The origin of iron silicides in ureilite meteorites

Ureilite meteorites contain iron silicide minerals including suessite (Fe,Ni)3Si, hapkeite (Fe2Si) and xifengite (Fe5Si3). Despite occurring mostly in brecciated varieties presumed to be derived from the regolith of the ureilite parent asteroid, suessite has also been confirmed in one lithology of a dimict ureilite (NWA 1241). In contrast, Si-bearing Fe-metals occur in both brecciated and unbrecciated ureilites, implying that they were formed throughout the ureilite parent asteroid. We examined major, minor and trace element data of Fe-metals in seven brecciated ureilites (DaG 319, DaG 999, DaG 1000, DaG 1023, DaG 1047, EET 83309, and EET 87720) in addition to the dimict ureilite NWA 1241. In this study we show that the silicides and Si-bearing metals in ureilites have similar siderophile trace element patterns; therefore, the precursors to the silicides were indigenous to the ureilite parent body. Si-free kamacite grains in brecciated ureilites show flatter, more chondritic siderophile element patterns. They may also be derived from the interior of the ureilite parent body, but some may be of exogenous origin (impactor debris), as are rare taenite grains. On Earth, iron silicides are often formed under high-temperature and strongly reducing conditions (e.g. blast furnaces, lightning strikes). On the Moon, hapkeite (Fe2Si) and other silicides have been found in the regolith where they were formed by impact-induced space weathering. In the Stardust aerogel, iron silicides derived from comet Wild2 were also formed by an impact-related reduction process. Silicides in ureilite regolith breccias may have formed by similar processes but ureilites additionally contain abundant elemental carbon which probably acted as a reducing agent, thus larger and more abundant silicide grains were formed than in the lunar regolith or cometary material. The origin of suessite in NWA 1241 may be analogous to that of reduced lithologies in the terrestrial mantle, although a regolith origin may also be possible since this sample is shown here to be a dimict breccia

UNetGE: A U-Net-Based Software at Automatic Grain Extraction for Image Analysis of the Grain Size and Shape Characteristics

The shape and the size of grains in sediments and soils have a significant influence on their engineering properties. Image analysis of grain shape and size has been increasingly applied in geotechnical engineering to provide a quantitative statistical description for grain morphologies. The statistic robustness and the era of big data in geotechnical engineering require the quick and efficient acquirement of large data sets of grain morphologies. In the past publications, some semi-automation algorithms in extracting grains from images may cost tens of minutes. With the rapid development of deep learning networks applied to earth sciences, we develop UNetGE software that is based on the U-Net architecture—a fully convolutional network—to recognize and segregate grains from the matrix using the electron and optical microphotographs of rock and soil thin sections or the photographs of their hand specimen and outcrops. Resultantly, it shows that UNetGE can extract approximately 300~1300 grains in a few seconds to a few minutes and provide their morphologic parameters, which will ably assist with analyses on the engineering properties of sediments and soils (e.g., permeability, strength, and expansivity) and their hydraulic characteristics

Quantum recoil in free-electron interactions with atomic lattices

The emission of light from charged particles underlies a wealth of scientific phenomena and technological applications. Classical theory determines the emitted photon energy by assuming an undeflected charged particle trajectory. In 1940, Ginzburg pointed out that this assumption breaks down in quantum electrodynamics, resulting in shifts—known as quantum recoil— in outgoing photon energies from their classically predicted values. Since then, quantum recoil in free-electron light-emission processes, including Cherenkov radiation and Smith–Purcell radiation, has been well-studied in theory, but an experimental demonstration has remained elusive. Here we present an experimental demonstration of quantum recoil, showing that this quantum electrodynamical effect is not only observable at room temperature but also robust in the presence of other electron-scattering mechanisms. By scattering free electrons off the periodic two-dimensional atomic sheets of van der Waals materials in a tabletop platform, we show that the X-ray photon energy is accurately predicted only by quantum recoil theory. We show that quantum recoil can be enormous, to the point that a classically predicted X-ray photon is emitted as an extremely low-energy photon. We envisage quantum recoil as a means of precision control over outgoing photon and electron spectra, and show that quantum recoil can be tailored through a host of parameters: the electron energy, the atomic composition and the tilt angle of the van der Waals material. Our results pave the way to tabletop, room-temperature platforms for harnessing and investigating qua- ntum electrodynamical effects in electron–photon interactions.Agency for Science, Technology and Research (A*STAR)National Research Foundation (NRF)Submitted/Accepted versionThis project was partially supported by the National Research Foundation (Project ID NRF2020-NRF-ISF004-3525) and the Agency for Science, Technology and Research (A*STAR) Science & Engineering Research Council (Grant No. A1984c0043). We acknowledge the Facility for Analysis, Characterisation, Testing and Simulation, Nanyang Technological University, Singapore, for use of their electron microscopy/X-ray facilities. Z.L. acknowledges the support from National Research Foundation, Singapore, under its Competitive Research Programme (CRP) (NRF-CRP22-2019-0007 and NRF-CRP26-2021-0004). This research is also supported by A*STAR under its AME IRG Grant (Project No. A2083c0052). L.J.W. acknowledges the Nanyang Assistant Professorship Start-up Grant

Quantum recoil in free electron-driven spontaneous emission from van der Waals crystals

We experimentally measure quantum recoil in Smith-Purcell radiation, achieved by scattering free electrons off the periodic lattice of van der Waals crystals to generate multimode coherent X-ray

Petrochronologic perspective on rhyolite volcano unrest at Laguna del Maule, Chile

Rhyolitic magmas have rarely erupted during historical times, thus we have a poor record of the signals of unrest that precede them. The Laguna del Maule volcanic field (LdM), Chile, is in the midst of a decade-long episode of unrest including surface inflation at more than 200 mm/yr. Geomorphic observations indicate that many similar deformation episodes occurred during the late Pleistocene and Holocene. During this time, approximately 40 km3 of rhyolite has erupted effusively and explosively from at least 24 vents distributed around a 300 km2 lake basin. The large volume, protracted eruptive history, and ongoing unrest of LdM offer an unusual opportunity to integrate petrologic reconstructions of recent rhyolite generation with geophysical and geodetic observations associated with an active, growing magma reservoir. New petrochronologic data shows that the most recent rhyolites, erupted during the last 3200 yr, each resided in the shallow crust for only decades following extraction from an underlying reservoir. The rhyolites contain only limited, cryptic evidence for magma replenishment and reheating in the form of Ba concentration spikes in plagioclase, which suggest biotite breakdown in a crystal-rich mush. The absence of evidence for substantial reheating or mixing with intruding magma preceding the rhyolitic eruptions indicates that they must have been triggered by another process. We propose the accumulation of fluids derived from the deeper degassing of mafic melts is capable of pressurizing eruptible magma bodies of low density rhyolite. This process likely continues to this day and is consistent with the best-fit models of the ongoing unrest. The striking absence of visible surface degassing accompanying the unrest at LdM suggests fluids are trapped beneath an impermeable carapace and could catalyze a future explosive eruption

The influence of stereochemically active lone-pair electrons on crystal symmetry and twist angles in lead apatite-2H type structures

Lead-containing (Pb-B-X)-2H apatites encompass a number of [AF ]4[AT ]6[(BO4)6]X 2 compounds used for waste stabilization, environmental catalysis and ion conduction, but the influence of the stereochemically active lone-pair electrons of Pb2+ on crystal chemistry and functionality is poorly understood. This article presents a compilation of existing structural data for Pb apatites that demonstrate paired electrons of Pb2+ at both the AF and AT results in substantial adjustments to the PbFO6 metaprism twist angle, ϕ. New structure refinements are presented for several natural varieties as a function of temperature by single-crystal X-ray diffraction (XRD) of vanadinite-2H (ideally Pb10(VO4)6Cl2), pyromorphite-2H (Pb10(PO4)6Cl2), mimetite-2H/M (Pb10(As5+O4)6Cl2) and finnemanite-2H (Pb10(As3+O3)6Cl2). A supercell for mimetite is confirmed using synchrotron single-crystal XRD. It is suggested the superstructure is necessary to accommodate displacement of the stereochemically active 6s2 lone-pair electrons on the Pb2+ that occupy a volume similar to an O2– anion. We propose that depending on the temperature and concentration of minor substitutional ions, the mimetite superstructure is a structural adaptation common to all Pb-containing apatites and by extension apatite electrolytes, where oxide ion interstitials are found at similar positions to the lone-pair electrons. It is also shown that plumbous apatite framework flexes substantially through adjustments of the Pb F O6 metaprism twist-angles (ϕ) as the temperature changes. Finally, crystal-chemical [100] zoning observed at submicron scales will probably impact on the treatment of diffraction data and may account for certain inconsistencies in reported structures.ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Assessing volcanic hazard and exposure to lava flows at remote volcanic fields: a case study from the Bolaven Volcanic Field, Laos

Southeast Asia is home to a large number of active and well-studied volcanoes, the majority of which are located in Indonesia and the Philippines. Northern Southeast Asia (Myanmar, Cambodia, Laos, Thailand and Vietnam) also hosts volcanoes that for several reasons (post-World War II conflicts, poor accessibility due to dense vegetation, no known historical activity) have been poorly studied. Systematic assessments of the threat these volcanoes pose to resident populations do not exist, despite evidence of numerous eruptions through the late Pleistocene and likely even during the Holocene. A recent study inferred the location of the Australasian meteorite impact to be beneath the Bolaven Volcanic Field in southern Laos; this study provided a wealth of data for the field: in particular, mapping of vents and flows, and their relative or absolute ages. The Bolaven Volcanic Field (16 Ma— 50 m in some locations. Building upon this foundation, we used the Bolaven Volcanic Field as a case study for assessing the potential exposure of populations and infrastructure to lava flows during future effusive eruptions. Our study uses remote sensing to map past flows and vents (i.e. scoria cones), lava-flow simulations from new simulated vents, and open-access exposure data, to assess hazards and exposure. Our results show that future vents are most likely to occur in a N-S band atop the Bolaven plateau, with some flows channelling into canyons and spilling down the plateau flanks onto lower plains that support more populated areas such as the provincial centre, Pakse. Our exposure assessment suggests that around 300,000 people could experience socio-economic impacts from future lava flow inundations. The largest impacts would be on two of the main economic sectors in the region, agriculture and hydropower. The potential also exists for life-threatening explosions from interactions between magma and surface waters, which are abundant in the region. We estimate an average recurrence interval of approximately 10,400 years, based on information from lava flows and scoria cones.Ministry of Education (MOE)National Research Foundation (NRF)Published versionThis research was supported by the Earth Observatory of Singapore via its funding from the National Research Foundation Singapore and the Singapore Ministry of Education under the Research Centers of Excellence initiative. This work comprises EOS contribution number 404

Magnetic Signature and Element Content of Upflow and Outflow Hotspring in Arjuno–Welirang Geothermal System

Research on magnetic properties and chemical element content of environmental deposits has been conducted for various purposes. This study focuses on characteristic magnetic susceptibility, magnetic mineral morphology, and the elemental composition of Cangar and Padusan hot springs in the Arjuno-Welirang geothermal system to differentiate upflow and outflow systems, respectively. The measurements were performed for better understand the relation between magnetic susceptibility, Fe-Silicate content, magnetic mineral morphology, surface temperature and compare these characteristics in two kinds of hot springs in the same mountain system. Magnetic susceptibility ranged (7.558 - 62.694 ) × 10-6 m3/kg with an average of 30.651 × 10-6 m3/kg for Cangar (upflow) and (11.821 - 28.101) × 10-6 m3/kg with an average of 18.148 × 10-6 m3/kg for Padusan (outflow). In situ magnetic minerals extracted of hot springs are averaged of magnetic susceptibility is 26.981 × 10-6 m3/kg for Cangar and 24.445 × 10-6 m3/kg for Padusan. The element content dominated by Al, Si, K, Ca, Ti and Fe, where Fe is more abundant in Cangar as an upflow. The higher magnetic susceptibility, the greater of Fe-silicate content in both of hot springs. The surface temperature ranged from 38 - 48°C, where the higher temperature, the magnetic susceptibility increased. In Cangar, extracted magnetic minerals tend show crystalline, especially hedralic shape with very fine surface, clean and free of impurities. Meanwhile, some magnetic minerals are also found in spherical shapes, especially in Padusan

Characterisation of defects generated during constant current InGaN-on-silicon LED operation

We studied the degradation of MOCVD-grown InGaN LEDs on Si substrates under constant current stressing. Characterisations using Deep Level Transient Spectroscopy and Electron Energy Loss Spectroscopy on active areas showed that the stressing had generated defects that have trap states at 0.26 eV below the conduction band edge (Ec – 0.26 eV) and that correlated with the active area's lower nitrogen content as compared to unstressed samples. The combination of Current-Voltage, Electroluminescence, Cathodoluminescence, and device simulations indicate that an increase in the density of these defects is correlated with an increase in the non-radiative carrier recombination that causes degradation in light emission. Preventing formation of these defects will be critical for improving InGaN-on-silicon LED reliability.NRF (Natl Research Foundation, S’pore)Accepted versio

Crystal structure and surface characteristics of Sr-doped GdBaCo2O6−δ double perovskites : oxygen evolution reaction and conductivity

A cheap and direct solution towards engineering better catalysts through identification of novel materials is required for a sustainable energy system. Perovskite oxides have emerged as potential candidates to replace the less economically attractive Pt and IrO2 water splitting catalysts. In this work, excellent electrical conductivity (980 S cm 1) was found for the double perovskite of composition GdBa0.6Sr0.4Co2O6 d which is consistent with a better oxygen evolution reaction activity with the onset polarisation of 1.51 V with respect to a reversible hydrogen electrode (RHE). GdBa1 xSrxCo2O6 d with increasing Sr content was found to crystallise in the higher symmetry tetragonal P4/mmm space group in comparison with the undoped GdBaCo2O6 d which is orthorhombic (Pmmm), and yields higher oxygen uptake, accompanied by higher Co oxidation states. This outstanding electrochemical performance is explained by the wider carrier bandwidth, which is a function of Co–O–Co buckling angles and Co–O bond lengths. Furthermore the higher oxygen evolution activity was observed despite the formation of non-lattice oxides (mainly hydroxide species) and enrichment of alkaline earth ions on the surface.Published versio