Crystal forms and surface textures of alluvial diamonds from the Western Region of the Central African Republic

The most common crystal forms of the diamonds from the alluvial deposits in the Western Region of the Central African Republic were examined by scanning electron microscope (SEM) in an attempt to determine their geological history. The marks observed are related to two distinct periods in the geological history of the diamonds: the magmatic episode and their hydraulic transport. The effects of significant magmatic corrosion undergone by the diamonds during their ascent from the upper mantle are shown by the predominance of rhombododecahedral forms over octahedral forms and the frequent occurrence on the crystal faces of pyramidal depressions with triangular (111) or square (100) bases, as well as of V-shaped figures (111) or stepped figures (on the faces around the ternary axes). Some impact marks probably occurred during the explosive episode of kimberlite extrusion. Other impact marks, the marks of general wear, and the high proportion of gemstone-quality diamonds indicate the lengthy transport. They thus also indicate that the diamonds have undergone a prolonged geological history after the erosion of the kimberlite. This suggests that the kimberlite are separated from the Carnot Sandstone Formation by a considerable distance. The diamonds were stored in Albian-Maastrichtian rocks before they become concentrated in the Recent alluvium.