Microanalytical studies of carbonado and synthetic polycrystalline diamond

Abstract

Transmission electron microscopy of carbon of diamond from Brazil and Central African Republic of mysterious origin, has revealed that euhedral diamonds are embedded in a finer grained diamond matrix. Characteristic defect lamellae are observed in both kinds of diamond grains. In addition, TEM has revealed the presence of inclusions such as Ti, SiC, florencite, and metallic Fe in both Brazilian and Central African carbonados. These microstructural similarities between the carbonados from the two locations indicate that they probably were produced by the same process during the Archean when the African and the South American continents were joined.

Carbonado is characterized by unusual carbon isotope compositions with bulk δ¹³C values clustered tightly between –23‰ and –30‰. In situ SIMS analyses of a Central African carbonado has revealed a bimodal distribution of δ¹³C values of –24‰ and –26‰. These δ¹³C distributions are correlated with variations in nitrogen abundance and with cathodoluminescence emission signatures. We believe that the bimodal microstructural and geochemical signatures are evidences for an initial slow-growth phase followed by a period of rapid diamond cementation.

In order to constrain the formation conditions of carbonado, we have compared the microstructures in sintered diamond compacts with those in carbonado. Compacts produced at 8 GPa and 1200°C exhibited defect lamellae and polygonalized networks that were very similar to features observed in carbonado, whereas lower-pressures did not succeed in replicating carbonadolike textures. Microstructures in the diamond compact synthesized from graphite exhibited stacking faults, which are absent in both carbonado and the sintered diamond compacts. Consequently, we conclude that carbonado was not produced by direct compression of graphite.

In order to assess the affects of oxygen fugacity on diamond growth, we investigated diamond precipitation under the catalytic influence of iron oxides and magnesium carbonate. Micro-Raman spectroscopy demonstrated that wüstite enhances diamond growth under the experimental conditions of 8 GPa and 1300°C. We propose that oxygen fugacity in the sample assembly reduced wüstite to metallic iron, which catalyzed the growth of diamond. Hematite, magnetite, and magnesite did not promote diamond growth. Our experiments also revealed that surface roughness in seed crystals is not an indicator for diamond growth.