After-coal diamonds: an enigmatic type of impact diamonds

Impact diamonds were discovered in the 70s and are usually accepted as being paramorphs after graphite, resulting in grains of extremely high mechanical quality. A diffusion-less mechanism for the graphite-to-diamond transition under huge pressure has been experimentally realized and theoretically explained. Besides, another type of impact product has received much less attention, namely diamonds formed after coal as a result of the impact. Here we describe after-coal impact diamonds from the giant Kara astrobleme (Pay-Khoy, Russia), which resulted from a large asteroid impact about 70 Ma ago. The impact created a large number of unusual impact diamonds, which are described here for the first time using high-resolution techniques including visible and UV Raman spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). Two main varieties of after-coal diamonds occur: micrograined (sugar-like, subdivided into coherent and friable) and, as a new type, paramorphs after organic relics. After-coal diamonds differ from after-graphite impact diamonds by the texture, the absence of lonsdaleite, a micro- and nanoporous structure. The sugar-like variety consists of tightly aggregated, well-shaped single nanocrystals. The after-organic diamond paramorphs are characterized by a well-preserved relict organic morphology, sub-nanocrystalline–amorphous sp³-carbon (ta-C) nanocomposites and other specific properties (optical transparence, brown color, very high luminescence, spectral features). Based on the description of after-coal diamonds, we propose a new, polystage formation mechanism: high-velocity coal pyrolysis with hetero-elements removal followed by diffusion-limited crystallization of pure carbon. The similarity of the after-coal diamonds features with carbonado is a strong piece of evidence in support of the impact hypothesis for the origin of carbonado.