Plastic deformation of natural diamonds by twinning: evidence from X-ray diffraction studies

A pink-purple diamond crystal from the Internatsional'naya kimberlite pipe (Siberia) was studied by single-crystal X-ray diffraction techniques using an area detector. Direct indexing of the diffraction pattern suggested a primitive hexagonal unit cell [a(hex) = 2.513(4), c(hex) = 6.172(11) angstrom], instead of the well known face-centred cubic unit cell (a(cub) similar to 3.567 angstrom). Theoretical considerations and diffraction pattern simulation showed that the hexagonal diffraction pattern is the result of the superposition of two diffraction patterns with cubic symmetry due to spinel-law twinning along (111). These data are in good agreement with previous analyses of deformation microtwins in natural pink-purple diamonds using electron paramagnetic resonance spectroscopy and optical microscopy. The results suggest that natural epigenetic plastic deformation of diamonds occurs not only by dislocation slipping but also as a result of mechanical twinning.