The name carbonado, meaning burnt or carbonized in Portuguese, was given to this material upon its discovery in Brazil in the 1840's. This dark gray to black variant of diamond has been mined in the Bahia Province of Brazil since that time. Due to its much greater hardness than typical diamond, it is used industrially for drills and for edges in cutting implements. Carbonado was subsequently found to occur in the same sedimentary geological horizon on a separate continentin the Ubangui region of the Central African Republic, and has also been reported to occur in Venezuela and the Soviet Union. Carbonado diamond is unique to these limited regions, and it has never been found during any conventional diamond mining and processing operations around the world. The largest recorded carbonado was found in Brazil and weighed 3,167 carats (633.4 grams).
In the ongoing debate to explain the circumstances surrounding the terrestrial occurrence of this unusual diamond, the theory suggesting its arrival during a Precambrian (late Archaean) impact event has been gaining ground. This conjectured impact delivery to Earth occurred 2.63.8 b.y. ago, at a time when the present-day continents of South America and Africa formed a unified supercontinent. A magnetic anomaly discovered in Central Africa, encompassing an area over 700,000 km², has also been dated to the Precambrian time. This impact feature, the largest known on Earth, might be associated with the delivery of the carbonado diamonds.
In a contrary opinion, Master (1998) has proposed that the Precambrian feature known as the Kogo Structure in Equatorial Guinea (1° 11' N., 10° 1' E.), measuring 4.67 km in diameter, would have been situated exactly between the Brazilian and Central African carbonado source locations at the time of its occurrence. He determined that a direct relationship exists between the size of the carbonados and their distance from the Kogo Structure, and he suggests that this is the probable impact point from which these diamonds were disseminated.
As described by Garai et al. (2006), carbonados are porous (~30%), polycrystalline aggregates of sub-µm- to µm-sized, mostly cuboid-shaped, diamond crystallites. The nodules have a polished surface rind reminiscent of a fusion crust. The interior is highly vesiculated, with some vesicles measuring up to 1 mm in size. They believe that entry- and impact-related processes, including partial ablation at extremely high temperatures in Earth's primordial, oxygen-poor atmosphere, melted an initially porous texture. Thereafter, secondary mineralization filled the remaining surface pores with silica-based minerals. In their magnetization studies of carbonado, Kletetschka et al. (2000) found that magnetic carriers are only present at the smooth surface of carbonado nodules, and that they are completely non-magnetic throughout the interior. This suggests that the magnetic carriers were not present at the initial site of carbonado formation, but instead, were added during the ablation process in Earth's atmosphere. This finding is contrary to what one would find given an origin within Earth's crust or mantle.
While the C- and N-isotopic compositions and N abundances of carbonados recovered from Central Africa and Brazil are indistinguishable from each other, they are unlike any known terrestrial-sourced diamond varieties (Shelkov et al., 1995); however, these unusual values, such as the isotopically light C content and the low N abundances, are consistent with an extraterrestrial origin.
Spectroscopic analyses of mineral inclusions in carbonado diamond matrices from Brazilian and Central African source locations reveal the presence of highly reduced metals and metal alloys, including Fe, Si, Ti, Ni, Ag, FeNi-metal, FeCr-metal, and NiCr-metal, as well as the carbide SiC (De, 1998; Garai et al., 2006 and references therein). This provides further evidence for an extraterrestrial origin for carbonado. The mineral osbornite (TiN) has also been identified in carbonado, a mineral previously found only in certain meteorites and recently acquired through NASA's Stardust mission to comet Wild 2 (Haggerty et al., 2006).
Theories of Origin
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 CH 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 HarvardSmithsonian 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 1015% 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, PBSNOVA). 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, 4043]).
Notably, the PbPb 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.