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Coesite, a high-pressure silica polymorph (pressure 3-10 GPa, temperature < 3000 K), is a diagnostic feature of shock metamorphism associated with impact cratering on quartz-bearing target rocks. It is preserved as a metastable phase in sedimentary target rocks that experienced peak pressures in excess of ~10 GPa, where it typically occurs as intergranular polycrystalline aggregates of microcrystals embedded in silica glass known as "symplectic regions." The presence of coesite in the symplectic regions of rocks experiencing shock conditions beyond the limits of the coesite stability field is a controversial issue. Through a combined scanning and transmission electron microscopy and Raman spectroscopy study of shocked quartzarenites from the 45-m-diameter Kamil Crater (southwest Egypt), we show that coesite in symplectic regions forms through direct subsolidus transformation from quartz, in contrast with the prevailing hypothesis for crystalline targets. The quartz-to-coesite transformation takes place during localized shock-wave reverberation at the beginning of the pore collapse process. Complete pore collapse generates the high temperature regimes responsible for the subsequent production of the embedding silica melts, in part at the expense of the previously formed coesite. This work documents the role of pore collapse in producing localized pressure-temperature-time gradients in shocked porous targets, as predicted by numerical models in the literature.

Small impact craters (<300 m in diameter) are rare on Earth and mostly deeply eroded, so that knowledge of their formation mechanism and the hazard small impactors constitute to human populations is largely based on physical models. We report on the geophysical investigation of the Kamil Crater we recently discovered in southern Egypt. The Kamil Crater is a <5 k.y. old impact crater 45 m in diameter, with a pristine ejecta ray structure. Such well-preserved structures have been previously observed only on extraterrestrial rocky or icy planetary bodies. This crater feature, and the association with an iron meteorite impactor and shock metamorphism, provides a unique impression of aspects of small-scale hypervelocity impacts on the Earth's crust. Contrary to current models, ground data indicate that iron meteorites with masses of tens of tons may be able to penetrate the atmosphere without substantial fragmentation.

We report the occurrence of microscopic inclusions of shocked quartz plus a Zr phase and trace of Fe oxide crystallites in Australasian microtektites recovered from deep-sea sediment cores within 2000 km of Indochina. The shocked quartz and the Zr phase are interpreted as relicts of the target rock. Furthermore, the internal homogeneity of Australasian microtektites in terms of abundance of relict mineral inclusions, vesicles, and schlieren increases with distance from Indochina. This finding strengthens the current hypothesis that the source crater of the largest and youngest tektite strewn field on Earth is located in the Indochina region, as internal heterogeneity characterizes normal impact glass found in or near the source crater. This finding also indicates that the Australasian microtektites with the longest trajectories underwent the highest temperatures or were heated longer. The definition of microtektites should include the possible occurrence of microscopic relict inclusions as an indication of proximity to the source crater.