Platelets, dislocation loops and voidites in diamond

Abstract

A type IaB diamond specimen containing partially decomposed platelets, dislocation loops and voidites has been investigated by transmission electron microscopy. The dislocation loops were found to be prismatic and interstitial in nature, some with Burgers vector ½ a <110> previously reported, but most with Burgers vector a <001>. Burgers vector analysis of the bounding dislocation of partially decomposed platelets shows that the a <001> loops are formed by transformation of the platelets, by nucleation and climb of a <00(1— f )> dislocation, combining with the a <00 f > dislocation bounding the platelet. The climb mechanism is driven by the need to generate vacancies for the decomposition of the platelets and to accommodate the nitrogen either in small clusters in solution in the lattice or in voidites. Glide dislocations interacting with the platelets are likely to act as nucleating centres for the climb process. The ½ a <110> dislocation loops are considered to be formed by dissociation of the a <001> loops, promoted by interaction with glide dislocations and involving prismatic slip and conservative climb. Voidites are assumed to originate as bubbles of fluid nitrogen formed at high pressure and temperature as a result of decomposition of the platelets; at room temperature they may be liquid or solid depending on the pressure, which cannot be estimated accurately. Electron diffraction patterns and microscope images of voidites prove that many consist of a solid phase at 300 K. It is suggested that the diamond has been subjected to a drop in pressure at high temperature, causing platelet decomposition and the generation of voidites, that may occur during ejection of the diamond to the earth’s surface.