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TiO2II, a high‐pressure polymorph of titanium dioxide, is a diagnostic indicator of shock metamorphism in impact rocks. Due to its typical micro‐to‐nanometer scale, there are no ab initio structure solutions of natural TiO2II, thereby generating uncertainty about its crystal structure and its known similarity with srilankite (Ti0.67,Zr0.33)O2. Nanoscale electron diffraction investigation of TiO2II from the Australasian tektite strewn field provides the first ab initio structure solution revealing a primitive orthorhombic lattice with cell parameters a = 4.547 Å, b = 5.481 Å, c = 4.891 Å, and space group Pbcn, that is, the same as srilankite and scrutinyite α‐PbO2. The linear a and c decrease, and b increase with Ti content indicate TiO2II as Zr‐free srilankite endmember in the binary system ZrO2‐TiO2. Thereby the name srilankite should be used referring to TiO2II, according to the International Mineralogical Association recommendations. We provide the first evidence for a topotactic subsolidus rutile‐to‐TiO2II transition, founding their finely intermixing nanocrystals in the same TiO2 crystal, where TiO2II is within the crystal and surrounded by rutile in direct contact. They also show recurrent iso‐orientation, with TiO2II [100] parallel to rutile [100], TiO2II [010] parallel to rutile [011], and TiO2II [001] parallel to rutile (0–11). The rutile‐TiO2II iso‐orientation suggests the compression of rutile (0–11) planes as a possible transition mechanism from rutile to TiO2II, with a consequent shortening of ~0.5 Å per cell. The presence of TiO2II in the distal (~1200 km) impact ejecta from the Australasian tektite strewn field indicates shock pressures of ~12–15 GPa and post‐shock temperatures below 500°C followed by rapid quenching. [ABSTRACT FROM AUTHOR]

Cosmic dust plays a dominant role in the universe, especially in the formation of stars and planetary systems. Furthermore, the surface of cosmic dust grains is the benchwork where molecular hydrogen and simple organic compounds are formed. We manipulate individual dust particles in a water solution by contactless and noninvasive techniques such as standard optical and Raman tweezers, to characterize their response to mechanical effects of light (optical forces and torques) and to determine their mineral compositions. Moreover, we show accurate optical force calculations in the T-matrix formalism highlighting the key role of composition and complex morphology in the optical trapping of cosmic dust particles. This opens perspectives for future applications of optical tweezers in curation facilities for sample-return missions or in extraterrestrial environments. [ABSTRACT FROM AUTHOR]

Reckling Peak (RKP) 17085 is a newly classified Antarctic CM chondrite that preserves a complex alteration history characterized by mild aqueous alteration (CM2.7), overprinted by a short‐lived thermal metamorphic event (heating stage III [<750°C]), and affected by low‐grade terrestrial weathering. This meteorite contains abundant Fe‐rich bands within the fine‐grained matrix, composed of micron‐scale Fe‐oxyhydroxide minerals. They are interpreted as “alteration fronts” arising due to the dissolution and transport of Fe (typically <500 μm) before being abruptly deposited. This alteration texture is relatively rare among hydrated carbonaceous chondrites, with only five reported instances to date (Murchison, Murray, Allan Hills 81002, Miller Range 07687, and Northwest Africa 5958). Evidence from RKP 17085 suggests that early aqueous alteration operated as multiple geochemically isolated microenvironments, which moved outwards from local point sources within the matrix. Low permeability fine‐grained rims on chondrules appear to have acted as barriers to fluid flow, controlling the migration of fluid across the parent body. Furthermore, the higher porosity regions within the altered fine‐grained matrix represent either void space generated by the dehydration of hydrated minerals during post‐hydration metamorphism and/or sites of ice accretion (water‐ice or C‐bearing ices) preserved within a mildly altered primitive matrix. [ABSTRACT FROM AUTHOR]

Introduction: Coesite is one of the most common and reliable indicator of shock metamorphism associated with impact cratering in quartz-bearing target rocks. For this reason, coesite is the subject of numerous studies aiming to better understand how silica polymorphs react under sudden and extreme P-T increases. In impact rocks, coesite is preserved as a metastable phase in crystalline rocks that experienced peak shock pressures above ~30-40 GPa [1] and in porous sedimentary rocks shocked at pressures as low as ~10 GPa [2]. Furthermore, impact-coesite generally forms aggregates of microcrystalline grains scattered within silica amorphous material known as "symplectic regiorns", and shows a characteristic polysynthetic twinning on (100) with the composition plane (010) [3]. There is a general consensus that the characteristic twinned impact-coesite is the result of crystallisation from a dense amorphous phase, either silica shock melt [1] or highly densified diaplectic silica glass [4], during shock unloading, when the pressure release path passes through the coesite stability field. In contrast to these models, the coesite transformation mechanism through a direct solid-state transition from quartz was suggested the first time by [5] studying shocked Coconino sandstones from the Barringer crater (Arizona, USA). Our recent FESEM-TEM study, performed on impact ejecta from Kamil crater in Egypt and the Australasian tektite strewn field, confirms that the impact coesite forms through direct quartz-coesite transformation [6,7,8]. Samples and Methods: Samples are from two different impact sites including a shocked porous sandstone from Kamil crater (Egypt) and two coesite-bearing quartz ejecta from the Australasian tektite/microtektite strewn field (ODP site 1144A and Sonne Core SO95-17957-2). All samples were investigated using field emission gun - scanning electron microscopy (FEG-SEM), and then 5 electron-transparent lamellae from the Kamil crater shocked sandstone and 3 lamellae from the Australasian coesite-bearing ejecta were exctracted using focused-ion beam (FIB). These lamellae were investigated by transmission electron microscopy (TEM) and 3D electron diffraction (3D ED) [9] for nano-petrographic and crystallographic analyses. Remarkably, both samples experienced relatively fast cooling, which preserved shock metamorphic mineralogies and textures only slightly altered by post-impact melting, (i.e., virtually unaffected by post-shock annealing and/or hydrothermal overprint). Results and discussion: In Kamil ejecta we found rounded single-crystal coesite domains of 200 nm or less. These domains are surrounded by shocked quartz, without any amorphous phase in-between. We also observed larger coesite domains. The larger the domains, the more they appear fragmented and progressively dispersed and resorbed in amorphous silica. Our observations suggest that coesite seeds nucleate and grow inside quartz during pressure uploading, probably favored by shock-wave reverberation. Later, when pressure is released and temperature is still high, coesite domains fragment and melt giving rise to the "symplectic regions" observed - for instance - at Barringer [5], Yilan [10], Lonar [11], Cheasapeak Bay [12]. In impact ejecta from the Australasian tektite strewn field, we again observe coesite crystals embedded in shocked quartz. Coesite crystals have well-developed euhedral habits, which grow at the expense of neighboring quartz and appear to postdate PDFs and other planar microstructures. In both ejecta, 3D electron diffraction reveals coesite grains displaying a recurrent crystallographic relation with quartz, with (010) coesite plane parallel to {10-11} or {-1011} of quartz. Such evidence suggests a topotactic relation between shocked quartz and impact coesite. The direct quartz to cosite subsolidus reaction is facilitated by the presence of preexisting and shock-induced discontinuities in the target. Shock wave reverberations can provide pressure and time conditions for coesite nucleation and growth. Because discontinuities occur in both porous and non-porous rocks and the coesite formation mechanism appears similar for small and large impacts, we infer that the proposed subsolidus transformation model is valid for all types of quartz-bearing target rocks. [ABSTRACT FROM AUTHOR]

Introduction: Chromite (FeCr2O4) is a mineral phase with a spinel structure found in different types of rocks across the planet and our solar system, including meteorites and micrometeorites [1]. Because of their strong resistance to weathering, chromite grains can survive in sedimentary successions in the stratigraphic record and advance our understanding of the variability of the extraterrestrial flux through time [2]. However, a full understanding of the origin of these sedimentary chromite grains and potential alteration processes requires detailed in situ analysis. A full characterization of individual chromites is necessary to comprehend what processes or reservoirs sedimentary spinels trace and how their abundance changes over time. Here, we characterize 27 chromite-bearing cosmic spherules (175-1000 µm) from two different Antarctic micrometeorite collections. Methodology: Twenty-three of the studied particles belong to the micrometeorite collection from the Transantarctic Mountains (Antarctica), and four particles are from the Sør Rondane Mountains (Antarctica) collection [3-4]. In total, 52 individual chromite minerals ranging in size from 5.3 to 120 µm are identified among these 27 cosmic spherules. Each micrometeorite was polished and their cross-sections were imaged using a JEOL JSM-IT300 scanning electron microscope equipped with an energy-dispersive X-ray spectrometer at the Vrije Universiteit Brussels (Belgium). All 27 micrometeorites were studied using a JEOL JXA-8200 electron microprobe analyzer at the National Institute of Polar Research (NIPR) in Tokyo (Japan). Both the individual chromite grains and groundmass were targeted to refine the parentage and alteration history of these cosmic spherules. A subset of 14 micrometeorites was selected for oxygen isotope analysis using a Cameca IMS 1207 ion microprobe at the Centre de Recherches Pétrographiques et Géochimiques (CRPG) in Nancy (France). Triple-oxygen isotope ratios are obtained for chromite minerals larger than 10 µm and the matrix of the cosmic spherules. Results and Discussion: Based on the bulk oxygen isotope composition, we recognize one particle with a carbonaceous chondritic parentage (D17O » -2.28 ‰ and δ18O » -4.46 ‰), one particle with ambiguous parentage (D17O » 0.09 ‰ and δ18O » -6.82 ‰) and 12 particles deriving from ordinary chondrite-like material (Δ17O » 0.80 - 1.49 ‰ and d18O » 3.06 - 11.69 ‰) [5]. In five particles with an ordinary chondrite origin, the chromite minerals (15 - 30 µm) have deviating d17O and d18O values suggesting these minerals could have been affected by alteration during atmospheric entry resulting in a shift to higher δ18O - values and are neoformed. Seven particles with ordinary chondrite parentage have values similar to chromites previously identified in cosmic spherules [6] and suggest a pristine nature. However, the observed difference between the two types of chromites was not observed by Rudraswami et al. [6]. This study confirms the presence of chromite minerals mainly in particles with an ordinary chondritic parentage, while particles with MgAl spinel grains are usually associated with micrometeorites deriving from carbonaceous chondritic parent bodies [7]. Spinel group minerals can therefore be an additional parameter to identify the parent body of a micrometeorite. Conclusion: Chromite is present in many types of extraterrestrial material, including cosmic spherules [1]. We identified 52 individual chromite minerals in 27 cosmic spherules from the Transantarctic and Sør Rondane Mountains (Antarctica). Seven micrometeorites with an ordinary chondritic origin contain pristine chromites with oxygen isotope values similar to the matrix. The chromite minerals in the five other particles have deviating d17O and d18O values suggesting these minerals were altered and even neoformed during atmospheric entry and terrestrial residence. [ABSTRACT FROM AUTHOR]

Micrometeorites are estimated to represent the main part of the present flux of extraterrestrial matter found on the Earth's surface and provide valuable samples to probe the interplanetary medium. Here, we describe large and representative collections of micrometeorites currently available to the scientific community. These include Antarctic collections from surface ice and snow, as well as glacial sediments from the eroded top of nunataks-summits outcropping from the icesheet-and moraines. Collections extracted from deep-sea sediments (DSS) produced a large number of micrometeorites, in particular, iron-rich cosmic spherules that are rarer in other collections. Collections from the old and stable surface of the Atacama Desert show that finding large numbers of micrometeorites is not restricted to polar regions or DSS. The advent of rooftop collections marks an important step into involving citizen science in the study of micrometeorites, as well as providing potential sampling locations over all latitudes to explore the modern flux. We explore their strengths of the collections to address specific scientific questions and their potential weaknesses. The future of micrometeorite research will involve the finding of large fossil micrometeorite collections and benefit from recent advances in sampling cosmic dust directly from the air. This article is part of the theme issue 'Dust in the Solar System and beyond'.