Edward P. Vicenzi

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

Abstract 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.

High-temperature phases in ternary Zr–O–N systems

Zirconium aerosol was ignited and burned in atmospheric pressure air in microgravity using a 2.2-s drop tower. Combustion products were collected and analyzed using electron microscopy. The elemental composition analyses indicated that combustion product compositions fell along two linear traces on a ternary Zr–O–N diagram. Currently, the equilibrium Zr–O–N phases are not characterized at temperatures above 2000 °C, typical of zirconium combustion in air, and it is suggested that the phases detected in zirconium combustion products can serve as a guide to further studies of the Zr–O–N system at high temperatures. It is also suggested that experimental metal combustion techniques can be adopted for studying high-temperature metal–gas phase equilibria.