Volume 60 (2018) Issue 1 Pages 9-16
The crystallization process of diamond in the graphite-diamond transformation was studied through detailed microtextural observation of synthetic and natural nano-polycrystalline diamonds （NPDs） by transmission electron microscopy. The transformation occurs by two types of processes, diffusion-controlled （nucleation and growth） process and diffusion-less （martensite） process, which produce different types of microtextures, typically, granular and lamellar （layered） textures. We found that the transformation process and the microtexture of diamonds are largely influenced by the crystallinity of the graphite starting material. This allows us to produce a variety of NPDs with novel microtextures by using graphitic starting materials with various degrees of crystalinity. We also studied impact diamonds from a large meteorite crater as a natural counterpart of NPD and revealed that they formed mainly by the martensitic transformation from single crystalline graphite which were originally contained in the host metamorphic rock at the impact site. Shock-induced fragmentation of the source graphite and subsequent rapid transformation to diamond in the limited time scale result in multiple diamond nucleation and suppression of the overall grain growth, producing the unique nanocrystalline texture of natural NPD.