A study of diamonds of cube and cube-related shape from the Jwaneng mine

Abstract

The Jwaneng mine in southwest Botswana is unusual inasmuch as about 8% of the diamonds produced have shapes which may be loosely described as “cubes”. Such shapes are normally very rare. The shapes vary from stones with heavily re-entrant cube faces and trigonally-symmetric Y-shaped octahedral faces, through more or less flat-faced cubes, to rounded cubes with growth protuberances at the cube corners. X-ray and cathodoluminescence topographic studies show that re-entrant cubes experienced mixed-habit growth with normal, faceted octahedral growth being confined to rather narrow sectors between which are regions of hummocky, non-crystallographic growth of mean orientation {100}, known as cuboid growth. Cuboid growth sectors contain regions with cloud-like light-scattering defects. Microscopic infrared absorption measurements and microscopic photoluminescence measurements, respectively, have shown that the occurence of the cloud-like defects correlates with the strength of the 3107 cm -1 absorption line and with a yellowish green luminescence band with a peak at about 525nm. There seems to be no correlation with the total nitrogen content as measured by the absorption coefficient at 1282 cm -1. The photoluminescence spectra measured at 77 K reveal sharp lines due to the S1 centre, which has zero-phonon lines at 503.4 nm (2.643 eV) and 510.7 nm (2.429 eV), and the S3 centre, which has a zero-phonon line at 496.7 nm (2.495 eV). Topographic evidence shows that rounded cubes are composed of normal, single-habit, octahedral growth surrounded by an overgrowth of cuboid form with narrow octahedral sectors producing the protuberances at the cube corners. Microscopic infrared measurements in the one-phonon region show that in the cuboid overgrowth the nitrogen is practically exclusively in the A form whereas in the octahedral core there is a significant amount of nitrogen in the B form, together with a platelet peak. These results suggest that rounded cubes underwent two distinct phases of growth separated, presumably, by a large time interval.