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

Abstract

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 lensshaped 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 b-cristobalite inverted to a-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.

Libyan Desert Glass area in western Egypt: Shocked quartz in bedrock points to a possible deeply eroded impact structure in the region

“…This was recently confirmed by the detection of mullite in LDG samples (Greshake et al. ). Magna et al.…”

“…Their findings also suggest that LDG formed during a short high-temperature event, as shown by melting of aluminum-rich orthopyroxene-bearing target material, which then formed the dark schlieren. This was recently confirmed by the detection of mullite in LDG samples (Greshake et al 2018). Magna et al (2011) noted that LDG carries high d 7 Li at ≥24.7&, which may represent the previous fluvial history of parental material that was perhaps deposited under lacustrine conditions or in coastal seawater.…”

Terrestrial target and melting site of Libyan Desert Glass: New evidence from trace elements and Sr isotopes

“…The recent discovery of mullite (Greshake et al. 2018) and the presence of high P‐T zircon transition phases (Kleinmann 1969; Cavosie et al. 2018a, 2018b; Cavosie and Koeberl 2019a, 2019b) suggest that, while high temperature melting and possible ejection of terrestrial material might have occurred at high pressure, the melt solidified quickly at atmospheric pressure.…”

Origin of β-cristobalite in Libyan Desert Glass: The hottest naturally occurring silica polymorph?

“…DOI: https://doi.org/10.2138/am-2021-7922. http://www.minsocam.org/ Cristobalite in LDG has been documented in optical images (Spencer, 1939;Kleinmann, 1969;Greshake et al, 2010;Swaenen et al, 2010;Fröhlich et al, 2013), and its composition determined by electron microscopy (Greshake et al, 2010(Greshake et al, , 2018. Identification of α-cristobalite was confirmed using Raman spectroscopy (Greshake et al, 2010;Swaenen et al, 2010;Aramendia et al, 2011) and Fourier transform infrared spectroscopy (Fröhlich et al, 2013).…”

“…Electron microprobe studies show LDG cristobalite is nearly pure silica (mostly >98 wt% SiO 2 ); Al is the second most abundant element (Al 2 O 3 = 0.2 to 1.1 wt%), followed by Fe and Ti (Barnes and Underwood, 1976;Greshake et al, 2010Greshake et al, , 2018. The LDG also contains little water (1100-1500 ppm, Frischat et al, 1984;Beran and Koeberl, 1997).…”