Mysterious carbonados - diamonds from Space
by space rocks uk

Every year, many tonnes of dark grey to black Carbonado from Precambrian strata in Brazil and the Central African Republic are mined for industrial use. This opaque, slightly porous substance is as hard as crystallized diamond (Moh 10) but not as brittle, and is therefore ideal for cutting-tools and drill bits. Its value has traditionally been considerably less than gem-quality diamond, but recent, concerted advertising has seen a steady rise in the price of both cut and raw stones.

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rock and gem magazine issue 61 autumn 2013

The majority of Carbonados are pea-sized, amorphous pieces, but extremely large examples up to 633g (3,160 carats) have been discovered. When cut and polished, Carbonados lack the ‘sparkle’ of white diamonds, since their black colouration absorbs light rather than refracting it: nevertheless, when faceted, stones reflect light with a pleasing luster. Perhaps unsurprisingly, the blackest stones are particularly appealing and sought-after for male jewellery. Since their discovery in Brazil in the 1840s, there has been much debate over the origin of Carbonados: not the least because they are not associated with the usual diamondbearing formations and minerals. It has been suggested that subduction of organic material, irradiation or impact metamorphosis of ancient organic deposits might have produced Carbonados, but none of these can account for their distribution within sedimentary rock strata.

Spectrographical analysis has revealed the presence of TiN (only previously encountered in meteorites and in cometary material retrieved by NASA’s ‘Stardust’ program) Furthermore, the relative abundances of carbon and nitrogen isotopes are identical to those found in nanodiamonds in Ureilites and some iron meteorites. These, and other factors, make a terrestrial origin unlikely.

The majority of Carbonados are found in sedimentary deposits around three and a half billion years old, suggesting a simultaneous arrival on an existing land surface. At that time, the Atlantic basin had not yet been opened by continental drift, so that the two separate fields we see today were originally much closer together: it has been suggested that the sites are centred around a massive impact crater, possibly the 4.7km Kogo Structure in Equatorial Guinea.

Accepting an extraterrestrial origin, where did Carbonados originate in the first place? One possibility is that all pre-solar diamond was formed in supernovae. Giant stars can finish their ‘lives’ in spectacular explosions that project vast amounts of gas and debris into interstellar space. These clouds condense to form new, second generation stars like our Sun, but could also accrete to form diamond-seeded planetissimals that were the source of meteoric nanodiamonds.

At the same time, planet-sized chunks of diamond formed during the collapse phase of a supernova could be thrown off: perhaps a kilometer-sized piece subsequently struck the Earth to create the Kogo Structure and the Carbonado strewn field.

Another exciting possibility is that Carbonados originated on one of the four ‘gas giant’ planets: Jupiter, Saturn, Uranus or Neptune. A suitably massive impactor such as a Kuiper Belt planetissimal or large comet could conceivably compress atmospheric hydrocarbon gases such as methane to produce diamond, which might be thrown into interplanetary space by the force of the impact.

Apossible candidate could be Uranus, which has an axial tilt of over ninety degrees, causing it to ‘roll around’ the Sun like a barrel! This tilt could, perhaps, have been caused by a collision with an Earth-sized proto-planet.

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