Mullite in Libyan Desert Glass: Evidence for high-temperature/low-pressure formation

1Ansgar Greshake,2Richard Wirth,3Jörg Fritz,4Tomasz Jakubowski,5Ute Böttger
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13030]
1Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Berlin, Germany
2Helmholtz Centre Potsdam, GFZ German Research Center for Geosciences, Potsdam, Germany
3Saalbau Weltraum Projekt, Heppenheim, Germany
4Wroclaw, Poland
5Deutsches Zentrum für Luft- und Raumfahrt, Institut für Optische Sensorsysteme, Berlin, Germany
Published by arrangement with John Wiley & Sons

Libyan Desert Glass (LDG) is a SiO2-rich natural glass whose origin, formation mechanism, and target material are highly debated. We here report on the finding of a lens-shaped whitish inclusion within LDG. The object is dominantly composed of siliceous glass and separated from the surrounding LDG by numerous cristobalite grains. Within cristobalite, several regions rich in mullite often associated with ilmenite were detected. Mineral assemblage, chemical composition, and grain morphologies suggest that mullite was formed by thermal decomposition of kaolinitic clay at atmospheric pressure and T ≥ 1600 °C and also attested to high cooling rates under nonequilibrium conditions. Cristobalite contains concentric and irregular internal cracks and is intensely twinned, indicating that first crystallized β-cristobalite inverted to α-cristobalite during cooling of the SiO2-rich melt. The accompanied volume reduction of 4% induced the high density of defects. The whitish inclusion also contains several partly molten rutile grains evidencing that at least locally the LDG melt was at T ≥ 1800 °C. Based on these observations, it is concluded that LDG was formed by high-temperature melting of kaolinitic clay-, rutile-, and ilmenite-bearing Cenozoic sandstone or sand very likely during an asteroid or comet impact onto Earth. While melting and ejection occurred at high pressures, the melt solidified quickly at atmospheric pressure.

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