On the origin of the rounded dodecahedral habit of natural diamond

Abstract

The morphology of natural rounded rhombic dodecahedral diamonds has been studied optically and by scanning electron microscopy, and their internal structures have been investigated by X-ray topographic techniques. That this rounded habit arises from dissolution of a sharp-edged octahedral growth form is indicated both by theoretical considerations and experimental evidence. The hypothesis that these diamonds grew throughout their history as rounded rhombic dodecahedra is directly refuted by the X-ray topographic studies. The information on dislocation distribution revealed by X-ray topographs also provides strong evidence against a current variant of the growth hypothesis, that the rounded rhombic dodecahedral habit developed upon an octahedral core. The manner by which dissolution of surfaces of rounded dodecahedral diamonds proceeds can be described on an atomic scale by motion of kinks along stable monomolecular steps parallel to 〈110〉 directions. It is shown that this process leads to the development of edges parallel to the minor diagonals of the rhombic surfaces, in accord with observation.