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

1Aaron J. Cavosie,2William D.A. Rickard,1,2Noreen J. Evans,2Kai Rankenburg,
3Malcolm Roberts,4Catherine A. Macris,5Christian Koeberl
American Mineralogist 107, 1325-1340 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2022/Abstracts/AM107P1325.pdf]
1School of Earth and Planetary Sciences, Curtin University, Perth, Western Australia 6102, Australia
2John de Laeter Centre, Curtin University, Perth, Western Australia 6102, Australia
3Centre for Microscopy, Characterisation, and Analysis, University of Western Australia, Perth, Western Australia 6009, Australia
4School of Science, Indiana University–Purdue University, Indianapolis, Indiana 46202, U.S.A.
5Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
Copyright: The Mineralogical Society of America

Identifying and determining the origin of β-cristobalite, a high-temperature silica polymorph, in
natural samples is challenging as it is rarely, if ever, preserved due to polymorphic transformation to
α-cristobalite at low temperature. Formation mechanisms for β-cristobalite in high-silica rocks are
difficult to discern, as superheating, supercooling, bulk composition, and trace element abundance all
influence whether cristobalite crystallizes from melt or by devitrification. Here we report a study of
α-cristobalite in Libyan Desert Glass (LDG), a nearly pure silica natural glass of impact origin found
in western Egypt, using electron microprobe analysis (EMPA), laser ablation inductively coupled mass
spectrometry (LA-ICP-MS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), scanning
electron microscopy (SEM), and electron backscatter diffraction (EBSD). The studied grains are
mostly 250 μm in diameter and consist of ~150 μm wide cores surrounded by ~50 μm wide dendritic
rims. Compositional layering in LDG continues across cristobalite grains and mostly corresponds to
variations in Al content. However, layering is disrupted in cores of cristobalite grains, where Al distribution records oscillatory growth zoning, whereas in rims the high Al occurs along grain boundaries.
Cristobalite cores thus nucleated within layered LDG at conditions that allowed mobility of Al into
crystallographically controlled growth zones, whereas rims grew when Al was less mobile. Analysis
of 37 elements indicates little evidence of preferential partitioning; both LDG and cristobalite are
variably depleted relative to the upper continental crust, and abundance variations correlate to layering in LDG. Orientation analysis of {112} twin systematics in cristobalite by EBSD confirms that
cores were formerly single β-cristobalite crystals. Combined with published experimental data, these
results provide evidence for high-temperature (>1350 °C) magmatic crystallization of oscillatory zoned
β-cristobalite in LDG. Dendritic rims suggest growth across the glass transition by devitrification, driven
by undercooling, with transformation to α-cristobalite at low temperature. This result represents the
highest formation temperature estimate for naturally occurring cristobalite, which is attributed to the
near pure silica composition of LDG and anomalously high temperatures generated during melting
by meteorite impact processes.


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