Microstructural observations of polycrystalline diamond: a contribution tothe carbonade conundrum

The formation mechanism of carbonade, a polycrystalline variety of diamond, remains unresolved. Here we report a microstructural analysis of systemati c defects and inclusions in carbonades from Brazil and the Central African Republic (CAR). Transmission electron microscopy (TEM) has revealed the pre sence of pervasive defect lamellae in both the CAR and Brazilian carbonades that are distinct from planar defects observed in synthetic polycrystallin e diamond (PCD). Analyses of included minerals by energy dispersive spectro scopy (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 Afr ican carbonades, suggesting formation within a united landmass during the l ate Archaean, The data do not preclude carbonade nucleation by meteoritic i mpact, though this issue remains contentious. The role of water in the grow th of carbonade merits further exploration. (C) 1998 Elsevier Science B.V. All rights reserved.