Crystallogenetic Causes of the Unique Shape of the Matryoshka Diamond: The Effect of Capturing a Diamond Inclusion of Twin Diamond Crystals

The unusual shape of the Matryoshka diamond, which is a diamond crystal with a cavity containing a diamond crystal freely moving in it, continues to attract keen interest of many researchers in the context of its seemingly paradoxical origin for a mantle mineral. The discovery sparked lively discussions and multiple attempts to explain the nature of the unique shape of this crystal. A comprehensive mineralogical and crystallographic analysis of the unusual specimen, as well as other analogous diamond crystals, suggests that it was formed as a consequence of the mutual disorientation of the crystals during their growth and the presence of a twin of diamond subindividuals that formed both the core (inclusion) and the sheath (host) diamonds. The twinning planes (111) in the contacting crystals of the inclusion and the host were in a sub-perpendicular position to each other during their simultaneous growth. The captured diamond of the inclusion prevented the normal development of the diamond that became the host. The diamond host rapidly grew along the direction of its own twin boundary and constantly generated new growth layers, which eventually converged around the small captured twin crystal cluster during metric selection. Analysis of diamond crystals of similar shape from the Nyurbinskaya pipe and from elsewhere worldwide confirms the ontogenic model of their origin as a consequence of the capture of diamond inclusions that hampered the rapid growth of the twin crystal (spinel-law twins) in the direction of the twin boundary.