Carbonado: Physical and chemical properties, a critical evaluation of proposed origins, and a revised genetic model

Carbonado-diamond is the most controversial of all diamond types and is found only in Brazil, and the Central African Republic (Bangui). Neither an affinity to Earth's mantle, nor an origin in the crust can be unequivocally established. Carbonado-diamond is at least 3.8 Ga old, an age about 0.5 Ga older than the oldest diamonds yet reported in kimberlites and lamproites on Earth. Derived from Neo- to Mid-Proterozoic meta-conglomerates, the primary magmatic host rock has not been identified. Discovered in 1841, the material is polycrystalline, robust and coke-like, and is best described as a strongly bonded micro-diamond ceramic. It is characteristically porous, which precludes an origin at high pressures and high temperatures in Earth's deep interior, yet it is also typically patinated, with a glass-like surface that resembles melting. With exotic inclusions of highly reduced metals, carbides, and nitrides the origin of carbonado-diamond is made even more challenging. But the challenge is important because a new diamondiferous host rock may be involved, and the development of a new physical process for generating diamond is possibly assured. The combination of micro-crystals and random crystal orientation leads to extreme mechanical toughness, and a predicable super-hardness. The physical and chemical properties of carbonado are described with a view to the development of a mimetic strategy to synthesize carbonado and to duplicate its extreme toughness and super-hardness. Textural variations are described with an emphasis on melt-like surface features, not previously discussed in the literature, but having a very clear bearing on the history and genesis of carbonado. Selected physical properties are presented and the proposed origins, diverse in character and imaginatively novel, are critically reviewed. From our present knowledge of the dynamic Earth, all indications are that carbonado is unlikely to be of terrestrial origin. A revised model for the origin of carbonado is developed based on: (1) new observations which show that white dwarf stars can have diamond cores; (2) C-rich exoplanets may have diamond-bearing mantles; and (3) new shock wave experiments on methane suggest that diamond may be present in large gaseous planets such as Saturn, Uranus, and Neptune where diamond in the core is solid, and in Jupiter where liquid diamond may be present. It is proposed that carbonado-diamond was delivered to Earth from extra-solar sources during the episode of Late Heavy Bombardment (4.1–3.8 Ga). If the currently held view that r-process heavy elements were added as a ‘late veneer’ to Earth, through the collision of white dwarfs in Kilonova explosions, then why not carbonado-diamond during this epic event?