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On Earth, diamonds usually occur in rocks derived from the Earth's mantle, e.g. kimberlites, or metamorphic rocks of mantle origin, having formed from fluids or melts in the upper mantle at very high pressures and temperatures (at depths in excess of 150km). However, diamonds have also formed in nature directly on the Earth's surface during impact cratering. The discovery of diamonds in clays marking the Cretaceous-Tertiary (K-T) boundary, which contained fairly unambiguous evidence of large-scale impact event, was unexpected. The nanometer-sized diamond grains were first interpreted as being remnants of the impacting meteorite, but carbon and nitrogen isotopic data pointed to an origin during the impact or from within the fireball. Speculations about the "true" origin of these clearly impact-associated diamonds were abundant.
Polycrystalline diamonds of up to about 1cm in size were discovered in impactites at a few Russian and Ukrainian impact structure. These diamonds, which usually contain appreciable amounts of losdaleite, have crystal chemical and isotopic signatures and preserve the macroscopical crystal habit of the graphite crystals from which the formed by shock metamorphism. Special investigation of impact diamonds from the Ries and Popigai craters using SEM, TEM and different X-ray methods were carried out. Attention was paid to the presence of cubic and hexagonal high-pressure carbon phases in polycrystalline aggregates, their mode of occurrence and interrelations. The studies showed a very complicated microtexture of the aggregates, probably caused by the orientation of packages of cubic microcrystals with sizes about 50nm. The principal pattern of these microcrystals is expressed in the form of numerous planar defects (sometimes only 10 A wide) - so called stacking faults. They represent specific type of dislocations of crystal lattice originated due to shock compression.
Another rare and unusual for of diamond on the Earth is "carbonado". They are extremely hard polycrystalline aggregates with a porous matrix of randomly oriented crystallites (0.5-20µm in size), which often enclose octahedral and cubic diamonds and other nocarbonaceous inclusions. In the present project, a variety of methods, including electron microscopy (secondary electron microscopy (SEM), and transmission electron microscopy (TEM)), neutron activation analysis, infrared spectroscopy, x-ray analysis, and isotope analyses were used to study diamond samples from a variety of craters, including carbonados. Our data show that impact diamonds from impact-derived rocks at the Ries crater in Germany and the Popigai impact structure in Siberia are similar in appearance, shape, physical properties. Both diamond types all have originates in to high-pressure transformation from graphite to diamond. The diamonds inherit the crystallographic characteristics of graphite from which they have formed and contain also the hexagonal polymorph of diamond (lonsdaleite). These findings indicate that the samples studied by us have originated by direct transformation from the graphite in the precursor rocks, and not by a chemical vapour deposition. Impact-derived rocks at the Gardnos (Norway) and Rochechouart (France) craters were searched for impact diamonds, with (so far) negative results. During our work, the presence of impact diamonds was established, for the first time, in suevites and impact melt breccias of the Lappajarvi crater (Finland).
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