Impact coesite: formation and survival

This is an overview of the results of our ongoing research (Folco et al., 2018; Campanale et al., 2019; Glass et al., 2019) 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 nanometre-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 (Chao et al., 1960), predict that coesite forms during crystallization from highly densified silica melts (Stöffler and Langenhorst, 1994; Fazio et al., 2017) or from diaplectic glass (Stähle et al., 2018) 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 electron diffraction (ED) evidence of subsolidus direct quartz-to-coesite transformation in quartzose impactites from different geological contexts (i.e. Kamil Crater, Egypt, and Australasian tektite/µtektite strewn field), 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. Furthermore, this work shows the potential of the emerging three-dimensional ED method for the structure characterization of materials available only as sub-micrometre-sized grains (Gemmi et al., 2019), thereby opening a new perspective in shock metamorphic studies and planetary science, given the typical micro-to-nanometre scale of shock metamorphic features and their defective nature. Interestingly, by using very mild illumination conditions, complete and high-resolution data can be collected on phases that normally deteriorate rapidly in high resolution TEM mode (such as high pressure SiO2 phases and nucleation seeds in amorphous areas). Likewise, the TEM-based phase/orientation mapping using precession-assisted crystal orientation mapping (PACOM) technique enables reliable data with a spatial resolution down to 2 nm when used with a field emission gun. Also, whilst yielding less precise orientation measurements when compared with Kikuchi lines in EBSD, spot diffraction patterns are less affected by the distortion induced by high dislocation densities (Viladot et al., 2013). Therefore, PACOM is particularly suited for investigating strongly plastically deformed materials like the shocked silica ejecta studied here.