Nitrogen and hydrogen aggregation in natural octahedral and cuboid diamonds

Abstract

Transformations of nitrogen and hydrogen defects in natural octahedral and cuboid diamonds after multi-stage annealing at P = 6 GPa and T = 2200°C are presented. It has been observed that nitrogen aggregation from A- to B-defects in octahedral diamonds has gradually increased. This transformation of nitrogen has proceeded more rapidly in cuboid diamonds. One may confirm that most of the cuboid diamonds characterized by a low nitrogen aggregation state were not annealed over a long period at mantle conditions and their formation occurred shortly before transportation to the Earth’s surface. Unlike octahedral diamonds, cuboid diamonds show a considerable increase in the intensity of the primary hydrogen-related peaks after the annealing, thus implying involvement of sites to which the hydrogen can bond and become IR-active, but these sites are simply not available in the octahedral diamonds. The magnitude to which peak 3107 cm^–1 is increased after annealing has been found to be correlated with the total nitrogen content in cuboid diamonds. This supports the suggestion that the vibrational system with a primary line at 3107 cm^–1 corresponds to a C-H vibration bonded to some form of aggregated nitrogen.