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Theories of Origin

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  Over the years, various non-impact theories have been put forth to 
explain the formation of these enigmatic diamonds. Some of these include 
1) chemical vapor deposition (CVD), 2) irradiation of carbonaceous 
material by highly energetic particles such as U and Th, 3) subduction 
of crustal organic matter into the mantle, and 4) impact metamorphism of 
Archaean rock containing concentrated organic biomass. In their studies 
of carbonado, Yokochi et al. (2008) found that the cathodoluminescence 
spectra and the 40Ar values are inconsistent with an impact-generated 
formation for this diamond, and that the relatively low concentration of 
Ar is inconsistent with a CVD origin of carbonado diamond. Ozima and 
Zashu (1991) conducted noble gas isotopic studies in order to study the 
mechanism of diamond formation based on irradiation by energetic 
particles. Their results did support the theory of diamond formation 
through high-energy irradiation. They found that carbonado contains an 
abundance of 4He, consistent with implantation through external 
radioactive decay processes (Ozima et al., 1991). Robinson (1998) and 
Vicenzi and Heaney (2001) studied the formation theory based on the 
subduction of a slab containing organic sediments. They determined that 
the C and N isotopes and the N abundances have values that do not 
support such an origin for carbonado. Moreover, it is not understood how 
such diamonds could eventually end up in placer deposits, or why their 
sizes should only reach to µm-size. Based on their examinations of 
carbonado and their discovery of lonsdaleite in one nodule from Yakutia 
(Soviet Union), Smith and Dawson (1985) favor the theory of impact 
metamorphism of Archaean crustal rock containing organic carbon or 
graphite. However, no evidence of high-pressure phases of silica such as 
coesite and stishovite have been identified in association with quartz.

An extraterrestrial origin for carbonado is supported by recent 
experiments utilizing Fourier transform infrared (FTIR) spectroscopy 
(Haggerty et al., 2006; Garai et al., 2006; Garai, 2012). By exposing 
carbonado to intense infrared light, they observed peaks primarily 
corresponding to C–H stretching of diamond hydride, spectra almost 
identical to that of presolar diamonds found in some meteorites. They 
assert that the hydrogen serves as the bonding agent (protonation) that 
sinters the microdiamonds together to form the carbonado. They argue 
that carbonado diamond is consistent with an origin in a hydrogen-rich 
environment similar to that of the solar nebula. Moreover, the presence 
of nitrogen mono-hydride substitution for C is more similar to that 
found in presolar diamonds than to terrestrial diamonds, providing a 
clear argument for an extraterrestrial origin. This substitution is 
inconsistent with the conditions under which conventional diamonds are 
formed, i.e., slow cooling over millions of years at high pressures.

As with the nanodiamonds present in some meteorites, carbonado diamonds 
may have also been produced in a supernova explosion. Over time, they 
would be accreted into a planetesimal or perhaps an iron core, as 
attested by their presence in the Canyon Diablo iron. On the other hand, 
perhaps the abundant carbonado that was produced broke up into 
asteroid-sized masses. The Florida International University and Case 
Western Reserve University research team has proposed that a 
carbonado-rich asteroid measuring ~1 km in diameter impacted the Earth 
billions of years ago when Africa and South America were part of a 
single supercontinent. They argue that the vesiculation present in 
carbonado was caused by gases escaping under conditions of low-pressure 
during formation, conditions which are inconsistent with the very high 
pressures existing at diamond formation depths of >150 km, but which do 
exist in space.

Other exotic mechanisms of carbonado formation have been proposed over 
the years. Scientists from Princeton University have postulated the 
existence of diamond layers within extrasolar carbon-rich planets. 
Haggerty (1996), supported by observations of astronomers from the 
Harvard–Smithsonian Astrophysical Observatory, presumes that carbon was 
transformed into diamond by the intense shock waves generated during the 
explosive collapse of a red giant star, resulting in a white dwarf and 
its accompanying planetary nebula. White dwarf stars constitute ~6% of 
the stars in the solar neighborhood. Either of these mechanisms of 
diamond formation may have resulted in the injection of massive diamond 
asteroids into the protosolar cloud which become gravitationally 
attracted to Earth.

Closer to home, the ice-giant planets Uranus and Neptune are considered 
by some planetary scientists to potentially produce diamond from 
methane, which constitutes 10–15% of their dense atmospheres. This 
diamond may surround the planets' cores. It was experimentally proven by 
M. Ross at Lawrence Livermore National Laboratory that the conversion of 
methane to diamond did occur at the high-temperature (at least 1649°C) 
and high-pressure (at least 200,000 Earth atmospheres) conditions that 
exist on Uranus and Neptune (P. Tyson, Diamonds in the Sky, PBS–NOVA). 
Under the ultrahigh-temperature and ultrahigh-pressure conditions that 
exist on Uranus and Neptune, oceans of liquid diamond with solid chunks 
of diamond floating atop are thought to be plausible, in a manner 
similar to the unusual behavior of water and its less-dense form of ice. 
This possibility was described in an article in the journal Nature 
Physics (Eggert et al., 2010 [1 January, vol 6, 40–43]).

Notably, the Pb–Pb age of carbonado coincides with the period of Solar 
System history known as the Late Heavy Bombardment, during which time it 
is thought that the gas giant planets Jupiter and Saturn, and the ice 
giant planets Uranus and Neptune, underwent orbital migrations under the 
influence of mutual resonances. Among the effects of the resulting 
gravitational instabilities was the perturbation of the smaller 
planetesimals into eccentric orbits, eventually leading some to 
intersect with the terrestrial planets. Significant collisions with 
Uranus and Neptune would be likely to occur at this time, and it has 
been argued that the large obliquity of Uranus is the result of a severe 
tangential collision with an Earth-sized proto-planet early in its 
history (Brunini, 2006). It may be conjectured that this unique 
cataclysmic event might also be responsible for the delivery of 
carbonado into an Earth-crossing orbit at this time.

While the exact origin of carbonado remains a mystery, the accumulated 
evidence for an extraterrestrial origin, or even possibly an extrasolar 
origin, is quite convincing. Much more study is required. The photo 
above shows a 1.07 carat (0.21 g) Brazilian carbonado nodule measuring 
6.1 × 5.5 × 4.3 mm and exhibiting a shiny, porous surface

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