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Carbonado, commonly known as the 'Black Diamond,' is a natural polycrystalline diamond found in alluvial deposits in the Central African Republic and Brazil. The carbonado was first recognized as a form of polycrystalline diamond as long ago as the 1840s, when it was discovered and placer mined in Brazil, S.America. It has since been found elsewhere, e.g., Venezuela, eastern Australia, and the Ubangui region of the Central African Republic, where it is known as `carbons' . A material called yakutite, perhaps related, is found in Russia. Its natural colour is black or dark grey, and it is more porous than other diamonds. 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. These and other characteristics that separate the carbonado from other diamonds have led to questions as to the carbonado's origin.
Generally it is found in dark masses of irregular polyhedral form, black, brown or dark-grey in colour, with a dull resinoid lustre; and breaking with a granular fracture, paler in colour, and in some cases much resembling that of fine-grained steel. Being slightly cellular, its specific gravity is rather less than that of crystallized diamond. It is found almost exclusively in the state of Bahia in Brazil, where it occurs in the cascalho or diamond-bearing gravel. Borneo also yields it in small quantity. Formerly of little or no value, it came into use on the introduction of Leschot's diamond-drills, and is now extremely valuable for mounting in the steel crowns used for diamond-boring. Having no cleavage, the carbon is less liable to fracture on the rotation of the drill than is crystallized diamond.
The origin of carbonado is controversial. Some proposed hypotheses are as follows:
- Direct conversion of organic carbon under high-pressure conditions in the Earth's interior, the most common hypothesis for diamond formation
- Shock metamorphism induced by meteoritic impact at the Earth's surface
- Radiation-induced diamond formation by spontaneous fission of uranium and thorium
- Formation inside an earlier-generation giant star in our area, that long ago exploded in a supernova.
- An origin in interstellar space, due to the impact of an asteroid, rather than being thrown from within an exploding star.
Supporters of an extraterrestrial origin of carbonados, such as Stephen Haggerty, a geoscientist from Florida International University, propose that their material source was a supernova which occurred at least 3.8 billion years ago. After coalescing and drifting through outer space for about one and a half billion years, a large mass fell to earth as a meteorite approximately 2.3 billion years ago, possibly fragmenting during entry into the Earth's atmosphere, and impacting in a region which would much later split into Brazil and the Central African Republic, the only two known locations of carbonado deposits.
Carbonado diamonds are typically pea-sized or larger porous aggregates of many tiny black crystals. The most characteristic carbonados have been found only in the Central African Republic and in Brazil, in neither place associated with kimberlite, the source of typical gem diamonds. Lead isotope analyses have been interpreted as documenting crystallization of carbonados about 3 billion years ago. The carbonados are found in younger sedimentary rocks.
Mineral grains included within diamonds have been studied extensively for clues to diamond origin. Some typical diamonds contain inclusions of common mantle minerals such as pyrope and forsterite, but such mantle minerals have not been observed in carbonado. In contrast, some carbonados do contain inclusions of minerals characteristic of the Earth’s crust: these inclusions do not necessarily establish formation of the diamonds in the crust, however, because while these obvious crustal inclusions occur in the pores that are common in carbonados, they may have been introduced after carbonado formation. Inclusions of other minerals, rare or nearly absent in the Earth’s crust, are found at least partly incorporated in diamond, not just in pores: among such other minerals are those with compositions of Si, SiC, and Fe‑Ni. No distinctive high-pressure minerals, including the hexagonal carbon polymorph, lonsdaleite, have been found as inclusions in carbonados, although such inclusions might be expected if carbonados formed by meteorite impact.