Evidence for subsolidus quartz-coesite transformation in impact ejecta from the Australasian tektite strewn field

1,2Fabrizio Campanale,2Enrico Mugnaioli,1 Luigi Folco,2Mauro Gemmi,3Martin R.Lee,3Luke Daly,4Billy P.Glass
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.08.014]
1Dipartimento di Scienze della Terra, Università di Pisa, V. S. Maria 53, 56126 Pisa, Italy
2Center for Nanotechnology Innovation@NEST, Istituto Italiano di Tecnologia (IIT), Piazza San Silvestro 12, 56127 Pisa, Italy
3Department of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK
4Department of Geosciences, University of Delaware, Newark, DE, USA
Copyright Elsevier

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 [11¯1]* vector of quartz near parallel to the [010]* 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¯011}
quartz planes, which would eventually turn into coesite (010) planes. This process further explains the structural relation observed between the characteristic (010) twinning and disorder of impact-formed coesite, and the 101¯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.

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