Growth and post-growth defects in a diamond from Finsch mine (South Africa)

A diamond from the Finsch mine (South Africa) was investigated by X-ray diffraction topography to characterize the extended defects and to reconstruct the growth history. The sample was a doubly polished triangular diamond slice with a rounded side in which re-entrant corners could be seen and the specimen exhibited anomalous birefringence. X-ray topographs show that the sample is actually a mosaic crystal that developed through aggregation of sub-individual parts during growth: some slightly rotated relative to one another and others twinned. The twin law is by reticular merohedry and consists of the well-known 2-fold rotation around [111]. Micro-Raman spectroscopy data revealed that the main inclusions are pyrope and enstatite trapped in the inner part of the individual. No dislocations nucleated from the silicate inclusions. Growth-sector boundaries, dislocations, grain boundaries and twinning indicate that these features formed during the growth of diamond. Throughout the whole sample and chiefly in the regions showing a lower density of inclusions, packed lamination lines parallel to an octahedral face can be observed. These laminations represent a polysynthetic twinning commonly observed in diamonds which have undergone plastic deformation and are considered to be a post-growth defect. The analysis of the structural defects allows a reconstruction of a complex growth history, characterized by a sequence of alternating episodes of growth and dissolution and, lastly, plastic deformation. This study represents a methodological contribution to the studies on the origin of diamond.