On the growth of natural octahedral diamond upon a fibrous core

International audienceWe document an unusual natural diamond by optical microscopy, X-ray topography, cathodoluminescence, infrared absorption and chemical analysis of the inclusions. These data prove that this diamond underwent a stage of fibrous growth before a stage of layered, octahedral growth, which is a succession of events reverse to that of the common “coated” diamonds. It is the first time that fibrous growth is described as occurring prior to regular octahedral growth. We propose three models to explain this. First, the fibrous core could have grown in a medium of high driving force, and then brought into an other medium of lower driving force where the layered, octahedral rim developed. Second, the fibrous diamond could have grown in an upcoming kimberlite that paused, so that the diamond could remain in the stability field of diamond in the upper mantle, and grow an octahedral layer; then, a second kimberlite transportation brought the whole diamond up to the surface. Third, the diamond could have grown from a single reservoir, e.g. a kimberlite fluid front, initially very favorable to diamond growth (fast, fibrous growth), but becoming progressively less favorable, so that the growth mode at some critical point switched to the slower layered growth