CUBO-OCTAHEDRAL GROWTH STRUCTURE IN A TYPE IB DOMINANT MIXED TYPE DIAMOND: A RARE DIAMOND FORMATION

Cubo-octahedral growth (cubic-octahedral mixed habit) is the typical structure for HPHT synthetic diamonds which are grown in a laboratory with fast growth rate (i.e. less than a week per carat) under P-T conditions of 5-6 GPa and 1300-1500 ºC. A natural diamond with a fast growth rate would also possess cubo-octahedral growth (cuboid-octahedral mixed habit) structure. In this research, we studied a Fancy Dark brown, 0.41 ct, gem-quality diamond with cubo-octahedral growth structure. It’s FTIR spectrum reveals that it is a mixed type (Ib > IaAB) diamond.

Nitrogen atoms are initially incorporated as single substitutional atoms (N_s⁰) during diamond formation. Time and temperature are critical in aggregation of nitrogen atoms. Long mantle residency time would cause nitrogen aggregation. Our sample is rich in single nitrogen atoms or ‘C defects’ with A (two nitrogen atoms) and B (four nitrogen atoms) aggregates. It suggests that it ascended to shallower depths in the mantle at lower temperatures <850 ºC by rapid tectonic exhumation (Smit et al., 2018) after forming minor nitrogen aggregates at higher temperatures. The cubo-octahedral growth structure of this diamond indicates that, prior to ascending to shallower depths of mantle, it grew rapidly in the diamond stability field, i.e. 150-200 km in depth within the Earth, compared to most gem-quality octahedral diamonds with slower growth rate.

The FTIR spectrum also shows radiation-related peaks at 1450 (H1a, di-nitrogen <001>-split interstitial) and 4935 (H1b, a complex form containing A aggregate) cm^-1. Therefore, this diamond was irradiated and annealed to achieve Fancy color grade. However, presence of amber center suggests that it was not experienced HPHT treatment. Hence, A and B aggregates are not modified or created by color-enhanced treatment.