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CARBONADO ....

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Carbonado

Carbonado, commonly known as the "black diamond", is the toughest form of natural diamond. It is an impure form of polycrystalline diamond consisting of diamondgraphite, and amorphous carbon.[1] It is found primarily in alluvial deposits in the Central African Republic and in Brazil. Its natural colour is black or dark grey, and it is more porous than other diamonds.

Carbonado
Sometimes cut as gemstones by lasers, but have a granular appearance. Usually cracked in high-pressure presses for industrial usage.
Three carbonados from the Central African Republic
General
Category Native minerals
Formula
(repeating unit)
C
Crystal system Isometric-hexoctahedral (cubic)
Identification
Formula mass 12.01 u
Color Typically black, can be grey, various shades of green and brown sometimes mottled.
Crystal habit Polycrystalline
Fracture Irregular torn surfaces
Mohs scale hardness 10
Luster Adamantine
Streak White
Specific gravity 3.52±0.01
Density 3.5–3.53 g/cm3
Polish luster Adamantine
Birefringence None
Pleochroism None

 

 

 
Unusual propertiesEdit

The characteristics of carbonado noted in this section are based mainly on the summary of Heaney et al. (2005),[2] unless otherwise noted.

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 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 SiSiC, and FeNi. No distinctive high-pressure minerals, including the hexagonal carbon polymorphlonsdaleite, have been found as inclusions in carbonados, although such inclusions might be expected if carbonados formed by meteorite impact.

Isotope studies have yielded further clues to carbonado genesis. The carbon