Carbonado
From Wikipedia, the free encyclopedia
- For the town, see Carbonado, Washington.
Carbonado is a natural polycrystalline diamond found in alluvial deposits in the Central African Republic and Brazil. This diamond looks black and is highly porous. Unlike other natural polycrystalline diamonds, carbonado has no mantle-derived inclusions and its carbon isotope value is very low. Additionally, carbonado exhibits strong luminescence (photoluminescence and cathodoluminescence) induced by nitrogen and by vacancies existing in the crystal lattice. Analysis of the luminescence suggests that radioactive inclusions existed in the formation process of carbonado.
The origin of carbonado is controversial, but several hypotheses have been proposed:
- Direct conversion of organic carbon under high-pressure conditions (the Earth's interior)
- Shock metamorphism induced by meteoritic impact at the Earth's surface
- Radiation-induced diamond formation by spontaneous fission of uranium and thorium
These hypotheses, however, still have several problems. First, if carbonado were formed by phase transformation of organic graphite inside the Earth, carbonado would be found all over the world. But carbonado appears only in the Central African Republic and Brazil. Second, the formation process of a shock metamorphic event also has problems, because shock-induced natural polycrystalline diamonds usually have hexagonal diamond (lonsdaleite) inside the samples and the presence of hexagonal diamond inside carbonado has not been reported. The third hypothesis, radiation-induced diamond formation, is questionable as well, because the energy of radiogenic fission is too small to create polycrystalline diamond of the large grain size of carbonado (up to 500 micrometers). These diamonds are currently being used in various designs and settings in the jewelry industry.
An analysis in 2006[1] indicates these diamonds formed in a hydrogen-rich interstellar environment, that is, outside the Solar System, or before the formation of the planets. In this sense, these diamonds are akin to carbon-rich cosmic dust, likely having formed in an environment near carbon stars. The diamonds were incorporated into solid bodies that subsequently fell to Earth as meteorites.
