Abstract
Diamonds were found in impact melt rocks and breccias at the Popigai impact structure in Siberia. The diamonds preserve the crystallographic habit and twinning of graphites in the preimpact target rocks, from which they formed by shock transformation. Secondary and transmission electron microscopy indicate that the samples are polycrystalline and contain abundant very thin lamellae, which could represent stacking faults, with local hexagonal symmetry, or microtwins. Microcrystalline units are ≤1 µm. Infrared spectroscopy indicates the presence of solid CO 2 and water in microinclusions in the diamonds, CO 2 being under a pressure greater than 5 GPa (at room temperature). Trace element and isotopic compositions confirm the derivation from graphite precursors.
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