  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.
