Space weathering on airless planetary bodies: clues from the lunar mineral hapkeite

Redox condition changes caused by impacts: Insights from Chang'e-5 lunar glass beads

Lunar materials are overall more reducing compared with their terrestrial counterparts, but the mechanism remains to be elucidated. In this study, we present a possible explanation for the changes in redox state of the lunar regolith caused by impact events, based on our investigations of the impact glass beads from Chang'e-5 mission. These glass beads contain iron metal grains and show concentration gradients of FeO and K2O (with or without Na2O) from their rims to centers. The compositional profiles exhibit error-function-like shapes, which indicates a diffusion-limited mechanism. Our numerical modeling results suggest that the iron metal grains on the surface of the glass beads were generated through the reduction of FeO by elemental K and (or) Na produced during the impact events. Meanwhile, the iron metal grains inside the bead may have formed due to oxygen diffusion driven by redox potential gradients. Furthermore, our study suggests that impact processes intensify the local reducing conditions, as evidenced by the presence of calcium sulfide particles within troilite grains that coexist with iron metal grains on the surface of the glass beads. This study provides insights into the oxygen diffusion kinetics during the formation of iron metal spherules and sheds light on the changes in redox conditions of lunar materials caused by impact events.

Vapor-deposited digenite in Chang'e-5 lunar soil

Frequent impacts on the Moon have changed the physical and chemical properties of the lunar regolith, with new materials deposited from the impact-induced vapor phase. Here, we combined nanoscale chemical and structural analysis to identify the mineral digenite (4Cu₂S·CuS) in Chang'e-5 lunar soil. This is the first report of digenite in a lunar sample. The surface-correlated digenite phase is undifferentiated in distribution and compositionally distinct from its hosts, suggesting that it originated from vapor-phase deposition. The presence of an Al-rich impact glass bead suggests that a thermal effect provided by impact ejecta is the main heat source for the evaporation of Cu-S components from a cupriferous troilite precursor, and the digenite condensed from these Cu-S vapors. A large pure metallic iron (Fe⁰) particle and high Cu content within the studied Cu-Fe-S grain suggest that this grain was most likely derived from a highly differentiated and reduced melt.

Sub-microscopic magnetite and metallic iron particles formed by eutectic reaction in Chang’E-5 lunar soil

Ferric iron as well as magnetite are rarely found in lunar samples, and their distribution and formation mechanisms on the Moon have not been well studied. Here, we discover sub-microscopic magnetite particles in Chang’E-5 lunar soil. Magnetite and pure metallic iron particles are embedded in oxygen-dissolved iron-sulfide grains from the Chang’E-5 samples. This mineral assemblage indicates a FeO eutectoid reaction (4FeO = Fe₃O₄ + Fe) for formation of magnetite. The iron-sulfide grains’ morphology features and the oxygen’s distribution suggest that a gas–melt phase reaction occurred during large-impact events. This could provide an effective method to form ubiquitous sub-microscopic magnetite in fine lunar soils and be a contributor to the presentation of ferric iron on the surface of the Moon. Additionally, the formation of sub-microscopic magnetite and metallic iron by eutectoid reaction may provide an alternative way for the formation of magnetic anomalies observed on the Moon.

Magnetite is rarely present on the Moon. Here the authors report the magnetite formed by eutectic reaction during the impact process in Chang’E-5 lunar soil, and the potential contribution of this magnetite formation to magnetic anomalies on the Moon.

Nitrile regio-synthesis by Ni centers on a siliceous surface: implications in prebiotic chemistry

By means of quantum chemistry (PBE0/def2-TZVPP; DLPNO-CCSD(T)/cc-pVTZ) and small, but reliable models of Polyhedral Oligomeric Silsesquioxanes (POSS), an array of astrochemically-relevant catalysis products, related to prebiotic and origin of life chemistry, has been theoretically explored. In this work, the heterogeneous phase hydrocyanation reaction of an unsaturated CC bond (propene) catalyzed by a Ni center complexed to a silica surface is analyzed. Of the two possible regioisomers, the branched iso-propyl-cyanide is thermodynamically and kinetically preferred over the linear n-propyl-cyanide (T = 200 K). The formation of nitriles based on a regioselective process has profound implications on prebiotic and origin of life chemistry, as well as deep connections to terrestrial surface chemistry and geochemistry.

Surface microstructures of lunar soil returned by Chang'e-5 mission reveal an intermediate stage in space weathering process

The lunar soils evolution over time is mainly caused by space weathering that includes the impacts of varying-sized meteoroids and charged particles implantation of solar/cosmic winds as well. It has long been established that space weathering leads to the formation of outmost amorphous layers (50-200 nm in thickness) embedded nanophase iron (npFe⁰) around the mineral fragments, albeit the origin of the npFe⁰ remains controversial . The Chang'e-5 (CE-5) mission returned samples feature the youngest mare basalt and the highest latitude sampling site , providing an opportunity to seek the critical clues for understanding the evolution of soils under space weathering. Here, we report the surface microstructures of the major minerals including olivine, pyroxene, anorthite, and glassy beads in the lunar soil of CE-5. Unlike the previous observations, only olivine in all crystals is surrounded by a thinner outmost amorphous SiO₂ layer (∼10 nm thick) and embedded wüstite nanoparticles FeO (np-FeO, 3-12 nm in size) instead of npFe⁰. No foreign volatile elements deposition layer and solar flare tracks can be found on the surface or inside the olivine and other minerals. This unique rim structure has not been reported for any other lunar, terrestrial, Martian, or meteorite samples so far. The observation of wüstite FeO and the microstructures support the existence of an intermediate stage in space weathering for lunar minerals by thermal decomposition.

Natural Iron Silicides: A Systematic Review

This review systematically presents all finds of geogenic, impact-induced, and extraterrestrial iron silicide minerals known at the end of 2021. The respective morphological characteristics, composition, proven or reasonably suspected genesis, and possible correlations of different geneses are listed and supported by the available literature (2021). Artificially produced iron silicides are only dealt with insofar as the question of differentiation from natural minerals is concerned, especially regarding dating to pre-industrial and pretechnogenic times.

Application of FIB-SEM Techniques for the Advanced Characterization of Earth and Planetary Materials

Advanced microanalytical techniques such as high-resolution transmission electron microscopy (HRTEM), atom probe tomography (APT), and synchrotron-based scanning transmission X-ray microscopy (STXM) enable one to characterize the structure and chemical and isotopic compositions of natural materials down towards the atomic scale. Dual focused ion beam-scanning electron microscopy (FIB-SEM) is a powerful tool for site-specific sample preparation and subsequent analysis by TEM, APT, and STXM to the highest energy and spatial resolutions. FIB-SEM also works as a stand-alone technique for three-dimensional (3D) tomography. In this review, we will outline the principles and challenges when using FIB-SEM for the advanced characterization of natural materials in the Earth and Planetary Sciences. More specifically, we aim to highlight the state-of-the-art applications of FIB-SEM using examples including (a) traditional FIB ultrathin sample preparation of small particles in the study of space weathering of lunar soil grains, (b) migration of Pb isotopes in zircons by FIB-based APT, (c) coordinated synchrotron-based STXM characterization of extraterrestrial organic material in carbonaceous chondrite, and finally (d) FIB-based 3D tomography of oil shale pores by slice and view methods. Dual beam FIB-SEM is a powerful analytical platform, the scope of which, for technological development and adaptation, is vast and exciting in the field of Earth and Planetary Sciences. For example, dual beam FIB-SEM will be a vital technique for the characterization of fine-grained asteroid and lunar samples returned to the Earth in the near future.

Effects of Lightning on the Magnetic Properties of Volcanic Ash

High-current impulse experiments were performed on volcanic ash samples to determine the magnetic effects that may result from the occurrence of volcanic lightning during explosive eruptions. Pseudo-ash was manufactured through milling and sieving of eruptive deposits with different bulk compositions and mineral contents. By comparing pre- and post-experimental samples, it was found that the saturation (i.e., maximum possible) magnetization increased, and coercivity (i.e., ability to withstand demagnetization) decreased. The increase in saturation magnetization was greater for compositionally evolved samples compared to more primitive samples subjected to equivalent currents. Changes in remanent (i.e., residual) magnetization do not correlate with composition, and show wide variability. Variations in magnetic properties were generally more significant when samples were subjected to higher peak currents as higher currents affect a greater proportion of the subjected sample. The electrons introduced by the current impulse cause reduction and devolatilization of the ash grains, changing their structural, mineralogical, and magnetic properties.

Room-Temperature Ferromagnetism in Two-Dimensional Fe2Si Nanosheet with Enhanced Spin-Polarization Ratio

Searching experimental feasible two-dimensional (2D) ferromagnetic crystals with large spin-polarization ratio, high Curie temperature and large magnetic anisotropic energy is one key to develop next-generation spintronic nanodevices. Here, 2D Fe₂Si nanosheet, one counterpart of Hapkeite mineral discovered in meteorite with novel magnetism is proposed on the basis of first-principles calculations. The 2D Fe₂Si crystal has a slightly buckled triangular lattice with planar hexacoordinated Si and Fe atoms. The spin-polarized calculations with hybrid HSE06 function method indicate that 2D Fe₂Si is a ferromagnetic half metal at its ground state with 100% spin-polarization ratio at Fermi energy level. The phonon spectrum calculation and ab initio molecular dynamic simulation shows that 2D Fe₂Si crystal has a high thermodynamic stability and its 2D lattice can be retained at the temperature up to 1200 K. Monte Carlo simulations based on the Ising model predict a Curie temperature over 780 K in 2D Fe₂Si crystal, which can be further tuned by applying a biaxial strain. Moreover, 2D structure and strong in-plane Fe-Fe interaction endow Fe₂Si nanosheet sizable magnetocrystalline anisotropy energy with the magnitude of at least two orders larger than those of Fe, Co and Ni bulks. These novel magnetic properties render the 2D Fe₂Si crystal a very promising material for developing practical spintronic nanodevices.

Low-voltage electron-probe microanalysis of Fe-Si compounds using soft X-rays

Conventional electron-probe microanalysis has an X-ray analytical spatial resolution on the order of 1-4 μm width/depth. Many of the naturally occurring Fe-Si compounds analyzed in this study are smaller than 1 μm in size, requiring the use of lower accelerating potentials and nonstandard X-ray lines for analysis. Problems with the use of low-energy X-ray lines (soft X-rays) of iron for quantitative analyses are discussed and a review is given of the alternative X-ray lines that may be used for iron at or below 5 keV (i.e., accelerating voltage that allows analysis of areas of interest <1 μm). Problems include increased sensitivity to surface effects for soft X-rays, peak shifts (induced by chemical bonding, differential self-absorption, and/or buildup of carbon contamination), uncertainties in the mass attenuation coefficient for X-ray lines near absorption edges, and issues with spectral resolution and count rates from the available Bragg diffractors. In addition to the results from the traditionally used Fe Lα line, alternative approaches, utilizing Fe Lβ, and Fe Ll-η lines, are discussed.

AFM for diagnosis of nanocrystallization of steels in hardening processes

INTRODUCTION

The aim of this study is to investigate the nanocrystallization of steels caused by the transformation from the austenitic to the martensitic phase induced by a severe plastic deformation (SPD) treatment. In this framework, we applied an air blast shot peening treatment, which is a simple protocol widely used for industrial purposes.

METHODS

AISI 286 and AISI 316 specimens were peened for different times and polished using diamond pastes in order to remove corrugations higher than 1 mum. The characterization of the steel surfaces was performed by atomic force microscopy (AFM) operating in contact mode. Additional EDXD measurements were performed to confirm the phase transition.

RESULTS AND DISCUSSION

An AFM-based characterization at nanometric level of the steel surfaces is provided. When the peening exceeds a threshold time that, as expected, depends on the steel composition, a uniform nanostructuration is detected. It is well known that such rearrangement is associated to the growth of a martensitic phase. To date, AFM has been employed in this field only for few applications and to solve specific problems. On the other hand, our results demonstrate that this is a useful technique for the characterization of hardened surfaces, especially when non-destructive sample preparation treatments are required. Moreover, we show that AFM can be a useful tool also for in situ industrial diagnostics of metallic parts.