Characterisation by X-ray diffraction of naturally occurring polycrystalline diamond samples from different origins

M.Phil. (Energy Studies)Non-destructive investigations of unusual natural polycrystalline diamond samples are reported. The samples originate from various international locations discovered in soils and sediments and may have been formed by di erent mechanisms which are the subject of debate. Analysis techniques include scanning electron microscopy, with energy dispersive spectroscopy and X-ray di raction. Sample composition, structure and surface stresses were investigated. The samples (two Egyptian nodules, two Brazillian carbonados, two Venetian ballas diamonds and a polycrystalline diamond compact sample consisting of a polycrystalline diamond layer sintered onto a Co-cemented tungsten carbide support) are cubic diamonds, with varying amounts of minority phases. In most cases the minority phases could not be identi ed unambiguously due to their low intensities and the overlap of Bragg peaks. The Egyptian nodule samples in particular showed a large presence of oxygen that could not be linked to speci c metal oxides. The Ballas samples had the lowest impurity content. Using a Rietveld based quanti cation analysis, the crystalline cubic diamond contents were determined respectively as 99 wt.%, 98 wt.% and 76 wt.% in the Ballas, Brazilian carbonado and Egyptian nodule samples. A signi cant result from this study is the substantial inhomogeneous peak broadening observed in the X-ray diffraction patterns of the Egyptian samples. This is indicative of the presence of nano-crystalline diamond grains in addition to substantial mechanical deformation with extensive accumulation of dislocations and crystalline defects. These observations are supportive of a recently proposed impact mechanism for their formation