¹Chi Ma,²Oliver Tschauner,¹John R. Beckett,³Vitali B. Prakapenka
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14302]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
²Department of Geoscience, University of Nevada, Las Vegas, Nevada, USA
³Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois, USA
Published by arrangement with John Wiley & Sons

High-pressure oxides like perovskite-type FeTiO3, CaTi2O4-type Fe2TiO4, and ferrous-ferric oxides that form polysomes between wüstite and CaFe2O4-type Fe3O4 are potential carriers of Fe, Ti, and other transition metals in the mantle and may play an important role in the redox budget of the deep Earth. Here, we report the occurrence of three of these phases as the new minerals: feiite (Sr2Tl2O5-type Fe2+2(Fe2+Ti4+)O5), liuite (FeTiO3 with a GdFeO3-type perovskite structure), and tschaunerite (CaTi2O4-type (Fe2+)(Fe2+Ti4+)O4), along with wangdaodeite (LiNbO3-type FeTiO3) in a transformed ulvöspinel clast entrained in a shock melt pocket in the Shergotty Martian meteorite. We show that reaction between the shocked ulvöspinel precursor and melt occurred at pressures between 20 and 25 GPa. The high-pressure Fe-, Ti-minerals lost Fe and O to the surrounding shock melt in exchange for Si, Mg, and Ca. Concentrations of Si and Mg in all of these clast phases and of Na in liuite are significant. They substantiate chemical interaction of the clast with melt during the shock event and highlight potential elemental distributions in complex Fe- and Ti-rich lithologies at pressures of the deep transition zone to shallow lower mantle.

¹Marina A. Ivanova,^2,3Sergey N. Britvin,⁴Roza I. Gulyaeva,⁴Sofia A. Petrova,⁵Nina G. Zinovieva,⁶Vladimir V. Kozlov,⁴Stanislav N. Tyushnyakov,⁷Anatoly V. Kasatkin
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14296]
¹Vernadsky Institute of Geochemistry of the Russian Academy of Sciences, Moscow, Russia
²Saint-Petersburg State University, St. Petersburg, Russia
³Kola Science Center, Russian Academy of Sciences, Apatity, Russia
⁴Institute of Metallurgy, Ural Branch, Russian Academy of Sciences, Yekaterinburg, Russia
⁵Lomonosov Moscow State University, Moscow, Russia
⁶Institute of Volcanology and Seismology, Far Eastern Branch of the Russian Academy of Sciences, Petropavlovsk-Kamchatsky, Russia
⁷Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia
Published by arrangement with John Wiley & Sons

A natural iron-bearing oxysulfide, named сafeosite after its chemical composition, is a unique example of a mineral that simultaneously contains iron in three oxidation states: Fe3+, Fe2+, and intermediate between Fe2+ and Fe0 involved in metallic-type FeFe bonding. Cafeosite was discovered in metamorphosed carbonaceous chondrite Dhofar 225, which is classified as CM-anomalous but likely related to the CY (Yamato-type) group. The mineral occurs as tiny anhedral grains that coalesce into irregular aggregates up to 20 μm, commonly encrusted by micrometer-thick troilite or pyrrhotite rims. The grains are randomly disseminated within a chondrite matrix composed of thermally altered phyllosilicates. Associated accessory minerals are troilite, pyrrhotite, Fe-rich, Al-bearing olivine, unknown Al-bearing Fe sulfide, Al-rich chromite, kamacite, awaruite, pentlandite, escolaite, and perovskite. In reflected light, cafeosite is gray, with no internal reflections. Anisotropy is moderate, bireflectance in gray hues. Infrared microspectroscopy did not reveal any bands attributable to (OH)−, H2O or CO32− vibrations. Owing to the small grain size, the crystal structure of the mineral has been studied using synthetic analog, which was found to be isostructural with natural cafeosite based on electron backscatter diffraction (EBSD) data. Cafeosite is orthorhombic, space group Cmce (#64), a 17.4856(9), b 11.1516(5), c 11.1543(5) Å, V 2175.0(2) Å3, Z = 8, Dx = 4.11 g cm−3. The crystal structure has been solved and refined to R1 = 0.039 for 1105 unique reflections. Chemical composition of both natural and synthetic cafeosite corresponds to the formula Ca4Fe2+3Fe3+2(□1−xFex)O6S4 where (□1−xFex) denotes structural vacancy partially occupied by semimetallic-type Fe (x = 0.2–0.3). The ideal endmember formula of the mineral is Ca4Fe2+3Fe3+2□O6S4. Cafeosite was likely formed from previously altered precursor material of Dhofar 225, which, like common CM chondrites, consisted of phyllosilicates, Ca-bearing carbonates, tochilinite-like sulfides–hydroxides and pyrrhotite. During thermal metamorphism at temperatures between 750 and 900°C, sulfides–hydroxides were partly sintered with calcined carbonates and iron oxides, resulting in cafeosite formation. Due to varying and redox-dependent contents of Fe3+ and Fe2+, as well as the presence of metallic-type Fe in the structure, cafeosite could be regarded as a single-phase redox indicator alternative to the known triple-phase buffers, for example, iron–magnetite–pyrrhotite (IM-Po), iron–wüstite–pyrrhotite (IW-Po) and magnetite–wüstite–pyrrhotite (MW-Po) systems. Discovery of cafeosite provides insight into a previously obscured aspect of CY-chondrite formation: the redox conditions of thermal metamorphism on carbonaceous asteroids.

¹Zélia Dionnet et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14304]
¹CNRS, Institut d’Astrophysique Spatiale, Université Paris-Saclay, Orsay, France
Published by arrangement with John Wiley & Sons

Understanding the processes of aqueous alteration within primitive bodies is crucial for unraveling the complex history of early planetesimals. To better identify the signs of this process and its consequences, we have studied the heterogeneity at a micrometric scale of the structure of the aliphatic organic compounds and its relationship to its mineralogical environment. Here, we report an analysis performed on two micrometric grains of Ryugu (C0002-FC027 and C0002-FC028). The samples were crushed in a diamond compression cell and analyzed using high-spatial resolution Fourier Transform InfraRed (FT-IR) hyperspectral imaging measurements conducted in transmission mode. We showed here the spatial distributions of the main components and the structural heterogeneity of the aliphatic organic matter highlighting a micrometer-scale variability in the methylene-to-methyl ratio. Moreover, we connected this heterogeneity to the one of the phyllosilicate band positions. Our findings indicate that the organic matter within Ryugu’s micrometric grains underwent varying degrees of aqueous alteration in distinct microenvironments resulting in an elongation of the length of their aliphatic chains, and/or a reduction in their branching and/or cross-linking.