Impact coesite: Formation and survival

This is an overview of the results of our ongoing research (in part published, [1], in part under review, [2] and [3]) aiming at better understanding of the formation and survival of impact coesite - a debated issue in impact cratering and shock metamorphism studies. Impact coesite occurs in the form of nanometer-sized grains with polysynthetic twinning on (010) grains, typically embedded in silica glass. Its presence in rocks that experienced shock conditions beyond the stability field is an intriguing and controversial issue. Models, widely accepted since its discovery in 1960 [4], predict that coesite forms during crystallization from highly densified silica melts [5], [6], [7] or from diaplectic glass [8] during shock unloading, when the decompression path intersects the coesite stability field (pressure 3–10 GPa, temperature <3000 K). In contrast to these mechanisms, we show mineralogical and petrographic evidence of subsolidus direct quartz-to-coesite transformation in quartzose impactites from different geological contexts, including a plausible mechanism for this polymorphic transformation. These results have implications on the reconstruction of the P-T-t paths experienced by target rocks and on the definition of impact scenarios