DIAMONDS are formed in more ways thanone. While it was believed earlier thatthe beautiful crystals were retrievedfrom deep mines of igneous or metamorphic rocks of the mantle formedunder conditions of high temperatureand pressure, new research reveals thatthere are other ways by which they areformed in nature.
Meteorite impaction on the earth'ssurface is one instance which has led tothe formation of these precious stones.Work conducted by R M Hough andothers from the department of earth sciences at the Open University in MiltonKeynes, UK, now adds another theoryregarding the origin of these surface diamonds. It is believed that diamondswere present in the parent meteoriteand were expelled onto the earth's surface as a result of the impact. Earlierstudies have shown the presence of thesecrystals in leaded meteorites and urelites. It was also assumed that diamondsin a meteorite originate due to the highgravitational pressure exerted on it during its fall and may have interstellar origins (Nature, Vol 378, No 6552).
Recently, scientists discovered tinydiamonds, identified from an elusivenoble-gas bearing component in chondritic (spheroidal mineral grain bearing) meteorites. The rare gas component gave them away as being interstellar in origin, formed due to vapour condensation in stellar atmospheres.
Hough et al conducted their studyon material collected from the 23 kmdiameter Reis crater in Germany, wherethey found small quantities of cubic andhexagonal diamonds alongwith cubicdiamonds interspersed with silicon carbide. The discovery of such diamondcomposites is the first of its kind,formed by undergoing condensation inthe hot vapour cloud of the impact.
Impact diamonds have also beenfound in clay, marking the boundarybetween the Cretaceous and Tertiaryperiods when the earth was bombardedby a large number of meteorites. Carbonand nitrogen isotopic data revealedthese diamonds to have been formeddue to impact or from within thefireball. The chemical structure andisotopic markings of these impact associated polycrystalline diamonds is distinct. They preserve the macroscopiccrystal layout of the graphite crystalsfrom which they originate due to shockmetamorphism.
Another rare and unusual form ofpolycrystalline black diamonds havebeen found in the sedimentary formations in Bahia, Brazil and in Ubangi,Central African Republic. These formations are called carbonados, which areextremely hard polycrystalline aggregates. Enclosed within their porousmatrix of randomly arranged crystalsare octahedral and cubic diamonds. Themechanism of formation of these diamonds is not yet clear, though suggestions range from their origin as carbonsubduction in the mantle, to shockmetamorphism during impact of irradiation of organic matter.