DIAMONDS FROM THE POPIGAI IMPACT STRUCTURE, RUSSIA

Diamonds were found in impact melt rocks and breccias at the Popigai i mpact structure in Siberia. The diamonds preserve the crystallographic habit and twinning of graphites in the preimpact target rocks, from w hich they formed by shock transformation. Secondary and transmission e lectron microscopy indicate that the samples are polycrystalline and c ontain abundant very thin lamellae, which could represent stacking fau lts, with local hexagonal symmetry, or microtwins. Microcrystalline un its are less than or equal to 1 mu m. Infrared spectroscopy indicates the presence of solid CO2 and water in microinclusions in the diamonds , CO2 being under a pressure greater than 5 GPa (at room temperature). Trace element and isotopic compositions confirm the derivation from g raphite precursors.