Direct quartz-coesite transformation in shocked porous sandstone from Kamil Crater (Egypt)

Abstract

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.

Crystal Structures and High-Temperature Vibrational Spectra for Synthetic Boron and Aluminum Doped Hydrous Coesite

“…Our result is essentially in agreement with the previous high-temperature Raman studies on SiO 2 -pure coesite [17,46]. The isobaric mode Grüneisen parameter (γiP) is defined as , (3) where α is the averaged volumetric thermal expansion coefficient (α = 8.4 × 10 −6 K −1 for coesite [11]). The calculated γiP parameters are shown in Figure 6A for the samples of R503, R663, R712, and R749.…”

“…Coesite, a high-pressure polymorph of SiO 2 , was firstly synthesized at 3.5 GPa (773-1073 K) in 1953 [1], and subsequently discovered in many locations, such as in the shocked sandstone ejecta samples from craters [2,3] as well as in eclogite [4][5][6]. Coesite is a very important index silica mineral for ultrahigh-pressure metamorphism [7,8], which provides key clue for the continental dynamics such as lithospheric subduction, exhumation, and reentry in extreme depths of more than 100 km.…”

Coesite in a Muong Nong‐type tektite from Muong Phin, Laos: Description, formation, and survival

“…The first two models envisage coesite formation during shock unloading (i.e., when the pressure release path passes via the coesite stability field) through crystallization from a silica shock melt (Langenhorst 2003;Chen et al 2010;Fazio et al 2017) or subsolidus nucleation from highly densified diaplectic silica glass (St€ ahle et al 2008). A third model involves the formation of coesite through direct subsolidus transformation from quartz, facilitated by localized reverberation (attenuation and amplification) of the compression shock wave due to discontinuities in the target (Kieffer et al 1976;Folco et al 2018;Campanale et al 2019).…”

Crystal structures and high-temperature vibrational spectra for synthetic boron and aluminum doped hydrous coesite

“…Temperature was monitored with a W5Re95-W2Re74 (C-type) thermocouple, and a graphite furnace was used in our experiments. Analytical reagent SiO 2 , Al(OH) 3 , B(OH) 3 (purity of >99.99%) were adopted as the starting materials to synthesize hydrous coesite samples with different compositions: Si-pure (Run 503), B-doped (R663), Al-doped (R749), and B,Al-doped (R694 and R712). Excess liquid water (1 µL) was added in each capsule to guarantee the water fugacity.…”