Iron-bearing Silicate Research Articles

Synergistic adsorption mechanism of novel combined collector in flotation separation of fayalite from magnetite: Experiments and MD simulations

As an iron-bearing silicate mineral, fayalite has similar surface properties to magnetite, which makes it challenging to efficiently separate fayalite from magnetite by flotation. In this study, magnetite and fayalite were separated using novel combined collector polyoxyethylene alkyl amine (PAE) and terpinol (AT) through reverse flotation. Combined with a variety of characterization analysis and molecular dynamics (MD) simulation, the synergistic adsorption mechanism of the combined collector on mineral surface was revealed from multiple perspectives. The optimum scheme and pH response range of the combined collector were determined by micro-flotation test. Compared with PAE alone, the difference value of flotation recovery between fayalite and magnetite using combined collector was increased by 10.16%, and the dosage was reduced by 20%. Additionally, an effectively adsorption of combined collector onto fayalite surface to improve its surface hydrophobicity. Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and MD simulations results showed synergistic adsorption was produced onto fayalite surface using combined collector mainly through non-polar interaction, hydrogen bond association and bridge adsorption. Furthermore, the spatial distribution and diffusion behavior of water molecules provide microscopic evidence for the enhancement of surface hydrophobicity of fayalite by combined collector.

Magnetic detection of anthropogenic fires at Xiaodong Rockshelter, Southwest China

The Xiaodong Rockshelter, located on the southwest edge of Yunnan Province, is known as Southeast Asia's oldest (>43.5 ka) and northernmost Hoabinhian technocomplex site. The rockshelter preserves a rich record of animals, plants, and lithic artifacts excavated from sediments with a thickness of 4.6 m. New dating reported here indicates that the stratigraphic sequence spans from 65 ka to 15 ka. Several layers in the sedimentary sequence show evidence of fire, representative of the earliest evidence of fire by Hoabinhian population in a tropical-subtropical area. Here, we use magnetic methods coupled with mineral analysis to differentiate natural material from anthropogenically fired sediment. Archaeological fire events are characterized by higher magnetic concentrations and coarser magnetic grains compared to natural sediments. Significant magnetic enhancements were caused by the transformation of paramagnetic iron-bearing silicates into ferrimagnetic, spherical-shaped magnetite with increasing temperatures. Notably, a pronounced magnetic enhancement was observed between 1.8 and 2.5 m, spanning between 42 and 34 ka, indicating intense and concentrated heating, with estimated firing temperatures reaching ca. 400 °C. Additionally, three thin layers exhibiting magnetic enhancement were detected at depths of 3.65 m, 4.45 m, and 4.55 m, dating to ca. 55.6 ka, 62.3 ka and 64.8 ka respectively. This suggests three short-term fired ash deposits with minimal vertical magnetic enhancement, indicative of fire temperatures at ca. 350 °C. The magnetic method proves effective in detecting anthropogenic fire in archaeological sediments and potentially estimating ancient fire temperatures.

Iron species and sulfur isotopic compositions of authigenic pyrite in deep-sea sediments at southern Hydrate Ridge, Cascadia margin (ODP Leg 204): implications for non-steady-state depositional and diagenetic processes

Two accretionary sediment sequences from Sites 1245 and 1252 recovered during Ocean Drilling Program (ODP) Leg 204 at southern Hydrate Ridge were investigated to explore the response of geochemical partitioning of iron and sulfur isotopic composition of authigenic pyrite to non-steady-state depositional and diagenetic scenarios. Five iron species were characterized by a modified sequential extraction procedure that covers almost all iron-bearing minerals in sediment cores, including: (1) iron-bearing carbonates, mainly siderite; (2) ferric (hydr)oxides, probably ferrihydrite and/or lepidocrocite; (3) magnetite; (4) iron-bearing silicates; and (5) pyrite. Highly reactive iron has been accumulated for a long-term steady-state history and its pyritization, to varying degrees, is limited by availability of dissolved sulfide. This causes pyrite and siderite occurred in the same sedimentary layer and shows an inverse relationship between their concentrations. From this, their proportions to highly reactive iron can be chosen for evaluating the degree of sulfidization. A significant change in sulfur isotopic composition of pyrite (-42.4 to +16.8‰ VCDT) indicates that the steady-state conditions are dramatically limited, where the δ34S values higher than -20‰ may result from an upward shift of SMT zone close to the seafloor or a sudden, massive depositional event. To explain the downcore sulfidization effects and pyrite δ34S values, we developed two categories of conceptual scenarios based on variations in sedimentation rate and methane flux. The geochemical features similar to those derived from each scenario were searched in the sediment columns and the non-steady-state events behind the scenarios were proved to be consistent with the real observations. Thus, iron species and pyrite δ34S values can be regarded as a proxy to differentiate different non-steady-state depositional and diagenetic controls on the sedimentary record.

Selective adsorption mechanism of polyoxyethylene alkyl amine in reverse flotation separation of fayalite and magnetite without depressant

Effective flotation separation of magnetite from iron-bearing silicate minerals is difficult with conventional collectors because of their similar surface properties. In this work, polyoxyethylene alkyl amine (PAE), which is easy to prepare and has high surface activity, was used for the reverse flotation separation of fayalite and magnetite for the first time, and its selective adsorption mechanism was revealed. The micro-flotation tests results show that compared with dodecylamine (DDA), the separation effect of PAE on fayalite and magnetite was significantly improved. When pH was 6.5 and PAE dosage was 20 mg/L, the selective separation of fayalite and magnetite can be achieved by reverse flotation without depressant. The results of zeta potential and contact angle measurement shown that PAE was significantly adsorbed on fayalite compared with magnetite, so the hydrophobicity of fayalite treated by PAE was effectively enhanced. In addition, the differential adsorption of PAE on fayalite and magnetite was further verified by Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and molecular dynamics (MD) simulation, where PAE was adsorbed on the surface of fayalite by electrostatic adsorption and hydrogen bonding, while electrostatic repulsion caused PAE to be removed from the surface of magnetite. The angle between the ethylene oxide chains of PAE at adsorption equilibrium reflects the stress state of NH+ and –OH. Therefore, the selective flotation separation of magnetite and fayalite was realized based on the electrical difference of mineral surface and PAE structural characteristics.

The structures of iron silicate glasses with varying iron redox ratios from molecular dynamics simulations and EXAFS analysis

Iron oxides are frequently found in natural and industrial glass compositions and can affect various physical and chemical properties of the glasses and their melts. Thus, a fundamental understanding of iron-bearing silicate melts and glasses is of both scientific and technological importance. This study investigates the structures of sodium iron silicate glasses with compositions of NaFeSiO4, NaFeSi2O6, NaFeSi3O8, and Na5FeSi4O12 using molecular dynamics simulations in combination with Extended X-ray Absorption Fine Structure (EXAFS) characterizations. Short and medium range structural features of these glasses support that ferrous (Fe2+) and ferric (Fe3+) ions play the roles of network modifier and network former, respectively, with the Fe oxidation states playing an important role in the polymerization of the glass network. These simulation results agree well with newly measured room temperature EXAFS spectra. The simulated glass structures were also compared to the melts structures with the same composition but different redox ratios. The average coordination numbers of the cations were found to be affected both by the melt temperature and iron redox ratio.

Thermodynamic assessment of iron and the expansion behavior of porous ceramsite in pure mineral systems relating to solid waste components

Herein, Factsage software and sintering experiment were conducted to predict and correlate thermodynamic calculations of Fe in a synthetic solid waste and the expansion behavior of ceramsite. With the increase of Fe2O3 content, the liquid phase content and viscosity in the multi-component system gradually decreased and the melting temperature range became wider. This can match with the generation of the liquid phase with proper viscosity in the ceramsite and the full expansion properties. At 400–1000 °C, iron as a network former was tetrahedrally coordinated with oxygen to form Hercynite (Al2FeO4) phase; at 1180–1200 °C, iron as a network modifier dissolved into the silicate mineral structure to form the eutectic composition of Diopside (Ca4Mg3.62Fe0.78Si7.6O24) and Ferrosolite (Si16Mg5.09Fe10.66Ca0.26O48). The formation of iron-bearing silicate facilitated the generation of the viscous layer, so that the outer layer of the ceramsite exhibited sealing performance. Finally, a preferable porous ceramsite with compressive strength of 10.7 MPa and an apparent density of 708 kg/m3 was obtained. This study analyzes the effect of Fe2O3 on the expansion behavior of porous ceramsite and provides new insight into how to analyze different solid wastes to produce ceramsite in a predictable way.

The role of iron-bearing minerals for the deep weathering of a hydrothermally altered plutonic rock in semi-arid climate (Chilean Coastal Cordillera)

Weathering is a fundamental process that controls the development of Earth's surface by the formation of erodible material and the release of mineral-bound nutrients. Weathering at depth has been predominantly studied in humid environments, where fluid flow sets mineralogical transformations, whereas (semi-)arid regions were barely investigated in this respect. In many studies the weathering of iron-bearing minerals was shown to play a major role for the advance of weathering at depth. Weathering of iron-bearing minerals is often superimposed on factors that precondition rocks for weathering such as hydrothermal alteration and fracturing by tectonic stress. The low degree of water-rock interactions in (semi-)arid climates offers the opportunity to single out the preconditioning factors and explore the role of iron-bearing minerals for weathering in detail. To this end the interactions of tectonic fracturing, hydrothermal alteration and weathering were investigated in a deep, weakly weathered profile of plutonic rock in a semi-arid region (Santa Gracia, Northern Chile). The profile was recovered by an 87-m-deep drill core and a 2-m-deep soil pit which were used to investigate the mineral composition, magnetic susceptibility, geochemical gradients, and Fe redox-state.Non-cemented fractures of tectonic origin were identified as pathways for the transport of fluids and reagents to depth that are fuelling the initial weathering reactions. Previous hydrothermal fluids mainly migrated along the same fractures and altered the properties of the original bedrock. Therefore, the weathering features along fractures depend on the preconditioning by hydrothermal alteration and tectonic stress which affected the vicinity of these fractures. From the fracture surfaces, water and O2 diffuse into the rock and initiate oxidation and dissolution-reprecipitation reactions. Our study shows that the transformation of iron-bearing silicates (especially biotite) to Fe(III) (oxyhydr)oxides and clay minerals is the dominant weathering-promoting process as it produces stress which leads to micro-fracturing of the rock and finally to the formation of saprolite.Deciphering the links between tectonic fracturing, hydrothermal alteration, and weathering-induced fracturing by iron-bearing silicates in semi-arid settings provides unique insights into the controls of deep weathering in regions with limited fluid flow.

New perspectives in iron ore flotation: Use of collector reagents without depressants in reverse cationic flotation of quartz

The reverse flotation of the iron ore with etheramine collector is significantly impacted when the iron-bearing silicates and the kaolinite are present in the silicates gangue mineral complex. This paper aims to propose a new amidoamine collector N-[3-(Dimethylamino)propyl]dodecanamide for iron ore reverse flotation with a potential of removing the iron oxides depressant. The contact angle and electrophoretic mobility measurements as well as the flotation experiments were performed on the pure quartz, kaolinite and hematite samples to compare its effect with the etheramine which is widely used as collector in reverse flotation of iron ores. Infrared spectroscopy was performed to study the adsorption of the amidoamine collector on the surface of the minerals studied.Amidoamine collector showed a significant selectivity towards quartz while the flotation of hematite and kaolinite were inhibited at pH 10 without the presence of a depressor. A total floatability of quartz was explained by increased contact angle from 10° to about 50° without impacting significantly the contact angle measured on the hematite (9.4° to 17°). Surface hindrance effect caused by the structure of the N-[3-(Dimethylamino)propyl]dodecanamide collector on the hematite surface is the most suitable hypothesis concerning the high selectivity regarding the iron ore reverse flotation process.

Atomistic insight into lithospheric conductivity revealed by phonon\u2013electron excitations in hydrous iron-bearing silicates

Amphiboles are essential components of the continental crust and subduction zones showing anomalous anisotropic conductivity. Rock properties depend on the physical properties of their constituent minerals, which in turn depend on the crystal phonon and electron density of states. Here, to address the atomic-scale mechanism of the peculiar rock conductivity, we applied in situ temperature-dependent Raman spectroscopy, sensitive to both phonon and electron states, to Fe2+-rich amphiboles. The observed anisotropic resonance Raman scattering at elevated temperatures, in combination with density-functional-theory modelling, reveals a direction-dependent formation of mobile polarons associated with coupled FeO6 phonons and electron transitions. Hence, temperature-activated electron-phonon excitations in hydrous iron-bearing chain and layered silicates are the atomistic source of anisotropic lithospheric conductivity. Furthermore, reversible delocalization of H+ occurs at similar temperatures even in a reducing atmosphere. The occurrence of either type of charge carriers does not require initial mixed-valence state of iron or high oxygen fugacity in the system.

The tungsten carbide-carbon monoxide-tungsten buffer and its use for synthesizing iron-bearing silicates in muffle furnaces.

A new "solid-state" oxygen buffering reaction (WC-COW), WC + 1/2O2 ⇌ CO + W, is presented. At atmospheric pressure, the oxygen fugacity in equilibrium with this buffer is approximated in the range of 600-1200 °C by logfO2 =1.53-21008TK±0.13bar. A simple method for maintaining oxygen fugacity in standard muffle furnaces using this buffer in off-the-shelf ceramic crucibles is described. The utility of the experimental arrangement in controlling oxygen fugacity during solid-state synthesis of iron-bearing silicates is demonstrated by rapid synthesis of fayalite, Fe2SiO4, from oxide starting mixtures at 1050 °C.

Dating the Hemudu Neolithic rice cultivation site, East China, by paleomagnetic chronostratigraphy

Hemudu is a famous archaeological site with the earliest rice remains unearthed in East China and represents a classic example of how Neolithic people managed coastal farmland. However, appreciation of human adaptation to Holocene sea-level changes has been hampered by low-resolution chronology due to lack of datable materials in coastal sediments. Correlation of paleomagnetic directions and intensity on a regional scale provides independent relative dating of such sediments. We did high-resolution paleomagnetic chronology work on core HMD13 (27 m in length) from Hemudu Neolithic site and discussed relevant early Holocene paleoclimate change of Southern China. Magnetic minerals identified by scanning electron microscopy are magnetite, iron-bearing silicates, and iron sulfide (pyrite or greigite). Rock magnetic measurements indicate that the primary remanence carrier is single-domain magnetite. Stepwise alternating field (AF) demagnetization experiments at 1 cm intervals on U-channel samples show that paleomagnetic inclinations fluctuate around 60°, with five major peaks and four troughs. A relative paleointensity (RPI) was estimated using normalizing natural remanent magnetization intensity to anhysteretic remanent magnetization over 20–40 mT AF range, which has four major peaks. Variations of inclination and RPI in core HMD13 resemble the data from the East China Sea and allowed the development of a refined age-depth model. The new adjusted age-depth model minimized dating errors of 14C ages among different datable materials. From the new age model, inclination and RPI of HMD13 are comparable to paleosecular variation (PSV) stack from East Asia and predicted results from geomagnetic models. Four cold events in the early Holocene recorded by geochemical proxies imply that sedimentary sequence of core HMD13 was influenced by global paleoclimate variation, while its long-term trend points to local paleoenvironmental change. Increases in sedimentation rates and the marine-derived sediment fractions suggest a massive transgression in the early Holocene (9–7.5 ka). Subsequently, ancient people developed and built pile dwellings since ~7 ka and cultivated wild rice from ~6.3 ka. Our study demonstrates that regional stratigraphic correlation with well-dated PSV and RPI records is a practical approach to establishing reliable age-depth models. A high-resolution chronology on continuous sedimentary sequence has excellent potential to elucidate prehistoric human-environment co-evolution.

Recycled calcium carbonate is an efficient oxidation agent under deep upper mantle conditions

Observations of high ferric iron content in diamond garnet inclusions and mantle plume melts suggest a highly heterogeneous distribution of ferric iron in the mantle. Recycling of oxidized materials such as carbonates from Earth’s surface by subduction could explain the observed variations. Here we present high-pressure high-temperature multi-anvil experiments to determine the redox reactions between calcium-, magnesium-, or iron-carbonate and ferrous iron-bearing silicate mineral (garnet or fayalite) at conditions representative of subduction zones with intermediate thermal structures. We show that both garnet and fayalite can be oxidized to ferric iron-rich garnets accompanied by reduction of calcium carbonate to form graphite. The ferric iron content in the synthetic garnets increases with increasing pressure, and is correlated with the Ca content in the garnets. We suggest that recycled sedimentary calcium carbonate could influence the evolution of the mantle oxidation state by efficiently increasing the ferric iron content in the deep upper mantle.

Rapid reduction of basaltic glasses in piston-cylinder experiments: a XANES study

Graphite capsules are commonly used in high-temperature, high-pressure experiments, particularly for nominally anhydrous experiments and iron-bearing silicate samples. Due to the presence of graphite in the sample assembly, the oxygen fugacity for these experiments is thought to be relatively low, typically at or below the graphite-CO-CO2 buffer (CCO). The detailed mechanism and kinetics of redox control in graphite capsule experiments are, however, poorly understood. This is especially problematic for short duration experiments (e.g. kinetic experiments), because it is uncertain whether the experimental product will preserve its initial oxygen fugacity, or become reduced during the experiment. In this study, a set of basaltic glasses after high-temperature experiments in graphite capsules were analyzed by micro X-ray absorption near-edge structure (µ-XANES) to obtain their Fe3+/ΣFe profiles near the graphite–melt interface. The results show rapid reduction of ferric iron in the basaltic melt, reaching near-equilibrium in half an hour for samples of 2 mm diameter and 1.3–1.9 mm thickness. Even for a “time-zero” experiment, which was quenched immediately after reaching the target temperature, the reduction profile is over 100 µm in length. By comparing experiments at the same temperature and pressure but with different durations, the reduction reaction progress is found to be linear to the square root of duration, indicating that the reduction process is diffusion-controlled. Such a rapid reduction of the basaltic melt requires a mechanism that is significantly faster than divalent cation diffusion or oxygen diffusion, and is best explained by molecular hydrogen diffusion. It has been shown by previous studies that nominally anhydrous high-pressure experiments could contain significant amounts of water. Thousands of ppm of H2O could remain in the graphite capsule even after drying at 120 °C for an extended time period. At high temperatures, H2O reacts with graphite to produce molecular hydrogen, which then diffuses into the basaltic glass and causes reduction. This mechanism is also supported by a compensating H2O profile of equivalent length in the basaltic glass, showing evidence for H2O produced by molecular hydrogen reacting with ferric iron. A quantitative model is proposed and it successfully reproduces the Fe3+/ΣFe profiles in our experiments. The model helps explain the kinetics of the reduction process and demonstrates that for a basaltic glass with reasonable initial FeO* content, Fe3+/ΣFe ratios, and thicknesses, the equilibrium oxidation state can usually be reached in one hour at ~ 1300 °C and ~ 0.5 GPa. Although extrapolating our conclusion to the large range of graphite capsule experiments requires knowledge on how H2 solubility and diffusivity varies as a function of silicate composition, temperature, and pressure, the reduction process is expected to be rapid in general because H2 diffusivity is high in silicate melts. Our study elucidates the mechanism and rate of oxygen fugacity change in graphite capsule experiments. Based on thermodynamic calculations, the reaction between graphite capsule and H2O is expected to produce a C-O-H fluid with an intrinsic oxygen fugacity of CCO −0.9, which agrees well with the measured Fe3+/ΣFe ratios in the basaltic glasses and the estimated oxygen fugacity for graphite and Pt–graphite capsule experiments from a previous study. Future studies are needed to better constrain the kinetics of this process at different temperature, pressure, and in silicate melts of different compositions. The dynamic process of H2 diffusing and reducing Fe3+ to Fe2+ shown in our experiments also provides a potential way to determine the diffusivity and solubility of molecular hydrogen in silicate melts, which are crucial in understanding volatile behaviors on reducing planetary bodies, such as the Moon and Mercury.

Non-stoichiometric amorphous magnesium-iron silicates in circumstellar dust shells

Aims.We study the growth of dust in oxygen-rich stellar outflows in order to find out to which extent dust growth models can quantitatively reconcile with the quantities and nature of dust as derived from observations of the infrared emission from circumstellar dust shells.Methods.We use a set of nine well-observed massive supergiants with optically thin dust shells as testbeds because of the relatively simple properties of the outflows from massive supergiants, contrary to the case of AGB stars. Models of the infrared emission from their circumstellar dust shells are compared to their observed infrared spectra to derive the essential parameters that rule dust formation in the extended envelope of these stars. The results are compared with a model for silicate dust condensation.Results.For all objects, the infrared emission in the studied wavelength range, between 6 and 25μm, can be reproduced rather well by a mixture of non-stoichiometric iron-bearing silicates, alumina, and metallic iron dust particles. For three objects (μCep, RW Cyg, and RS Per), the observed spectra can be sufficiently well reproduced by a stationary and (essentially) spherically symmetric outflow in the instantaneous condensation approximation. For these objects, the temperature at the onset of massive silicate dust growth is of the order of 920 K and the corresponding outflow velocity of the order of the sound velocity. This condensation temperature is only somewhat below the vapourisation temperature of the silicate dust and suggests that the silicate dust grows on the corundum dust grains that formed well inside of the silicate dust shell at a much higher temperature. The low expansion velocity at the inner edge of the silicate dust shell further suggests that, for these supergiants, the region inside the silicate dust shell has an only subsonic average expansion velocity, though a high degree of supersonic turbulence is indicated by the widths of spectral lines.Conclusions.Our results suggest that for the two major problems of dust formation in stellar outflows, that is (i) formation of seed nuclei and (ii) their growth to macroscopic dust grains, we are gradually coming close to a quantitative understanding of the second item.

Collecting Agent–Mineral Interactions in the Reverse Flotation of Iron Ore: A Brief Review

Froth flotation has been widely used in upgrading iron ores. Iron ore flotation can be performed in two technical routes: direct flotation of iron oxides and reverse flotation of gangue minerals with depression of iron oxides. Nowadays, reverse flotation is the most commonly used route in iron ore flotation. This review is focused on the reverse flotation of iron ores, consisting of reverse cationic flotation and reverse anionic flotation. It covers different types of collecting agents used in reverse iron ore flotation, the surface characteristics of minerals commonly present in iron ores (e.g., iron oxides, quartz, alumina-bearing minerals, phosphorus-bearing minerals, iron-bearing carbonates, and iron-bearing silicates), and the adsorption mechanisms of the collecting agents at the mineral surface. The implications of collecting agent–mineral interactions for improving iron ore flotation are discussed.

Investigation of the specularite/chlorite separation using chitosan as a novel depressant by direct flotation

Chlorite is one of the representative iron-bearing silicates gangue minerals existed in the specularite ores which the traditional depressants are incapable of action in specularite/chlorite separation flotation. An attempt was conducted for the separation of specularite/chlorite with chitosan as a novel depressant through microflotation tests, Zeta potential measurements, adsorption tests, FT-IR, and XPS analysis. The microflotation results show that chitosan selectively depresses chlorite while specularite still keeps in high floatability. Zeta potential measurements and adsorption tests indicate that chitosan mainly adsorbed on chlorite surface, hindering the subsequent adsorption of dodecan-1-amine and leading the hydrophobicity distinction. The FT-IR spectra of chlorite validate the adsorption of chitosan on chlorite. The results of XPS illustrate that electrons partially transferred from chitosan to the aluminum, iron, magnesium, silicon, and adjacent oxygen atoms of silicon atoms in chlorite during the adsorption process.

Fate of Hydrocarbons in Iron-Bearing Mineral Environments during Subduction

Subducted sediments play a key role in the evolution of the continental crust and upper mantle. As part of the deep carbon cycle, hydrocarbons are accumulated in sediments of subduction zones and could eventually be transported with the slab below the crust, thus affecting processes in the deep Earth’s interior. However, the behavior of hydrocarbons during subduction is poorly understood. We experimentally investigated the chemical interaction of model hydrocarbon mixtures or natural oil with ferrous iron-bearing silicates and oxides (representing possible rock-forming materials) at pressure-temperature conditions of the Earth’s lower crust and upper mantle (up to 2000(±100) K and 10(±0.2) GPa), and characterized the run products using Raman and Mössbauer spectroscopies and X-ray diffraction. Our results demonstrate that complex hydrocarbons are stable on their own at thermobaric conditions corresponding to depths exceeding 50 km. We also found that chemical reactions between hydrocarbons and ferrous iron-bearing rocks during slab subduction lead to the formation of iron hydride and iron carbide. Iron hydride with relatively low melting temperature may form a liquid with negative buoyancy that could transport reduced iron and hydrogen to greater depths.

Hubble Ultraviolet Spectroscopy of Jupiter Trojans

Abstract We present the first ultraviolet spectra of Jupiter Trojans. These observations were carried out using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope and cover the wavelength range 200–550 nm at low resolution. The targets include objects from both of the Trojan color subpopulations (less-red and red). We do not observe any discernible absorption features in these spectra. Comparisons of the averaged UV spectra of less-red and red targets show that the subpopulations are spectrally distinct in the UV. Less-red objects display a steep UV slope and a rollover at around 450 nm to a shallower visible slope, whereas red objects show the opposite trend. Laboratory spectra of irradiated ices with and without H2S exhibit distinct UV absorption features; consequently, the featureless spectra observed here suggest H2S alone is not responsible for the observed color bimodality of Trojans, as has been previously hypothesized. We propose some possible explanations for the observed UV-visible spectra, including complex organics, space weathering of iron-bearing silicates, and masked features due to previous cometary activity.

The degree of polymerization and structural disorder in (Mg,Fe)SiO3 glasses and melts: Insights from high-resolution 29Si and 17O solid-state NMR

Probing the degree of polymerization and structural disorder of (Mg,Fe)SiO3 glasses and melts remains a challenging problem, despite their strong implications for the properties of natural volcanic glasses and dense melts in Earth’s surface and interior. Here, we explore the iron-induced changes in silicon and the oxygen configurations of (Mg1−XFeX)SiO3 glasses (Fe3+/ƩFe = 0.22 ± 0.07) with varying XFe [=Fe/(Mg + Fe), containing up to 13.5 wt% FeO] using high-resolution 29Si and 17O MAS, and 2D 17O 3QMAS NMR, unveiling the effect of Fe2+ on the extent of disorder in these glasses. The 29Si MAS NMR spectra show the apparent peak broadening in less polymerized species (i.e., Q1) and an increase in the fraction of highly polymerized (i.e., Q3) species with increasing XFe. These changes imply the selective decrease in NMR signal intensity of Q1 and Q2 due to their spatial proximity to Fe2+ (i.e., paramagnetic effect), and slight increase in the degree of polymerization due to the presence of minor fraction of Fe3+. The 2D 17O NMR spectra revealed that the fraction of Si-O-Si apparently increases with XFe and more significant changes in peak width for Mg-O-Si are observed with varying XFe. The results indicate that Mg-O-Si is more strongly affected by Fe2+ than Si-O-Si, suggesting a moderate degree of preferential proximity between Fe2+ and non-bridging oxygen. The results allow us to conceptualize the scheme to account for paramagnetic interaction between the nuclear spins and Fe2+. The scheme provides a systematic protocol to interpret the NMR spectra of diverse iron-bearing silicate glasses. The current NMR results, in conjunction with the results in the previous study for Fe3+-dominant sodium silicate glasses, suggest that paramagnetic effect depends on the valence state of Fe, indicating the presence of a moderate degree of preferential interaction between Fe2+ and NBO and less polymerized Si units as well as that between Fe3+ and highly polymerized Si units and BO. Though qualitative, the current NMR results for (Mg,Fe)SiO3 glasses shed light on an opportunity to explore the detailed structures of iron-bearing natural volcanic glasses and can improve understanding of the transport properties of the basaltic melts.

Bulk chemical characteristics of soluble polar organic molecules formed through condensation of formaldehyde: Comparison with soluble organic molecules in Murchison meteorite

Carbonaceous chondrites contain up to 2 wt% organic carbon, which is present as acid and solvent insoluble solid organic matter (IOM) and solvent soluble organic matter (SOM). The extraterrestrial organic matter should record chemical processes occurred in different environments in the early history of the Solar System, and the role of parent body aqueous alteration in the synthesis or subsequent modification of IOM and SOM still requires accurate constraints. We conducted hydrothermal experiments to simulate the synthesis of organic molecules during aqueous alteration on small bodies. Bulk chemical characteristics of soluble organic matter synthesized from formaldehyde in aqueous solutions were studied to compare them with that of chondritic SOM. We found that the redox state of synthesized organic molecules depends on temperature; the molecules become richer in hydrogen at higher temperatures. This can be explained by a cross-disproportionation reaction between organic molecules and formic acid, which occurs as a side reaction of the aldol condensation and works more effectively at higher temperatures. Comparison of the bulk chemical characteristics between the synthesized molecules and SOM extracted from the Murchison meteorite with methanol shows that the soluble organic molecules in Murchison are more reduced than the synthesized molecules. Considering the temperature condition for aqueous alteration on the CM parent body that is lower than or equivalent to the experimental temperatures, the reduced nature of Murchison organic molecules requires a reducing environment for them to be formed during hydrothermal alteration or imply that processes other than hydrothermal alteration were responsible for their synthesis. In case of hydrothermal synthesis, reducing conditions might be established by the interaction between water and iron-bearing silicates or metals on the parent body.