Carbonado
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Carbonado
Carbonado
Sometimes cut as gemstones - often requiring lasers -, but have a granular appearance. Usually cracked in high-pressure presses for industrial usage.
Three carbonados from the Central African Republic
General
CategoryNative minerals
Formula
(repeating unit)
C
Crystal systemIsometric-hexoctahedral (cubic)
Identification
Formula mass
ColorTypically black, can be grey, various shades of green and brown sometimes mottled.
Crystal habitPolycrystalline
FractureIrregular torn surfaces
Mohs scale hardness10
LusterAdamantine
StreakWhite
Specific gravity
Density3.5-
Polish lusterAdamantine
BirefringenceNone
PleochroismNone

Carbonado, commonly known as black diamond, is one of the toughest forms of natural diamond. It is an impure, high-density, micro-porous form of polycrystalline diamond consisting of diamond, graphite, and amorphous carbon, with minor crystalline precipitates filling pores and occasional reduced metal inclusions.[1] It is found primarily in alluvial deposits where it is most prominent in mid-elevation equatorial regions such as Central African Republic and in Brazil, where the vast majority of carbonado diamondites have been found. Its natural colour is black or dark grey, and it is more porous than other diamonds.

Unusual properties

Carbonado diamonds are typically pea-sized or larger porous aggregates of many tiny black crystals. The most characteristic carbonados are mined 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; yet carbonado is found in younger sedimentary rocks.[2]

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 contain authigenic inclusions of minerals characteristic of the Earth's crust; the inclusions do not necessarily establish formation of the diamonds in the crust, but because while the obvious crystal 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.[2]

Isotope studies have yielded further clues to carbonado origin. The carbon isotope value is very low (little carbon-13 compared to carbon-12, relative to typical diamonds).[2]

Carbonado exhibits strong luminescence (photoluminescence and cathodoluminescence) induced by nitrogen and by vacancies existing in the crystal lattice. Luminescence halos are present around radioactive inclusions, and it is suggested that the radiation damage occurred after formation of the carbonados,[3] an observation perhaps pertinent to the radiation hypothesis listed below.

Hypotheses for origin

The origin of carbonado is controversial, and some proposed hypotheses are as follows:

  1. Direct conversion of organic carbon under high-pressure conditions in the Earth's interior, the most common hypothesis for diamond formation
  2. Shock metamorphism induced by meteoritic impact at the Earth's surface
  3. Radiation-induced diamond formation by spontaneous fission of uranium and thorium
  4. accumulated local formation in reduced organic-rich sediment over long geologic periods due to pyrometamorphic-rapid processes associated with superbolt-long duration lightning strikes, known to be coincident in global distribution with carbonado diamondite deposits, at similar elevations.
  5. Formation inside an earlier-generation giant star in our area, that long ago exploded in a supernova.[4]
  6. An origin in interstellar space, due to the impact of an asteroid, rather than being thrown from within an exploding star.[4]

None of these hypotheses for carbonado formation had come into wide acceptance in the scientific literature by 2008.[5] 13C/12C ratios in carbonado are identical to local sediments.

Extraterrestrial origin hypothesis

Supporters of an extraterrestrial origin of carbonados propose that their material source was a supernova which occurred at least 3.8 billion years ago.[6][7] 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. It possibly fragmented during entry into the Earth's atmosphere and impacted in a region which would much later split into Brazil and the Central African Republic, assumed to be the only two known locations of carbonado deposits (which is not an accurate representation of the true distribution of carbonado diamondite).

See also

References

  1. ^ Kroschwitz], [executive editor, Jacqueline I. (2004). Kirk-Othmer encyclopedia of chemical technology (5th ed.). Hoboken, N. J.: J. Wiley. p. 10. ISBN 9780471484943.
  2. ^ a b c Heaney, P. J.; Vicenzi, E. P.; De, S. (2005). "Strange Diamonds: the Mysterious Origins of Carbonado and Framesite". Elements. 1 (2): 85. doi:10.2113/gselements.1.2.85.
  3. ^ Kagi, H., Sato, S., Akagi, T., and Kanda, H., 2007 (2007). "Generation history of carbonado inferred from photoluminescence spectra, cathodoluminescence imaging, and carbon-isotopic composition" (PDF). American Mineralogist. 92: 217-224. Bibcode:2007AmMin..92..217K. doi:10.2138/am.2007.1957.CS1 maint: multiple names: authors list (link)
  4. ^ a b Garai, Jozsef; Haggerty, Stephen E.; Rekhi, Sandeep; Chance, Mark (2006). "Infrared Absorption Investigations Confirm the Extraterrestrial Origin of Carbonado Diamonds". The Astrophysical Journal. 653 (2): L153. arXiv:physics/0608014. Bibcode:2006ApJ...653L.153G. doi:10.1086/510451. Archived from the original on 2007-08-09.. This study suggested that infrared absorption spectra of carbonado are similar to diamonds of extraterrestrial origin; selected significant peaks are due to trace abundances of the elements nitrogen and hydrogen. The researchers concluded with the assumption that the mineral necessarily formed in an interstellar environment. In this sense, carbonado are theorized to be akin to carbon-rich cosmic dust, likely having formed in an environment near carbon stars. The diamonds were suggested to have been fragments of a body of asteroid size that subsequently fell to Earth as meteorites.
  5. ^ Rondeau, B; Sautter, V; Barjon, J (2008). "New columnar texture of carbonado: Cathodoluminescence study". Diamond and Related Materials. 17 (11): 1897. Bibcode:2008DRM....17.1897R. doi:10.1016/j.diamond.2008.04.006.
  6. ^ Broad, William J. (1996-09-17). "Giant Black Diamonds Of Mysterious Origin May Hail From Space". The New York Times. ISSN 0362-4331. Retrieved .
  7. ^ Garai, Jozsef; Haggerty, Stephen E.; Rekhi, Sandeep; Chance, Mark (2006-12-20). "Infrared Absorption Investigations Confirm the Extraterrestrial Origin of Carbonado-Diamonds". The Astrophysical Journal. 653 (2): L153-L156. arXiv:physics/0608014. Bibcode:2006ApJ...653L.153G. doi:10.1086/510451. ISSN 0004-637X.

External links


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Carbonado
 



 



 
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