Abstract
Abstract Coesite, a high-pressure silica polymorph, is a diagnostic indicator of impact cratering in quartz-bearing target rocks. The formation mechanism of coesite during hypervelocity impacts has been debated since its discovery in impact rocks in the 1960s. Electron diffraction analysis coupled with scanning electron microscopy and Raman spectroscopy of shocked silica grains from the Australasian tektite/microtektite strewn field reveals fine-grained intergrowths of coesite plus quartz bearing planar deformation features (PDFs). Quartz and euhedral microcrystalline coesite are in direct contact, showing a recurrent pseudo iso-orientation, with the [1 1 ¯ 1] * vector of quartz near parallel to the [0 1 0]* vector of coesite. Moreover, discontinuous planar features in coesite domains are in textural continuity with PDFs in adjacent quartz relicts. These observations indicate that quartz transforms to coesite after PDF formation and through a solid-state martensitic-like process involving a relative structural shift of { 1 ¯ 0 1 1 } quartz planes, which would eventually turn into coesite (0 1 0) planes. This process further explains the structural relation observed between the characteristic (0 1 0) twinning and disorder of impact-formed coesite, and the 1 0 1 ¯ 1 PDF family in quartz. If this mechanism is the main way in which coesite forms in impacts, a re-evaluation of peak shock pressure estimates in quartz-bearing target rocks is required because coesite has been previously considered to form by rapid crystallization from silica melt or diaplectic glass during shock unloading at 30–60 GPa.
Highlights
Quartz is one of the most common minerals in Earth’s continental crust
Electron diffraction analysis coupled with scanning electron microscopy and Raman spectroscopy of shocked silica grains from the Australasian tektite/microtektite strewn field reveals fine-grained intergrowths of coesite plus quartz bearing planar deformation features (PDFs)À
As outlined in the Introduction, it is generally believed that coesite in impact rocks forms by the crystallisation of an amorphous phase. This process would take place during the decompression stage either from silica melt with shortrange order and silicon in fourfold coordination (e.g., Stoffler and Langenhorst, 1994), or through a solid-state transformation of diaplectic silica glass (Stahle et al, 2008). Both models are based on direct observations of natural and experimental non-porous rocks and on theoretical considerations: (1) in non-porous rocks, coesite only occurs in association with amorphous silica material (Stoffler, 1971); (2) coesite cannot be produced in shock experiments, possibly due the too short pressure-pulse length reached in laboratory conditions (Stoffler and Langenhorst, 1994); (3) direct quartz-to-coesite transformation is reconstructive and is presumed to be too time-consuming to take place during compression in impact cratering events, this because the complete collapse of the crystal structure to glass in the solid state is the only possible response to rapid shock compression (e.g. Langenhorst and Deutsch, 2012)
Summary
Quartz is one of the most common minerals in Earth’s continental crust. Under shock metamorphism it displays⇑ Corresponding author at: Dipartimento di Scienze della Terra, Universitadi Pisa, V. Quartz is one of the most common minerals in Earth’s continental crust. ⇑ Corresponding author at: Dipartimento di Scienze della Terra, Universitadi Pisa, V. A wide range of effects including mechanical twins, planar fractures (PFs), planar deformation features (PDFs), diaplectic glass (densified glass), and lechatelierite (silica glass).
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