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Mullite in Libyan Desert Glass: Evidence for high‐temperature/low‐pressure formation
Author(s) -
Greshake Ansgar,
Wirth Richard,
Fritz Jörg,
Jakubowski Tomasz,
Böttger Ute
Publication year - 2018
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.13030
Subject(s) - cristobalite , mullite , rutile , ilmenite , mineral , mineralogy , geology , coesite , stishovite , materials science , composite material , metallurgy , quartz , tectonics , ceramic , paleontology , eclogite , subduction
Libyan Desert Glass ( LDG ) is a SiO 2 ‐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 SiO 2 ‐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.