Skeletal crystallization of quartz from SiO2 impact melt
Authors: Fazio Agnese , Folco Luigi 
Published: October 2020

. IntroductionA recent study on a Muong Nong-type tektite [1] revealed the occurrence of quartz with a &#8220;spongy texture&#8221;. The orientation and distribution of the voids in quartz were supposed to be crystallographically controlled.Similar structures were not reported before in the literature except for a glass impactite from the 45-m-Kamil crater, Egypt (sample L09) [2]. No Raman or TEM studies were carried out on this lapillus and due to the occurrence of voids, this SiO2-rich material was interpreted as glass.The renewed interest induced us to check again these structures in Kamil impactites. They were found along borders and fractures in numerous quartz relicts occurring in impact melt lapilli. In this work, we present the preliminary results of the characterization of these structures.2. Impactite L09The present study focuses on the SiO2 glass-rich impactite L09 (max length ~5 cm). This sample was described in [2] as white glass, i.e., a glass derived from the melting of the target rock without interaction with the projectile (Ni below detection limit). The glass is locally stained by reddish-brownish material. Among Kamil white glasses, L09 is the only sample showing large quartz relicts (up to a few millimeters).3. MethodsThe sample was optically characterized through polarized microscope and Raman spectroscopy. Successively, it was investigated by scanning electron microscopy (SEM) and spot chemical analyses from the vesicular glass and quartz relicts were acquired through electron microprobe (EMP).4. ResultsUnder crossed polarized light the quartz relicts have a very low birefringence and show sporadic birefractive domains. In the birefractive domains, planar deformation features (PDFs) have been observed. Raman spectra of quartz relicts vary between a pure SiO2 glass spectrum with the typical large asymmetric band at ~490 cm-1 and a glass-free shocked-quartz spectrum (main peak at 460 cm-1). Shocked-quartz is easily recognized by Raman spectroscopy: the peaks are shifted towards lower frequencies due to the increase in the angle spanning two tetrahedra [3,4]. Up to two sets of PDFs were recognized at the SEM. PDFs are irregular, closely spaced, enlarged, and tend to coalesce. The PDF areas are in spatial continuity with PDF-free amorphous regions. The PDF-free amorphous regions can contain sporadic vesicles. No flux textures have been recognized in quartz relicts. Occasionally, quartz relicts are crossed by vesicular glass veins chemically enriched in Al2O3 and FeO and open fractures.The margins of the quartz relicts and fractures are mostly surrounded by a layer (usually 30-&#181;m-thick; exceptionally up to 100-&#181;m-thick) of unshocked quartz (Raman main peak at 465 cm-1). Quartz appears in form of skeletal hexagonal aggregates. The numerous voids left out from the skeletal growth are partially filled by iron oxides. The number of the voids increases and their size decreases towards the quartz relicts describing a kind of rim. Small concentric fractures are frequent in the quartz relicts, marking the rim of the skeletal quartz layers. No cristobalite or high-pressure SiO2 polymorphs (i.e., coesite and stishovite) have been detected by Raman.5. Preliminary discussionThe occurrence of skeletal quartz is indicative of a rapid crystallization of the quartz from a fluid phase. Due to the size of the impact event and size of the lapillus, a hydrothermal post-shock alteration could be ruled out. Thus, it is plausible that the quartz crystallized from a SiO2-rich melt. Due to its viscosity, this melt should have been formed at the same location where the skeletal quartz occurs. The melt formed at the expense of the highly-shocked-quartz/diaplectic-glass grains with a minor involvement of the Si-Fe-Al surrounding melt/glass. The melting should have occurred immediately after the pressure release, in the early decompression stage. The subsequent drop of the temperature induced the fast (skeletal) crystallization of the quartz. That probably started in the stability field of &#946;-quartz (4.5 GPa