The formation of impact coesite

Abstract

Coesite in impact rocks is traditionally considered a retrograde product formed during pressure release by the crystallisation of an amorphous phase (either silica melt or diaplectic glass). Recently, the detailed microscopic and crystallographic study of impact ejecta from Kamil crater and the Australasian tektite strewn field pointed in turn to a different coesite formation pathway, through subsolidus quartz-to-coesite transformation. We report here further evidence documenting the formation of coesite directly from quartz. In Kamil ejecta we found sub-micrometric single-coesite-crystals that represent the first crystallization seeds of coesite. Coesite in Australasian samples show instead well-developed subeuhedral crystals, growing at the expenses of hosting quartz and postdating PDF deformation. Coesite (010) plane is most often parallel to quartz {10–11} plane family, supporting the formation of coesite through a topotactic transformation. Such reaction is facilitated by the presence of pre-existing and shock-induced discontinuities in the target. Shock wave reverberations can provide pressure and time conditions for coesite nucleation and growth. Because discontinuities occur in both porous and non-porous rocks and the coesite formation mechanism appears similar for small and large impacts, we infer that the proposed subsolidus transformation model is valid for all types of quartz-bearing target rocks.

TiO2 II: The high‐pressure Zr‐free srilankite endmember in impact rocks

“…Moreover, a dynamic compression study through a membrane‐driven diamond anvil cell (mDAC) at temperatures up to 1160 K on quartz powder shows the coesite formation happening at 760–900 K and between 2 and 11 GPa (Carl et al., 2018). The peak shock pressure of ~12–15 GPa estimated here by the presence of TiO 2 II in the Australasian ejecta is well in agreement with pressure estimation of ~15 GPa suggested by the orientation of PDFs reported in these rocks (Campanale et al., 2019, 2021; Fazio et al., 2014; Folco et al., 2018).…”

“…In supporting the subsolidus topotactic hypothesis, a similar incomplete transformation has already been observed in the case of quartz‐to‐coesite transition in some ejecta particles from the same deep‐sea sediment core (ODP site 1144A) where the particles 1144_12 and 1144_16 studied here are from (Campanale et al., 2019), as well as in the shocked sandstones from Kamil crater (Egypt; Campanale et al., 2021). In particular, these silica ejecta particles show that the coesite (010) plane is usually parallel to the quartz {10–11} or {−1011} plane families, that is, one of the most recurrent orientations for planar fractures and PDFs in shocked quartz.…”

“…As trace minerals, they show rutile, pyrite, Fe oxide (possibly magnetite), ilmenite, zircon, titanite, garnet, Caphosphate (possibly apatite), and dolomite. Shock metamorphism is documented here by the presence of TiO 2 II (Glass & Fries, 2008), coesite, and planar deformation features (PDFs) in quartz (Campanale et al, 2019(Campanale et al, , 2021.…”

Pervasive shock melting at >65 GPa in a Martian basalt, the shergottite Northwest Africa 14672

“…There has been much discussion of the possible formation mechanisms of coesite. Folco et al (2018) and Campanale et al (2021) proposed that coesite at Kamil formed by direct subsolidus transformation from quartz. At Meteor Crater, coesite was interpreted as the product of the inversion of stishovite formed at pressures near 30 GPa (St€ offler & Langenhorst, 1994).…”

3D electron diffraction study of terrestrial iron oxide alteration in the Mineo pallasite

“…In Earth sciences, 3D ED has been used for the structure elucidation of nanoscopic inclusions (Xiong et al , 2020), mineral seeds (Németh et al , 2018), hydrated phases (Mugnaioli et al , 2020a; Krysiak et al , 2021) and modulated systems (Lanza et al , 2019; Steciuk et al , 2020), even when characterised by very large asymmetric units (Rozhdestvenskaya et al , 2010; Mugnaioli et al , 2020b). 3D ED has been applied to geological samples in different contexts (Mugnaioli and Gemmi, 2018), and in particular for the characterisation of meteorites (Pignatelli et al , 2017, 2018; Suttle et al , 2021), impactites (Campanale et al , 2021), cryptocrystalline oxides (Koch-Müller et al , 2014) and hydroxides (Viti et al , 2016).…”