of yakutite are dominated by dislocation-related defects. Optical microscopy supported by X-ray diffraction reveals the presence of single crystal diamonds with sizes of up to several tens of microns embedded into nanodiamond matrix. The presence of single crystal grains in impact diamond may be explained by CVD-like growth in a transient cavity and/or a seconds-long compression stage of the impact process due to slow pressure release in a volatile-rich target. For the first time, protogenetic mineral inclusions in yakutites represented by mixed monoclinic and tetragonal ZrO2 are observed. This implies the presence of baddeleyite in target rocks responsible for yakutite formation.