Microstructural observations of polycrystalline diamond: a contribution to the carbonado conundrum

The formation mechanism of carbonado, a polycrystalline variety of diamond, remains unresolved. Here we report a microstructural analysis of systematic defects and inclusions in carbonados from Brazil and the Central African Republic (CAR). Transmission electron microscopy (TEM) has revealed the presence of pervasive defect lamellae in both the CAR and Brazilian carbonados that are distinct from planar defects observed in synthetic polycrystalline diamond (PCD). Analyses of included minerals by energy dispersive spectroscopy (EDS) yielded strong evidence for metallic Fe, Ti, and Si, as well as SiC and Fe–Cr alloys, within the diamond matrix. Our results offer strong evidence for a close genetic relationship between Brazilian and Central African carbonados, suggesting formation within a united landmass during the late Archaean. The data do not preclude carbonado nucleation by meteoritic impact, though this issue remains contentious. The role of water in the growth of carbonado merits further exploration.