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.

¹Priscille Cuvillier,^2,3Noël Chaumard,¹Hugues Leroux,^2,4,5Brigitte Zanda,^2,4Roger H. Hewins¹Damien Jacob,⁶Bertrand Devouard
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13032]
¹Unité Matériaux et Transformations, Université Lille 1 and CNRS, Villeneuve d’Ascq, France
²Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Université, Muséum National d’Histoire Naturelle, UPMC Université Paris 06, IRD & CNRS, Paris, France
³WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin, USA
⁴Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
⁵Institut de Mécanique Céleste et de Calcul des Ephémérides, Observatoire de Paris, Paris Cedex, France
⁶Aix-Marseille Université, CNRS, IRD, CEREGE UM34, Aix en Provence, France
Published by arrangemenr with John Wiley & Sons

We conducted a transmission electron microscope study of the exsolution microstructures of Ca-rich pyroxenes in type I chondrules from the Paris CM and Renazzo CR carbonaceous chondrites in order to provide better constraints on the cooling history of type I chondrules. Our study shows a high variability of composition in the augite grains at a submicrometer scale, reflecting nonequilibrium crystallization. The microstructure is closely related to the local composition and is thus variable inside augite grains. For compositions inside the pyroxene miscibility gap, with a wollastonite (Wo) content typically below 40 mole%, the augite grains contain abundant exsolution lamellae on (001). For grain areas with composition close to Wo₄₀, a modulated texture on (100) and (001) is the dominant microstructure, while areas with compositions higher than Wo₄₀ do not show any exsolution microstructure development. To estimate the cooling rate, we used the spacing of the exsolution lamellae on (001), for which the growth is diffusion controlled and thus sensitive to the cooling rate. Despite the relatively homogeneous microstructures of augite grains with Wo < 35 mole%, our study of four chondrules suggests a range of cooling rates from ~10 to ~1000 °C h⁻¹, within the temperature interval 1200–1350 °C. These cooling rates are comparable to those of type II chondrules, i.e., 1–1000 °C h⁻¹. We conclude that the formation of type I and II chondrules in the proto-solar nebula was the result of a common mechanism.

¹Tomasz Brachaniec
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13035]
¹Department of Geochemisty, Mineralogy and Petrography, Faculty of Earth Sciences, University of Silesia, Sosnowiec, Poland
Published by arrangement wit John Wiley & Sons

Reworking and redeposition of tektites is a highly complex and multistage geological process including many factors. A tumbling experiment was therefore undertaken with the aim of estimating a distance of transport that such moldavites can withstand. Though the experiment probably did not accurately mimic natural conditions, our results proved that moldavites can withstand considerable transport only over a distance of a few kilometers. Observed abrasion of tektites was significant in the early stage of experimental transport; the rate of abrasion decreased correlatively with increasing distance of transport as usual. Overall, given the results obtained from this experimental study and their state of preservation described in the literature, it is very likely that Polish tektites were reworked and redeposited by rivers from the Sudetes Mountains. Based on the paleoreconstruction of river flows, it can be assumed that the Polish tektites originated from two independent sediment supply areas.

^1,2Mona Weyrauch,¹Marian Horstmann,¹Addi Bischoff
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13025]
¹Institut für Planetologie, Universität Münster, Münster, Germany
²Institut für Mineralogie, Universität Hannover, Hannover, Germany
Published by arrangement with John Wiley & Sons

In this study, the metal and sulfide compositions of 45 enstatite chondrites were analyzed to determine possible mineral-chemical trends correlated with the petrologic type. Data for 35 additional samples were taken from the literature. Considering the data from this huge number of different E chondrite samples (80 in total), none of the trends previously described in the literature could be clearly confirmed. Also, among the opaque phases of enstatite chondrites, no other “new” correlations between mineral chemistry and the petrologic type were found. However, major differences in the sulfide and metal chemistry became obvious. Specifically, a certain number of chondrites in the EH and the EL groups have Cr in troilite above 2 wt%, Fe in niningerite or alabandite above 20 wt%, and lack abundant daubréelite. Differences were also found for Ni concentrations in kamacite. Thus, we propose a system for classifying E chondrites by defining four major subgroups: EHa, ELa, EHb, and ELb. All subgroups show full petrologic sequences that are similar to each other. This observation, in combination with the differences in sulfide and metal chemistry, suggests an origin of the samples from different parent bodies. Considering the anomalous E chondrite samples that neither fit in the previous classification scheme nor in the new one described here, the samples investigated in this study require at least eight different parent bodies.

¹Georgy A. Belyanin, ¹Jan D. Kramers, ²Marco A.G. Andreoli, ^1,3Francesco Greco, ^1,4,5Arnold Gucsik, ¹Tebogo V. Makhubela, ^6,7Wojciech J. Przybylowicz, ⁸Michael Wiedenbeck
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.12.020]
¹Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa
²School of Geosciences, University of the Witwatersrand, PO Box 3, Wits 2050, South Africa
³Dipartimento di Scienze Biologiche, Geologiche ed Ambientali, Università di Bologna, Via Zamboni 67, 40126 Bologna, Italy
⁴Department of Nonlinear and Laser Optics, Wigner Research Institute for Physics, Hungarian Academy of Sciences, Konkoly-Thege Miklós út 29-33, Budapest, H-1121, Hungary
⁵Department of Mineralogy and Geology, Cosmochemistry Research Group, University of Debrecen, Egyetem tér 1., H-4032, Hungary
⁶iThemba Labs, National Research Foundation, P.O. Box 722, Somerset West 7129, South Africa
⁷AGH University of Science and Technology, Faculty of Physics & Applied Computer Science, 30-059 Kraków, Poland
⁸Deutsches GeoForschungsZentrum GFZ, D14473 Potsdam, Germany
Copyright Elsevier

The stone named “Hypatia” found in the Libyan Desert Glass area of southwest Egypt is carbon-dominated and rich in microdiamonds. Previous noble gas and nitrogen isotope studies suggest an extraterrestrial origin. We report on a reconnaissance study of the carbonaceous matrix of this stone and the phases enclosed in it. This focused on areas not affected by numerous transecting fractures mostly filled with secondary minerals. The work employed scanning electron microscopy (SEM) with energy-dispersive (EDS) and wavelength-dispersive (WDS) electron microprobe (EMPA) analysis, Proton Induced X-ray Emission (PIXE) spectrometry and micro-Raman spectroscopy. We found that carbonaceous matrices of two types occur irregularly intermingled on the 50-500 μm scale: Matrix-1, consisting of almost pure carbonaceous matter, and Matrix-2, containing Fe, Ni, P and S at abundances analyzable by microprobe. Matrix-2 contains the following phases as inclusions: (i) (Fe,Ni) sulphide occurring in cloud-like concentrations of sub-μm grains, in domains of the matrix that are enriched in Fe and S. These domains have (Fe+Ni)/S (atomic) = 1.51 ± 0.24 and Ni/Fe = 0.086 ± 0.061 (both 1SD); (ii) grains up to ∼5 μm in size of moissanite (SiC); (iii) Ni-phosphide compound grains up to 60 μm across that appear cryptocrystalline or amorphous and have (Ni+Fe)/P (atomic= 5.6. ± 1.7 and Ni/Fe = 74 ± 29 (both 1SD), where both these ratios are much higher than any known Ni-phosphide minerals; (iv) rare grains (observed only once) of graphite, metallic Al, Fe and Ag, and a phase consisting of Ag, P and I. In Matrix-2, Raman spectroscopy shows a prominent narrow diamond band at 1340 cm-1. In Matrix-1 the D and G bands of disordered carbon are dominant, but a minor diamond band is ubiquitous, accounting for the uniform hardness of the material. The D and G bands have average full width at half maximum (FWHM) values of 295 ± 19 and 115 ± 19 cm-1, respectively, and the D/G intensity ratio is 0.75 ± 0.09 (both 1SD). These values are similar to those of the most primitive solar system carbonaceous matter. The diamond phase is considered to be a product of shock. The (Fe,Ni) sulphide phase is probably pyrrhotite and a shock origin is likewise proposed for it. Moissanite is frequently associated with the Ni-phosphide phase, and a presolar origin for both is suggested. The lack of recrystallization of the Ni-phosphide phase suggests that the Hypatia stone did not experience long-lasting thermal metamorphism, in accord with the Raman D-G band characteristics.

A lack of silicate matter sets the stone apart from interplanetary dust particles and known cometary material. This, along with the dual intermingled matrices internal to it, could indicate a high degree of heterogeneity in the early solar nebula.