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Origin and formation of iron silicide phases in the aerogel of the Stardust mission
Abstract
Abstract— Suessite along with hapkeite and more Fe-rich iron-silicides up to Fe7Si2 formed near the entrance of aerogel track #44. These phases are ˜100 nm, quenched-melt spheres, but the post-impact cooling regime was such that melt vitrification produced a poly cry stalline mixture of Fe silicides and kamacite. The compositional similarities of the impact-produced Fe-Si phases and the Fe-Ni-S phases scattered throughout the aerogel capture medium strongly supports the idea that Fe silicides resulted from a reaction between molten Fe-Ni-S phases and aerogel at very high heating and cooling rates. Temperatures of around 1500 °C are inferred from the observed compositions had the silicide spheres formed at thermodynamic equilibrium, which seems unlikely. When the conditions were kinetically controlled, they could have been similar to those leading to the formation of solids with predictable deep metastable eutectic compositions in laboratory condensation experiments.
... The numerous, typically <100 nm electron-opaque Fe-Ni-S inclusions Rietmeijer at al. 2008;Tomeoka et al. 2008;Zolensky et al. 2008) were derived from Wild 2 sulfides. The low-Ni Fe-S and high-S Fe-S eutectic and deep metastable eutectic compositions of the inclusions suggest Sirich melt temperatures >1200 °C (Rietmeijer 2008;Rietmeijer et al. 2008). ...
... The Si-rich glass clumps appear to be porous agglomerates of smaller (quenched) melt droplets (see images in Nakamura et al. 2008;Rietmeijer et al. 2008). It is tempting to think that the extracted clumps are the "frozen" agglomerates by an instantaneous thermal transformation without chemical or physical mixing of structurally coherent, low-porosity nanometer scale agglomerates of smaller Wild 2 minerals and amorphous materials. ...
... Aggregate IDP L2011A9 is no exception with its simple binary mixture of Mg,Fe-silicates and Fe,Nisulfides that is the common feature of the hierarchy of dust aggregation. It is also present in the Si-rich Stardust glass with the multitude of Fe-Ni-S nanograins wherein this binary hierarchical petrologic makeup was transformed and redistributed as immiscible "silicate" and "sulfide" melts and metastable Fe,Ni and S separation in chemically zoned Fe(Ni)S compounds (Zolensky et al. 2006;Leroux et al. 2008b;Rietmeijer 2007Rietmeijer , 2008aRietmeijer et al. 2008). It is reasonable that some fraction of small Wild 2 loosely bonded grains or agglomerates had fused into denser agglomerates that resembled a dense agglomerate mass in IDP L2011A9 prior to interacting with the hot aerogel. ...
Abstract— Many of the nanometer-scale grains from comet 81P/Wild 2 did not survive hypervelocity capture. Instead, they melted and interacted with silica melt derived from the aerogel used by the Stardust mission. Their petrological properties were completely modified, but their bulk chemistry was preserved in the chemical signatures of mostly vesicular Si-rich glass with its typical Fe-Ni-S compound inclusions. Chondritic aggregate IDP L2011A9 that experienced atmospheric pre-entry thermal modification was selected as an analog to investigate these Wild 2 chemical signatures. The chemical, petrologic, and mineralogical properties of the individual constituents in this aggregate IDP are presented and used to match the chemical signatures of these Wild 2 grains. Mixing of comet material and pure silica, which is used in a diagram that recognizes this mixing behavior, is used to constrain the probable petrologic and minerals that caused the Wild 2 signatures. The Wild 2 nanometer-scale grain signatures in Si-rich glass allocations from three different deceleration tracks resembled mixtures of ultrafine-grained principal components and dense agglomerate-like material, Mg-rich silicates (<500 nm) and Fe,Ni-sulfides (<100 nm), and Si-rich amorphous material. Dust resembling the mixed matrix of common chondritic aggregate IDPs was present in Jupiter-family comet Wild 2.
... Transmission electron microscope (TEM) imagery from numerous extensively melted, glassy grains reveals fractured grains in which voids and breakage exhibit a preferred orientation parallel to the long dimension of the particle. The chattering (which is more distinct in higher-magnification images) is an artifact of flexural stresses fracturing brittle material during ultramicrotomy (Bradley 1988;Zolensky et al. 2008a;Leroux et al. 2008a;Tomeoka et al. 2008;Rietmeijer et al. 2008), as it does not occur on the grain in the potted butt from which the ultramicrotome slices were taken or in amorphous grains examined by synchrotron microtomography . Extensively melted grains consist of vesicular glassy material with dark inclusions as described by Zolensky et al. (2006, Figs. ...
... 1A and B), Leroux et al. (2008aLeroux et al. ( , 2009) and Tomeoka et al. (2008). Energy dispersive spectroscopy (EDS) analyses of low-magnification fields-of-view suggest broadly chondritic ratios of elements indigenous to the cometary particles (Leroux et al. 2008aTomeoka et al. 2008;Rietmeijer et al. 2008); for example, Leroux et al. (2008aLeroux et al. ( , 2009 found bulk major element compositions of glassy Stardust grains (including sulfur) to be a near-perfect match with CI composition. Grains affected by partial or complete melting are compositionally heterogeneous, reflecting the small size and heterogeneous distribution of minerals in the pre-capture comet-dust particle, and extensive mixing of partly to completely melted comet material with molten aerogel Leroux et al. 2008a). ...
... Rounded objects darker (electron-opaque at the applied accelerating voltage) in bright-field TEM than the vesicular glass are ubiquitous in all grains from Track 35 examined here, as in many similar grains (Leroux et al. 2008a;Tomeoka et al. 2008). Rounded objects (beads) are very common and widely distributed in the five grains allocated for this study, as in many other grains Ishii et al. 2008a;Leroux et al. 2008a;Rietmeijer et al. 2008;Tomeoka et al. 2008). The beads range in diameter from tens of nanometers or smaller to more than 100 nm (Leroux et al. 2008a). ...
Abstract— Five amorphous (extensively melted) grains from Stardust aerogel capture Track 35 were examined by transmission electron microscopy (TEM); two from the bulb, two from near the bulb-stylus transition, and one from near the terminal particle. Melted grains consist largely of a texturally and compositionally heterogeneous emulsion of immiscible metal/sulfide beads nanometers to tens of nanometers in diameter in a silica-rich vesicular glass. Most metal/sulfide beads are spherical, but textures of non-spherical beads indicate that some solidified as large drops during stretching and breaking while in translational and rotational motion, and others solidified from lenses of immiscible liquid at the silicate-melt/vesicle (vapor) interface.
Melted grains appear to become richer in Fe relative to Mg, and depleted in S relative to Fe and Ni with increasing penetration distance along the aerogel capture track. Fe/S ratios are near unity in grains from the bulb of Track 35, consistent with the dominance of Fe-monosulfide minerals inferred by previous research on Stardust materials. Near-stoichiometric Fe/S in melted grains from the bulb suggests that Fe-sulfides in the bulb were dispersed and melted during formation of the bulb but did not lose S. Along-track increases in Fe/S in melted grains from the bulb through the bulb-stylus transition and continuing into the stylus indicate that S initially present as iron monosulfide may have been progressively partially volatilized and lost from the melted grains with greater penetration of the grains deeper into the aerogel during capture-melting of comet dust. Extensively melted grains from the bulbs of aerogel capture tracks may preserve better primary compositional information with less capture-related modification than grains from farther along the same capture tracks.
... The allocation described here came from a large (>10 µm), irregular (popcorn-like) clump of Si-rich glass with many macroscopic voids (cf. Rietmeijer et al. 2008;Nakamura et al. 2008). Its morphology gives the impression of an agglomerated mass of quenched glass blobs. ...
... Its morphology gives the impression of an agglomerated mass of quenched glass blobs. Internally it is a silica-rich glass with numerous nanometer Fe,Ni metal and Fe-Ni-S compound inclusions very similar to previously studied Stardust glasses Nakamura et al. 2008;Rietmeijer et al. 2008;Tomeoka et al. 2008;Zolensky et al. 2008). The Si-rich matrix consists mostly of highly vesicular shards but non-vesicular "clean" glass shards are present. ...
... The compositions of Si-rich glasses extracted from the deceleration tracks are linear mixtures of CI-like comet dust and melted silica aerogel . I assume that (1) chondritic pieces of comet Wild 2 dust interacting with silica melt and aggregate IDPs are both binary mixtures of (Rietmeijer 2008;Rietmeijer et al. 2008). I then use this graphical presentation (Fig. 8) to explore trends in background-corrected compositions. ...
Abstract— Flight aerogel in Stardust allocation C2092,2,80,47,6 contains percent level concentrations of Na, Mg, Al, S, Cl, K, Ca, Cr, Mn, Fe, and Ni that have a distinctive Fe- and CI-normalized distribution pattern, which is similar to this pattern for ppb level chemical impurities in pristine aerogel. The elements in this aerogel background were assimilated in non-vesicular and vesicular glass with the numerous nanometer Fe-Ni-S compound inclusions. After correction for the background values, the chemical data show that this piece of comet Wild 2 dust was probably an aggregate of small (<500 nm) amorphous ferromagnesiosilica grains with many tiny Fe,Ni-sulfide inclusions plus small Ca-poor pyroxene grains. This distinctive Fe- and CI-normalized element distribution pattern is found in several Stardust allocations. It appears to be a common feature in glasses of quenched aerogel melts but its exact nature is yet to be established.
... This is discussed in more detail in Grossemy et al. (2008) and Burchell and Kearsley (2009) and Burchell et al. (2006a). Briefly, Rietmeijer et al. (2008) and Leroux et al. (2008) used analytical TEM analyses of samples from along cometary tracks to show that the most frequent microstructure consists of a silica-rich glassy matrix containing a large number of vesicles and abundant Fe-Ni-S inclusions. This suggests that a large proportion of incoming dust particles were fully melted and mixed with molten aerogel. ...
... As some glassy aerogel was attached to the iron oxides, grain sizes were difficult to measure accurately but ranged up to approximately 100-200 nm. A second elemental signature for some grains was Mg-Fe-O-(Si), consistent with the common presence of forsterite olivine in many of the Stardust samples and characteristic of silica-rich glass in the Stardust samples (Zolensky et al. 2006;Leroux et al. 2008;Rietmeijer et al. 2008). ...
... There have been conflicting arguments for whether capture heating in aerogel causes oxidation or reduction. Rietmeijer et al. (2008) described Fe silicides near the entrance of track 44: they suggested these resulted from the reduction of melted Fe-Ni sulfide grains. Marcus et al. (2008) also reported light-gas gun experiments where ferric iron had been reduced to ferrous compounds and considered that this was associated with capture heating in the presence of carbon within the aerogel. ...
Abstract– We have used synchrotron Fe-XANES, XRS, microRaman, and SEM-TEM analyses of Stardust track 41 slice and track 121 terminal area slices to identify Fe oxide (magnetite-hematite and amorphous oxide), Fe-Ti oxide, and V-rich chromite (Fe-Cr-V-Ti-Mn oxide) grains ranging in size from 200 nm to ∼10 μm. They co-exist with relict FeNi metal. Both Fe-XANES and microRaman analyses suggest that the FeNi metal and magnetite (Fe2O3FeO) also contain some hematite (Fe2O3). The FeNi has been partially oxidized (probably during capture), but on the basis of our experimental work with a light-gas gun and microRaman analyses, we believe that some of the magnetite-hematite mixtures may have originated on Wild 2. The terminal samples from track 121 also contain traces of sulfide and Mg-rich silicate minerals. Our results show an unequilibrated mixture of reduced and oxidized Fe-bearing minerals in the Wild 2 samples in an analogous way to mineral assemblages seen in carbonaceous chondrites and interplanetary dust particles. The samples contain some evidence for terrestrial contamination, for example, occasional Zn-bearing grains and amorphous Fe oxide in track 121 for which evidence of a cometary origin is lacking.
... The Stardust mission to comet 81P/Wild 2 (size: 5.5 km × 4.0 km × 3.3 km; density: 0.6 g/cm 3 ; mass: 2.3 × 10 13 kg) provided evidence of iron silicides, but these are very likely to be secondarily produced. In aerogel track #44 within three grains (C2004,1,44,1,0, C2004,1,44,2,0, and C2004,1,44,3,0 all approximately 15 µm × 20 µm sized), Fe-Si phases (~100 nm, quenched-melt spheres) Fe 2 Si (hapkeite), Fe 3 Si (suessite), up to (Fe,Ni,Cr)Si alloy (Fe 3.35 Ni 0.13 Cr 0.05 )(Si) 1.0 , corresponding a Fe 7 Si 2 (unnamed), were detected [243]. Suessite was also found in 16 grains of track #35 [244]. ...
... Ca 2 Al 2 O 5 was also detected in the xenoliths of the Ettringer Bellerberg volcanic system (Ettringen, Mayen-Koblenz, Rhineland-Palatinate, Germany, 50 • 21 0.88 N, 7 • 13 41.65 E), dated c. 0.215 ± 0.004 to 0.190 ± 0.004 mya [434]. In addition, the iron silicide suessite (Fe,Ni) 3 Si formed from the matrix at more than 2000 K, and cubic moissanite ([β]3C-SiC) as well as nanodiamonds indicated high shock pressure [243]. Xifengite (Fe 5 Si 3 ) and carbon spherules within amorphous carbon were found in the glazed enamel skin of a pebble from crater #004 in the field. ...
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.
... Silicon-rich iron silicide minerals are more commonly found in fulgurites, which in general are much richer in Si vs. Fe [2,19,22,25,38]. Iron-rich iron silicide minerals are more common in extraterrestrial materials, as iron metal is generally more abundant [30,[46][47][48][49]. However, it is also plausible that fulgurites enriched in iron also contain iron-rich iron silicide [3], and silicon-rich iron silicides can be formed in the solar system [50] (Figure 4). ...
... Silicides are also found in at least one Lunar meteorite, which are believed to be formed during impact. Thus, this iron silicide mineral formation route in our solar system requires low oxygen fugacity and ultrahigh-temperature conditions [31,46], which are like the fulgurite-forming conditions [2]. ...
Iron silicide minerals (Fe-Si group) are found in terrestrial and solar system samples. These minerals tend to be more common in extraterrestrial rocks such as meteorites, and their existence in terrestrial rocks is limited due to a requirement of extremely reducing conditions to promote their formation. Such extremely reducing conditions can be found in fulgurites, which are glasses formed as cloud-to-ground lightning heats and fuses sand, soil, or rock. The objective of this paper is to review reports of iron silicides in fulgurites, note any similarities between separate fulgurite observations, and to explain the core connection between geological environments wherein these minerals are found. In addition, we also compare iron silicides in fulgurites to those in extraterrestrial samples.
... They have also been identified in several ureilite meteorites (e.g., Keil et al. 1982; Herrin et al. 2008; Ross et al. 2009; Smith et al. 2010 ). Fesilicide beads similar in size to those we observe in the cronstedtite residue (<100 nm) have also previously been identified near the entrance of track #44 in the Stardust aerogel cell C2004 (Rietmeijer et al. 2008). It is generally believed that those observed in the ureilite and lunar samples were produced during impacts occurring on the parent body (although for ureilites it has also been suggested they may instead represent samples of the parent body core, e.g., Ross et al. 2009; Smith et al. 2010). ...
... The Fe-silicide phases are thought to have formed as a result of melting and mixing of Fe and Si (from the materials involved) under reducing conditions. The reducing conditions in each case are thought to be produced by the materials involved: e.g., in the case of Stardust, it was suggested that carbonaceous material in the impacting cometary grain and/or the aerogel itself may have decreased oxygen fugacity in the melted material, thus promoting the reduction of silicates (Rietmeijer et al. 2008). In the case of our cronstedtite impacts into Al foils, the reducing conditions may have been produced by the Al foil target, which upon impact formed an Oscavenging melt. ...
Comet 81P/Wild 2 samples returned by NASA's Stardust mission provide an unequalled opportunity to study the contents of, and hence conditions and processes operating on, comets. They can potentially validate contentious interpretations of cometary infrared spectra and in situ mass spectrometry data: specifically the identification of phyllosilicates and carbonates. However, Wild 2 dust was collected via impact into capture media at ~6 km s-1, leading to uncertainty as to whether these minerals were captured intact, and, if subjected to alteration, whether they remain recognizable. We simulated Stardust Al foil capture conditions using a two-stage light-gas gun, and directly compared transmission electron microscope analyses of pre- and postimpact samples to investigate survivability of lizardite and cronstedtite (phyllosilicates) and calcite (carbonate). We find the phyllosilicates do not survive impact as intact crystalline materials but as moderately to highly vesiculated amorphous residues lining resultant impact craters, whose bulk cation to Si ratios remain close to that of the impacting grain. Closer inspection reveals variation in these elements on a submicron scale, where impact-induced melting accompanied by reducing conditions (due to the production of oxygen scavenging molten Al from the target foils) has resulted in the production of native silicon and Fe- and Fe-Si-rich phases. In contrast, large areas of crystalline calcite are preserved within the calcite residue, with smaller regions of vesiculated, Al-bearing calcic glass. Unambiguous identification of calcite impactors on Stardust Al foil is therefore possible, while phyllosilicate impactors may be inferred from vesiculated residues with appropriate bulk cation to Si ratios. Finally, we demonstrate that the characteristic textures and elemental distributions identifying phyllosilicates and carbonates by transmission electron microscopy can also be observed by state-of-the-art scanning electron microscopy providing rapid, nondestructive initial mineral identifications in Stardust residues.
... Potential contaminants may derive from spacecraft materials, landing site soils, or carbon intrinsic to the aerogel itself , although such contaminants have yet to be observed within capture tracks. Hypervelocity impact ($6 km s )1 ) into aerogel may heat the captured particles to a peak temperature greater than 1200°C Rietmeijer et al. 2008), which could vaporize organic matter. However, much evidence exists that suggests that some organic matter can survive hypervelocity impact Noguchi et al. 2007;Spencer et al. 2009;Berger et al. 2011) and that primitive cometary organic matter is present within the capture tracks (Matrajt et al. 2008;Wirick et al. 2009;De Gregorio et al. 2010). ...
... Cometary dust from Wild 2 impacted the Stardust collector array at a velocity of 6.1 km s )1 and was brought to rest after several mm by aerogel compression (Dominguez et al. 2004). Frictional heating during this process may heat the captured particle and surrounding aerogel to temperatures in excess of 1200°C, causing melting of mineral grains and silica aerogel Rietmeijer et al. 2008). Gas-gun test shots using coated polystyrene grains showed significant ablation and mass loss during aerogel capture (Burchell et al. 2009). ...
Carbonaceous matter in Stardust samples returned from comet 81P/Wild 2 is observed to contain a wide variety of organic functional chemistry. However, some of this chemical variety may be due to contamination or alteration during particle capture in aerogel. We investigated six carbonaceous Stardust samples that had been previously analyzed and six new samples from Stardust Track 80 using correlated transmission electron microscopy (TEM), X-ray absorption near-edge structure spectroscopy (XANES), and secondary ion mass spectroscopy (SIMS). TEM revealed that samples from Track 35 containing abundant aliphatic XANES signatures were predominantly composed of cometary organic matter infilling densified silica aerogel. Aliphatic organic matter from Track 16 was also observed to be soluble in the epoxy embedding medium. The nitrogen-rich samples in this study (from Track 22 and Track 80) both contained metal oxide nanoparticles, and are likely contaminants. Only two types of cometary organic matter appear to be relatively unaltered during particle capture. These are (1) polyaromatic carbonyl-containing organic matter, similar to that observed in insoluble organic matter (IOM) from primitive meteorites, interplanetary dust particles (IDPs), and in other carbonaceous Stardust samples, and (2) highly aromatic refractory organic matter, which primarily constitutes nanoglobule-like features. Anomalous isotopic compositions in some of these samples also confirm their cometary heritage. There also appears to be a significant labile aliphatic component of Wild 2 organic matter, but this material could not be clearly distinguished from carbonaceous contaminants known to be present in the Stardust aerogel collector.
... Potential contaminants may derive from spacecraft materials, landing site soils, or carbon intrinsic to the aerogel itself (Sandford et al. 2010), although such contaminants have yet to be observed within capture tracks. Hypervelocity impact ($6 km s )1 ) into aerogel may heat the captured particles to a peak temperature greater than 1200 °C (Leroux et al. 2008; Rietmeijer et al. 2008), which could vaporize organic matter. However, much evidence exists that suggests that some organic matter can survive hypervelocity impact (Burchell et al. 2006; Noguchi et al. 2007; Spencer et al. 2009; Berger et al. 2011) and that primitive cometary organic matter is present within the capture tracks (Matrajt et al. 2008; Wirick et al. 2009; De Gregorio et al. 2010). ...
... The position of the characteristic graphite exciton photoabsorption is denoted by ''g.'' melting of mineral grains and silica aerogel (Leroux et al. 2008; Rietmeijer et al. 2008). Gas-gun test shots using coated polystyrene grains showed significant ablation and mass loss during aerogel capture (Burchell et al. 2009). ...
Abstract– Carbonaceous matter in Stardust samples returned from comet 81P/Wild 2 is observed to contain a wide variety of organic functional chemistry. However, some of this chemical variety may be due to contamination or alteration during particle capture in aerogel. We investigated six carbonaceous Stardust samples that had been previously analyzed and six new samples from Stardust Track 80 using correlated transmission electron microscopy (TEM), X-ray absorption near-edge structure spectroscopy (XANES), and secondary ion mass spectroscopy (SIMS). TEM revealed that samples from Track 35 containing abundant aliphatic XANES signatures were predominantly composed of cometary organic matter infilling densified silica aerogel. Aliphatic organic matter from Track 16 was also observed to be soluble in the epoxy embedding medium. The nitrogen-rich samples in this study (from Track 22 and Track 80) both contained metal oxide nanoparticles, and are likely contaminants. Only two types of cometary organic matter appear to be relatively unaltered during particle capture. These are (1) polyaromatic carbonyl-containing organic matter, similar to that observed in insoluble organic matter (IOM) from primitive meteorites, interplanetary dust particles (IDPs), and in other carbonaceous Stardust samples, and (2) highly aromatic refractory organic matter, which primarily constitutes nanoglobule-like features. Anomalous isotopic compositions in some of these samples also confirm their cometary heritage. There also appears to be a significant labile aliphatic component of Wild 2 organic matter, but this material could not be clearly distinguished from carbonaceous contaminants known to be present in the Stardust aerogel collector.
... Natural iron silicide materials have been found in various, generally extremely reducing, environments, e.g., in lightning-induced fulgurites (Essene & Fisher, 1986), in micro-meteoritic impactformed lunar regolith (Gopon et al., 2013), in rocks formed deep in the Earth's mantle (Shiryaev et al., 2011), and in Project Stardust samples (Rietmeijer et al., 2008). Their study is of great importance to understand the formation mechanism of the materials in which they occur. ...
The recent availability of Schottky-type field emission electron microprobes provides incentive to consider analyzing micrometer-sized features. Yet, to quantify sub-micrometer-sized features, the electron interaction volume must be reduced by decreasing accelerating voltage. However, the K lines of the transition elements (e.g., Fe) then cannot be excited, and the L lines must be used. The Fe L α1,2 line is the most intense of the L series but bonding effects change its atomic parameters because it involves a valence band electron transition. For successful traditional electron probe microanalysis, the mass absorption coefficient (MAC) must be accurately known, but the MAC of Fe L α1,2 radiation by Fe atoms varies from one Fe-compound to another and is not well known. We show that the conventional method of measuring the MAC by an electron probe cannot be used in close proximity to absorption edges, making its accurate determination impossible. Fortunately, we demonstrate, using a set of Fe–silicide compounds, that it is possible to derive an accurate calibration curve, for a given accelerating voltage and takeoff angle, which can be used to quantify Fe in Fe–silicide compounds. The calibration curve can be applied to any spectrometer without calibration and gives accurate quantification results.
... Their stoichiometry varies widely, from Me/Si = 3 to 0.42. Most of them are identified in meteorites and cosmic dust, such as suessite and hapkeite (Anand et al. 2004;Rietmeijer et al. 2008), as well as the manganese silicide, brownleeite MnSi (Nakamura-Messenger et al. 2010); some are known from terrestrial formations, frequently as inclusions in moissanite along with native Si 0 (Shiryaev et al. 2011). ...
A series of polycrystalline diamond grains were found within the Valizhgen Peninsula in Koryakia, northern Kamchatka, Russia. A grain from the Aynyn River area is studied in detail with TEM. Diamond crystallites, 2-40 μm in size are twinned and have high dislocation density. They are cemented with tilleyite Ca 5 (Si 2 O 7)(CO 3) 2 , SiC, Fe-Ni-Mn-Cr silicides, native silicon, graphite, calcite, and amorphous material. Among SiC grains, three polymorphs were discriminated: hexagonal 4H and 6H and cubic C3 (β-SiC). Silicides have variable stoichiometry with (Fe,Ni,Mn,Cr)/Si = 0.505-1.925. Native silicon is an open-framework allotrope of silicon S 24 , which has been observed, to date, as a synthetic phase only; this is a new natural mineral phase. Three types of amorphous material were distinguished: a Ca-Si-CO material, similar in composition to tilleyite; amorphous carbon in contact with diamond, which includes particles of crystalline graphite; and amorphous SiO 2. No regularity in the distribution of the amorphous material was observed. In the studied aggregate, diamond crystallites and moissanite are intensively twinned, which is characteristic for these minerals formed by gas phase condensation or chemical vapor deposition (CVD) processes. The synthetic analogs of all other cementing compounds (β-SiC, silicides, and native silicon) are typical products of CVD processes. This confirms the earlier suggested CVD mechanism for the formation of Avacha diamond aggregates. Both Avacha and Aynyn diamond aggregates are related not to "classic" diamond locations within stable cratons, but to areas of active and Holocene volcanic belts. The studied diamond aggregates from Aynyn and Avacha, by their mineralogical features and by their origin during the course of volcanic eruptions via a gas phase condensation or CVD mechanism, may be considered a new variety of polycrystalline diamond and may be called "kamchatite." Kamchatite extends the number of unusual diamond localities. It increases the potential sources of diamond and indicates the polygenetic character of diamond.
... Even interplanetary dust particles are also composed of these materials [1]. Organic materials, amorphous and crystalline silicate materials are found in comets and IDPs [21]. Amorphous silicates are also found in interstellar and circumstellar medium and constitute 2/3 of the mass of the interstellar dust. ...
In this work, we study the light scattering properties of dust aggregates (0.7µm< R c < 2.0µm) having a wide range of porosity (P = 0.59 to 0.98). The simulations are executed using the Superposition T-matrix code with BCCA, BA, BAM1 and BAM2 clusters of varying porosity. We investigate the nature and dependencies of the different scattering parameters on porosity, size and composition of the aggregated particles for wavelengths 0.45 µm and 0.65 µm. We find that the scattering parameters are strongly correlated with the porosity of the aggregated structures. Our results indicate that, when the porosity of the aggregates decreases, keeping characteristic radius of the aggregates (R c) same for all structures, there is an enhancement in the negative polarization branch (NPB) which is accompanied by a substantial increase in the anisotropies present in the material. Also at the exact backscattering region, the anisotropies are found to be linearly correlated with the porosity of the aggregated structure. The computational study reveals that, for low absorbing materials (k ≤ 0.1), the negative polarization minimum (P min) is strongly correlated with the associated anisotropies. Finally, we put forward a qualitative comparison between our computationally obtained results and some selected data from the Amsterdam Light Scattering Database for both low and high absorbing materials. The experimental results also suggest that an increase in the NPB is always accompanied by an enhancement in the anisotropy at the backscattering region.
... As discussed 567 by Essene and Fisher (1986) and Rowan and Ahrens (1994), possible reducing agents include C and 568 N, which may effectively scavenge oxygen from a melt and locally facilitate reduction of oxides to 569 produce elemental metals. This form of "smeltering" was also discussed for the formation of iron 570 silicides in dust particles from comet 81P/Wild 2, which were trapped in the silica aerogel capture 571 medium of the Stardust spacecraft (Rietmeijer et al., 2008). Thus, we raise the question whether 572 organic matter could have played a role in Khatyrka's history as well. ...
We analyzed the interaction of spherical, 6.36-mm-diameter, Cu-bearing aluminum projectiles with quartz sand targets in hypervelocity impact experiments performed at NASA Ames Vertical Gun Range. Impact velocities and inferred peak shock pressures varied between 5.9-6.5 km/s and ~41-48 GPa, respectively. Shocked particles ("impact melt particles") coated with thin crusts of molten projectile material were recovered from the floors of the ca. 33-cm-diameter craters and the respective ejecta blankets. Through petrographic and chemical analyses (optical microscopy, FE-EMPA, SEM-EDX, and XRF analysis) we show that these particles have a layered structure manifested in distinct layers of decreasing shock metamorphism. These can be characterized by the following physical and chemical reactions and alteration products: (i) complete melting and subsequent recrystallization of the projectile, forming a distinct crystallization texture in the fused metal crust; (ii) projectile-target mixing, involving a redox reaction between Cu-bearing Al alloy und SiO2, leading to formation of khatyrkite (CuAl2), Al2O3 melt, euhedral silicon crystals, and spherical droplets of silicon; (iii) melting of quartz to lechatelierite and formation of planar deformation features in relic quartz grains; and (iv) shock lithification of quartz grains with fracturing of grains, grain-boundary melting, planar deformation features, and complete loss of porosity. To our knowledge, this is the first report of khatyrkite formed experimentally in hypervelocity impact experiments. These results have implications for the understanding of a similar redox reaction between Al-Cu metal and siliceous impact melt recently postulated for the Khatyrka CV3 carbonaceous chondrite. Moreover, these results bear on the processes that lead to layers of regolith on the surfaces of planetary bodies without atmospheres, such as asteroids in the main belt (e.g., 4 Vesta), and on the Moon. Specifically, impacts of mm-sized projectiles at velocities between 4-6 km/s into regolith-covered, asteroidal surfaces in the main belt should yield similar impact melt particles that feature a continuum of shock effects, i.e., partially to completely molten projectile remnants adhering to impact-melted regolith agglomerates, as well as projectile-contaminated impact melts and local shock melting along grain boundaries.
... Two spherules matching this description were reported by Yu (1984) and appear as small (100-500 μm diameter) spherules containing an Fe-Silicide core, mantled by a concentric layer of Fe-Ni metal and coated by Feoxides (wüstite, maghemite and magnetite) (Yu, 1984). These rare grains most likely represent ablation spherules, rather than cosmic spherules, derived by melt extraction from the fusion crust of an infalling ureilite (Rietmeijer et al., 2008). Instead, the Cretaceous Fe-silicide spherules, identified in this study, most likely formed by a similar process as the Fe-oxide spherules, owing to their similar textural and trace element properties. ...
We report the discovery of fossil micrometeorites from Late Cretaceous chalk. Seventy-six cosmic spherules were recovered from Coniacian (87±1 Ma) sediments of the White Chalk Supergroup. Particles vary from pristine silicate and iron-type spherules to pseudomorphic spherules consisting of either single-phase recrystallized magnetite or Fe-silicide. Pristine spherules are readily identified as micrometeorites on the basis of their characteristic mineralogies, textures and compositions. Both magnetite and silicide spherules contain dendritic crystals and spherical morphologies, testifying to rapid crystallisation of high temperature iron-rich metallic and oxide liquids. These particles also contain spherical cavities, representing weathering and removal of metal beads and irregular cavities, representing vesicles formed by trapped gas during crystallization; both features commonly found among modern Antarctic Iron-type (I-type) cosmic spherules. On the basis of textural analysis, the magnetite and Fe-silicide spherules are shown to be I-type cosmic spherules that have experienced complete secondary replacement during diagenesis (fossilization). Our results demonstrate that micrometeorites, preserved in sedimentary rocks, are affected by a suite of complex diagenetic processes, which can result in disparate replacement minerals, even within the same sequence of sedimentary beds. As a result, the identification of fossil micrometeorites requires careful observation of particle textures and comparisons with modern Antarctic collections. Replaced micrometeorites imply that geochemical signatures the extraterrestrial dust are subject to diagenetic remobilisation that limits their stratigraphic resolution. However, this study demonstrates that fossil, pseudomorphic micrometeorites can be recognised and are likely common within the geological record.
... Silicide group minerals are rare on Earth, and other than inclusions in terrestrial SiC they mostly occur in meteorites, impactites (terrestrial rocks subjected to meteorite impacts), and interplanetary dusts (e.g. Anand et al., 2004;Rietmeijer et al., 2008). Native silicon is a very rare compound, and has been reported as inclusions in gold (Novgorodova et al. 1989) and SiC (Di Pierro et al., 2003). ...
Here, we present studies of natural SiC that occurs in situ in tuff related to the Miocene alkaline basalt formation deposited in northern part of Israel. Raman spectroscopy, SEM and FIB-assisted TEM studies revealed that SiC is primarily hexagonal polytypes 4H-SiC and 6H-SiC, and that the 4H-SiC polytype is the predominant phase. Both SiC polytypes contain crystalline inclusions of silicon (Sio) and inclusions of metal-silicide with varying compositions (e.g. Si58V25Ti12Cr3Fe2, Si41Fe24Ti20Ni7V5Zr3, and Si43Fe40Ni17). The silicides crystal structure parameters match Si2TiV5 (Pm-3m space group, cubic), FeSi2Ti (Pbam space group, orthorhombic), and FeSi2 (Cmca space group, orthorhombic) respectively. We hypothesize that SiC was formed in a local ultra-reduced environment at respectively shallow depths (60-100 km), through a “desilification” reaction of SiO2 with highly reducing fluids (H2O-CH4–H2–C2H6) arisen from the mantle “hot spot” and passing through alkaline basalt magma reservoir. SiO2 (melt) interacting with the fluids may originate from the walls of the crustal rocks surrounding this magmatic reservoir. The “desilification” process led to the formation of SiC and the reduction of metal-oxides to native metals, alloys, and silicides. The latter were trapped by SiC during its growth. Hence, interplate “hot spot” alkali basalt volcanism can now be included as a geological environment where SiC, silicon, and silicides can be found.
... Sulfides are the only mineral group found in all extraterrestrial materials. Fe-Ni sulfides are also ubiquitous in the Wild 2 grains, grading from apparently unmodified FeS and pentlandite ((Fe,Ni) 9 S 8 ) (Fig. 5), to sulfides that apparently melted (and was reduced) during collection and separated into a mixture of sulfide and metal (Fig. 6) (Leroux et al. 2008a;Rietmeijer et al. 2008). Several tracks (e.g., track 59) have FeS-or pentlandite-dominated terminal grains. ...
... As discussed 567 by Essene and Fisher (1986) and Rowan and Ahrens (1994), possible reducing agents include C and 568 N, which may effectively scavenge oxygen from a melt and locally facilitate reduction of oxides to 569 produce elemental metals. This form of "smeltering" was also discussed for the formation of iron 570 silicides in dust particles from comet 81P/Wild 2, which were trapped in the silica aerogel capture 571 medium of the Stardust spacecraft (Rietmeijer et al., 2008). Thus, we raise the question whether 572 organic matter could have played a role in Khatyrka's history as well. ...
We analyzed the interaction of spherical, 6.36-mm-diameter, Cu-bearing aluminum projectiles with quartz sand targets in hypervelocity impact experiments performed at NASA Ames Vertical Gun Range. Impact velocities and inferred peak shock pressures varied between 5.9 and 6.5 km/s and ∼41 and 48 GPa, respectively. Shocked particles (“impact melt particles”) coated with thin crusts of molten projectile material were recovered from the floors of the ca. 33-cm-diameter craters and the respective ejecta blankets. Through petrographic and chemical (optical microscopy, FE-EMPA, SEM-EDX, and XRF) analysis we show that these particles have a layered structure manifested in distinct layers of decreasing shock metamorphism. These can be characterized by the following physical and chemical reactions and alteration products: (i) complete melting and subsequent recrystallization of the projectile, forming a distinct crystallization texture in the fused metal crust; (ii) projectile–target mixing, involving a redox reaction between Cu-bearing Al alloy und SiO2, leading to formation of khatyrkite (CuAl2), Al2O3 melt, euhedral silicon crystals, and spherical droplets of silicon; (iii) melting of quartz to lechatelierite and formation of planar deformation features in relic quartz grains; and (iv) shock lithification of quartz grains with fracturing of grains, grain-boundary melting, planar deformation features, and complete loss of porosity. To our knowledge, this is the first report of khatyrkite formed experimentally in hypervelocity impact experiments. These results have implications for the understanding of a similar redox reaction between Al–Cu metal and siliceous impact melt recently postulated for the Khatyrka CV3 carbonaceous chondrite. Moreover, these results bear on the processes that lead to layers of regolith on the surfaces of planetary bodies without atmospheres, such as asteroids in the main belt (e.g., 4 Vesta), and on the Moon. Specifically, impacts of mm-sized projectiles at velocities between 4 and 6 km/s into regolith-covered, asteroidal surfaces in the main belt should yield similar impact melt particles that feature a continuum of shock effects, i.e., partially to completely molten projectile remnants adhering to impact-melted regolith agglomerates, as well as projectile-contaminated impact melts and local shock melting along grain boundaries.
... They also concluded that some reduction together with S-redistribution had occurred during melting. Rietmeijer et al. (2008) described Fe silicides near the entrance of Track 44 and suggested these resulted from the reduction of melted Fe-Ni sulphide grains. ...
X-ray Absorption Fine Structure techniques have been used on Comet Wild2/81P tracks from the Stardust mission. Fe-XANES and EXAFS have been performed on aerogel sections from Tracks 41 and 162 as well as the mid and terminal positions of Track 134. This is the first use of EXAFS in the study of early Solar System materials. With EXAFS, we have measured Fe–O and Fe–S bond lengths and thus, together with complementary XANES measurements, identified Fe-rich phases. In particular, we show that ferric-rich phases in 2 Tracks (41, 162) have Fe–O bond 1st shell bond lengths of 1.99–2.01 Å and Fe K absorption edge and pre edge centroid positions consistent with being hematite-dominated grains. These iron oxides can be clearly distinguished from a magnetite grain, present in Track 134. We also demonstrate the identification of the Mg-rich end member olivine using EXAFS with XANES in Track 162. The terminal grain of Track 134 is pyrrhotite, its first atomic shell has an Fe–S structure, with 4 nearest neighbouring S atoms at a distance of 2.29 ± 0.05 Å.Our XANES results show the presence of Fe3+-bearing grains along the Stardust tracks and suggest either flash-cooling of an Fe–S–SiO–O2 gas during capture or the presence of a Fe–Ni–S–O melt along the cometary tracks during impact capture in the aerogel, rather than the capture process being solely associated with reduction of cometary phases. Accurate determination of Comet Wild2 redox conditions requires the identification of phases, in particular terminal grains, which have not experienced this melting. For instance, the larger hematite-rich grains (>10 μm) are more likely to be cometary in origin. EXAFS provides a valuable new analytical technique to study fine-grained early Solar System materials.
... In comparison, the lower limit of crystallization temperature of reduced spherules (objects 6, 7, and 10) ranges from ∼1,600°C to 1,700°C (Fig. S5). Object 6 shows silica flow texture (lechatelierite, temperature >1,713°C), object 7 contains droplets of iron (temperature >1,536°C) and corundum and mullite (cocrystallization temperature of ∼1,800°C for the inferred bulk composition of this spherule), and object 10 has suessite that requires a crystallization temperature in excess of 2,000°C (20,42). The estimated viscosity of molten precursors of these objects is also quite high (at 1,200°C log η = 5.1, 4.2, and 4.8 for objects 6, 7, and 10, respectively) and their glass transition temperature is ∼800°C. ...
Significance
This study ties the spherules recovered in Pennsylvania and New Jersey to an impact in Quebec about 12,900 y ago at the onset of Younger Dryas. Our discovery resulted from an exhaustive search that examined the question of whether there is any evidence of extraterrestrial platinum group metals present in the bulk sediments, magnetic grains, and spherules recovered from the Younger Dryas boundary (YDB). We find that the spherules are likely quenched silicate melts produced following the impact at the YDB. The source of spherule osmium, however, is likely terrestrial and not meteorite derived.
... This configuration suggests abrasion and ablation or breaking up of the incident particles during the deceleration in the aerogel. For number of samples, in particular those extracted from track walls, the microstructure shows clear evidences of thermal modification in addition to strong intermixing with melted aerogel, showing that the particles suffered thermal alteration during the capture process (Leroux et al. , 2009Ishii et al. 2008b;Rietmeijer et al. 2008). ...
Abstract— Four particles extracted from track 80 at different penetration depths have been studied by analytical transmission electron microscopy (ATEM). Regardless of their positions within the track, the samples present a comparable microstructure made of a silica rich glassy matrix embedding a large number of small Fe-Ni-S inclusions and vesicles. This microstructure is typical of strongly thermally modified particles that were heated and melted during the hypervelocity impact into the aerogel. X-ray intensity maps show that the particles were made of Mg-rich silicates (typically 200 nm in diameter) cemented by a fine-grained matrix enriched in iron sulfide. Bulk compositions of the four particles suggest that the captured dust particle was an aggregate of grains with various iron sulfide fraction and that no extending chemical mixing in the bulb occurred during the deceleration. The bulk S/Fe ratios of the four samples are close to CI and far from the chondritic meteorites from the asteroidal belt, suggesting that the studied particles are compatible with chondritic-porous interplanetary dust particles or with material coming from a large heliocentric distance for escaping the S depletion.
Since the pioneer lunar sample return missions, the development of advanced techniques for the laboratory analysis of retrieved samples has run in parallel with the advances in space studies. All along the last decades, a common goal of these developments has been to maximize the scientific outcomes of laboratory studies while minimizing the loss of precious extraterrestrial samples. We provide an overview of the techniques and instruments used so far to analyze, characterize and study returned extraterrestrial samples. Considering the limited amount of material that is retrieved by the space missions, a multi-analytical sequence from less destructive techniques to more destructive techniques needs to be established. The return of Hayabusa2 and OSIRIS-REx samples will probably trigger a wave of laboratory studies, which, in turn, will support new developments of space instruments for observations of Solar System bodies.
This article presents a model of conjugated Gaussian random particles, which are convenient for simulation of irregular particles that constitute cometary dust. Computer simulation is conducted for the polarimetric properties of these particles; phase dependences are calculated for the linear polarization degree. The calculated results are used to interpret cometary polarimetric observations and determine possible physical and chemical characteristics of comets as well as the variation range within which the model adequately describes the observed data. The model calculations are used to refine the empirical formula that describes the phase dependence of the linear polarization degree for the cometary continuum.
Geochemical indicators in meteorites imply that most formed under relatively oxidizing conditions. However, some planetary materials, such as the enstatite chondrites, aubrite achondrites, and Mercury, were produced in reduced nebular environments. Because of large-scale radial nebular mixing, comets and other Kuiper Belt objects likely contain some primitive material related to these reduced planetary bodies. Here, we describe an unusual assemblage in a dust particle from comet 81P/Wild 2 captured in silica aerogel by the NASA Stardust spacecraft. The bulk of this ~20 μm particle is comprised of an aggregate of nanoparticulate Cr-rich magnetite, containing opaque sub-domains composed of poorly graphitized carbon (PGC). The PGC forms conformal shells around tiny 5–15 nm core grains of Fe carbide. The C, N, and O isotopic compositions of these components are identical within errors to terrestrial standards, indicating a formation inside the solar system. Magnetite compositions are consistent with oxidation of reduced metal, similar to that seen in enstatite chondrites. Similarly, the core–shell structure of the carbide + PGC inclusions suggests a formation via FTT reactions on the surface of metal or carbide grains in warm, reduced regions of the solar nebula. Together, the nanoscale assemblage in the cometary particle is most consistent with the alteration of primary solids condensed from a C-rich, reduced nebular gas. The nanoparticulate components in the cometary particle provide the first direct evidence from comets of reduced, carbon-rich regions that were present in the solar nebula.
The bulbous Stardust track #80 (C2092,3,80,0,0) is a huge cavity. Allocations C2092,2,80,46,1 nearest the entry hole and C2092,2,80,47,6 about 0.8 mm beneath the entry hole provide evidence of highly chaotic conditions during capture. They are dominated by nonvesicular low-Mg silica glass instead of highly vesicular glass found deeper into this track which is consistent with the escape of magnesiosilica vapors generated from the smallest comet grains. The survival of delicate (Mg,Al,Ca)-bearing silica glass structures is unique to the entry hole. Both allocations show a dearth of surviving comet dust except for a small enstatite, a low-Ca hypersthene grain, and a Ti-oxide fragment. Finding scattered TiO2 fragments in the silica glass could support, but not prove, TiO2 grain fragmentation during hypervelocity capture. The here reported dearth in mineral species is in marked contrast to the wealth of surviving silicate and oxide minerals deeper into the bulb. Both allocations show Fe-Ni-S nanograins dispersed throughout the low-Mg silica glass matrix. It is noted that neither comet Halley nor Wild 2 had a CI bulk composition for the smallest grains. Using the analogs of interplanetary dust particles (IDPs) and cluster IDPs it is argued that a CI chondritic composition requires the mixing of nonchondritic components in the appropriate proportions. So far, the fine-grained Wild 2 dust is biased toward nonchondritic ferromagnesiosilica materials and lacking contributions of nonchondritic components with Mg-Fe-Ni-S[Si-O] compositions. To be specific, “Where are the GEMS”? The GEMS look-alike found in this study suggests that evidence of GEMS in comet Wild 2 may still be found in the Stardust glass.
Conventional electron-probe microanalysis (EPMA) 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 non-standard X-ray lines for analysis. The 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 smaller than 1 μm). Problems include the 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 (MAC) 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.
We developed a novel technique called "analytical dual-energy
microtomography" that uses the linear attenuation coefficients (LACs) of
minerals at two different X-ray energies to nondestructively obtain
three-dimensional (3D) images of mineral distribution in materials such
as rock specimens. The two energies are above and below the absorption
edge energy of an abundant element, which we call the "index element".
The chemical compositions of minerals forming solid solution series can
also be measured. The optimal size of a sample is of the order of the
inverse of the LAC values at the X-ray energies used. We used
synchrotron-based microtomography with an effective spatial resolution
of >200 nm to apply this method to small particles (30-180 μm)
collected from the surface of asteroid 25143 Itokawa by the Hayabusa
mission of the Japan Aerospace Exploration Agency (JAXA). A 3D
distribution of the minerals was successively obtained by imaging the
samples at X-ray energies of 7 and 8 keV, using Fe as the index element
(the K-absorption edge of Fe is 7.11 keV). The optimal sample size in
this case is of the order of 50 μm. The chemical compositions of the
minerals, including the Fe/Mg ratios of ferromagnesian minerals and the
Na/Ca ratios of plagioclase, were measured. This new method is
potentially applicable to other small samples such as cosmic dust, lunar
regolith, cometary dust (recovered by the Stardust mission of the
National Aeronautics and Space Administration [NASA]), and samples from
extraterrestrial bodies (those from future sample return missions such
as the JAXA Hayabusa2 mission and the NASA OSIRIS-REx mission), although
limitations exist for unequilibrated samples. Further, this technique is
generally suited for studying materials in multicomponent systems with
multiple phases across several research fields.
Organic nanoglobules are submicrometer spherical, often hollow organic
grains ubiquitously distributed throughout primitive solar materials,
such as carbonaceous chondrites. Until now, organic nanoglobules have
been examined by TEM only after sectioning by ultramicrotomy so it has
not been possible to determine whether fluids or mineral grains occur in
the hollow cores. H2O-rich fluids might be present in hollows
of the nanoglobules if they originate from dust particles composed of
organic materials and ice prior to or in an early stage of the solar
system formation or fluids incorporated into nanoglobules during aqueous
alteration on the asteroidal parent body. In order to determine whether
or not any fluids or mineral grains are present in the nanoglobules, a
carbonaceous chondrite sample (Tagish Lake C2 meteorite) was observed
non-destructively using synchrotron radiation-based X-ray CT (computed
tomography), and then microtomed sections were observed using a
transmission electron microscope (TEM). We observed three-dimensional
shapes of thirty-eight organic nanoglobules in the meteorite sample.
Their size and shape distributions are consistent with a hypothesis that
nanoglobules originate from icy dust particles. Nanoglobule candidates
observed in CT images were confirmed by the TEM images. However, the
presence or absence of fluid could not be judged because CT images of
nanoglobules are affected by X-ray refraction. Simulation of CT images
by considering X-ray refraction shows that the presence or absence of
water in nanoglobules cannot be distinguished with CT images alone.
However the outer shapes of nanoglobules can be determined
quantitatively and nanoglobules containing silicate cores can be easily
identified. The thirty-eight nanoglobules we examined did not have
silicate cores.
The oxidation state of transition metal elements is an indicator of the
environmental conditions during formation and history of
extraterrestrial materials. We studied the iron valence state of
fine-grained material from a bulbous track extracted from the Stardust
cometary collector. It likely originated from primitive material of the
comet Wild 2. We used synchrotron-based Scanning Transmission X-ray
Microscopy (STXM) to collect Fe L3-XANES spectra at a spatial
resolution of about 20 nm. Maps of Fe valence state were combined with
the elemental maps recorded by energy dispersive X-ray spectroscopy
(EDS) with a transmission electron microscope (TEM), on the same areas
and with a comparable electron probe size (5–20 nm). As for most
Stardust fine-grained material, the samples are severely damaged by the
hypervelocity impact in the aerogel collector blocks. They show of a
wide range of oxidation state at a micrometer scale, from Fe metal to
Fe3+. This heterogeneity of oxidation state can be due to the
extreme conditions of the collection. Two major parameters can favor
changes in redox state. The first is the high temperature regime, known
to be highly heterogeneous and to have locally reached extreme values
(up to 2000 K). The second is the local chemical environment. It may
contain elements that could favor a reduction or oxidation reaction
within the flash-heated Wild 2 fragments. Comparison of maps by STXM and
EDS shows evidence for several correlation trends between element
concentrations and the iron valence state. These observations, together
with the study of a melted rim of a larger particle, suggest that the
redox state was not completely redistributed within the impact melts.
These local signatures are compatible with precursors that could have
been close to primitive matrix material of chondrites or to chondritic
interplanetary dust particles. On average, the fine-grained material
from Wild 2 displays a molar fraction (Fe2+oxide +
Fe3+oxide)/(total Fe) equal to 0.80 ± 0.10.
It appears more oxidized than the average value measured for the comet,
when done on larger particles (Westphal et al., 2009). This fine-grained
material from Wild 2 does not seem to have sampled reducing environments
in the solar nebulae in contrast with the larger particles of Wild 2.
This observation confirms the high degree of diversity of materials in
Wild 2 and is in good agreement with the dual distribution of high
temperature minerals and matrices in carbonaceous chondrites.
The silica glass extracted from the bulbous parts of Stardust tracks is
riddled by electron-opaque nanograins with compositions that are mostly
between pyrrhotite and metallic iron with many fewer nanograins having a
Fe-Ni-S composition. Pure taenite nanograins are extremely rare, but
exist among the terminal particles. Assuming that these Fe-Ni-S
compositions are due to mixing of pyrrhotite and taenite melt droplets,
it is remarkable that the taenite melt grains had discrete Fe/Ni ratios.
This paper presents the data from an igneous pyrrhotite/taenite fragment
of cluster IDP L2011#21, wherein the taenite
compositions have the same discrete Fe/Ni clusters as those inferred for
the Stardust nanograins. These Fe/Ni clusters are a subsolidus feature
with compositions that are constrained by the Fe-Ni phase diagram. They
formed during cooling of the parent body of this cluster
IDP fragment. These specific Fe/Ni ratios, 12.5,
24, 40, and 53 atom% Ni, were preserved in asteroidal taenite that
survived radially outward transport to the Kuiper Belt where it accreted
into the (future) comet Wild 2 nucleus.
We report the presence of preserved primitive fine-grained material containing an enstatite whisker with the crystallographic characteristics of a primary condensate in a sample of the Jupiter-family comet Wild 2, returned to earth by NASAʼs Stardust mission. The preserved primitive material is composed of silica-rich amorphous material embedded with iron sulfides and silicates. It is in close association with a type II chondrule-like object in the track C2052,2,74 (Ogliore et al., 2012). The close association of a chondrule and a primary condensate shows they must have formed in different environments and probably met in the comet-forming region. The first observation of an enstatite whisker with properties indicating primary condensation in a comet is a new link between comets and Chondritic Porous IDPs (CP-IDPs).
In this study, the three-dimensional (3-D) microstructure of 48 Itokawa regolith particles was examined by synchrotron microtomography at SPring-8 during the preliminary examination of Hayabusa samples. Moreover, the 3-D microstructure of particles collected from two LL6 chondrites (Ensisheim and Kilabo meteorites) and an LL5 chondrite (Tuxtuac meteorite) was investigated by the same method for comparison. The modal abundances of minerals, especially olivine, bulk density, porosity, and grain size are similar in all samples, including voids and cracks. These results show that the Itokawa particles, which are surface materials from the S-type asteroid Itokawa, are consistent with the LL chondrite materials in terms of not only elemental and isotopic composition of the minerals but also 3-D microstructure. However, we could not determine whether the Itokawa particles are purely LL5, LL6, or a mixture of the two. No difference between the particles collected from Rooms A and B of the sample chamber, corresponding to the sampling sequence of the spacecraft's second and first touchdowns, respectively, was detected because of the statistically small amount of particles from Room B.
The deceleration tracks in the Stardust aerogel display a wide range of
morphologies, which reveal a large diversity of incoming particles from
comet 81P/Wild 2. If the large and dense mineral grains survived the
extreme conditions of hypervelocity capture, this was not the case for
the fine-grained material that is found strongly damaged within the
aerogel. Due to their low mechanical strength, these assemblages were
disaggregated, dispersed, and flash melted in the aerogel in walls of
bulbous deceleration tracks. Their petrologic and mineralogical
properties are found significantly modified by the flash heating of the
capture. Originating from a quenched melt mixture of comet material and
aerogel, the representative microstructure consists of silica-rich
glassy clumps containing Fe-Ni-S inclusions, vesicles and "dust-rich"
patches, the latter being remnants of individual silicate components of
the impacting aggregate. The average composition of these melted
particle fragments is close to the chondritic CI composition. They might
originate from ultrafine-grained primitive components comparable to
those found in chondritic porous IDPs. Capture effects in aerogel and
associated sample biases are discussed in terms of size, chemical and
mineralogical properties of the grains. These properties are essential
for the grain survival in the extremely hot environment of hypervelocity
impact capture in aerogel, and thus for inferring the correct properties
of Wild 2 material.
To constrain the effects of capture modification processes, the size
distribution of nanoscale refractory Fe-Ni-S inclusions ("droplets") was
measured in five allocations extracted from throughout the depth of
Stardust Track 35. The Fe/S ratio has been previously shown to increase
significantly with penetration depth in this track, suggesting
increasing capture-related modification along the track. Astronomical
image analysis tools were employed to measure the sizes of more than
8000 droplets from TEM images, and completeness simulations were used to
correct the distribution for detection bias as a function of radius. The
size distribution characteristics are found to be similar within
independent regions of individual allocations, demonstrating uniformity
within grains. The size distribution of the Fe-Ni-S droplets in each
allocation is dominated by a mode near 11 nm, but is coarse-skewed and
leptokurtic with a mean of ˜17 nm and a standard deviation of
˜9 nm. The size distribution characteristics do not vary
systematically with penetration depth, despite the strong trend in bulk
Fe/S ratio. This suggests that the capture modification process is not
primarily responsible for producing the morphology of these nanoscale
droplets. The Stardust Track 35 droplet size distribution indicates
slightly smaller sizes, but otherwise resembles those in carbonaceous
chondrite Acfer 094, and chondritic porous interplanetary dust particles
that escaped nebular annealing of sulfides. The size distribution of
metal-sulfide beads in Stardust's quenched melted-grain emulsions
appears to be inherited from the size distribution of unmelted sulfide
mineral grains in comet-dust particles of chondritic character.
We analyzed carbonaceous materials in the two main morphological types of Stardust tracks (A, and B). We analyzed 71 particles (∼1–10 μm in size) distributed along these tracks with transmission electron microscopy (TEM) and found carbon associated with 16 of them. The carbonaceous materials occur in five distinct morphologies: graphitic, smooth, dirty, spongy and globular, covering most of the range of morphologies observed in primitive meteorites and interplanetary dust particles. We measured N and C isotopic compositions on 5 of these particles and found that all but one have terrestrial isotopic compositions. The anomalous particle had a moderate 15N enrichment (δ15N = 150 ± 36‰, 2σ) and both globular and spongy morphologies. The carbonaceous materials are not preferentially associated with particles of a particular size but are randomly distributed in all three tracks.
We have characterized by transmission electron microscopy the mineralogy of samples extracted from the walls of the Stardust track 80. More than 500 fragments were studied using conventional imaging, electron diffraction and EDX microanalysis. Two categories of particles are distinguishable in equal proportions (wt%). The first one is comprised of relatively large crystalline grains (≈1 μm on average), dominated by silicates (olivine and pyroxene). They display a wide range of compositions and microstructures comparable to those found in terminal particles. Minor phases including magnetite and apatite are also present. Their occurrence suggests that the Wild 2 material underwent aqueous alteration to some extent. The second type of particle, called GEMS-like material, is made of silica-rich glassy clumps embedding iron sulfide beads and vesicles. Their microstructure is characteristic of thermally modified particles that have suffered strong interaction with the silica aerogel during the hypervelocity impact. This melted material may form by shedding of melted and vaporized material, but given the shape of the impact track and high diversity of surviving mineral compositions, much of it originated from fine-grained aggregates that disaggregated during the collection. Chemical mapping at the nano-scale allowed the localization of individual components within the silica-rich glass. They are dominated by Mg-rich components with a size less than 300 nm. The average composition of this thermally modified material is close to the solar abundance for the major elements Fe, Mg and S. The fine-grained material has probably not been chemically fractionated in the protoplanetary disk before its incorporation in comet Wild 2 unlike the sulfur depleted matrix of chondrites. From these two categories of particles, we deduce that Wild 2 is likely made of an assemblage of relatively large evolved grains (first category) cemented by a fine-grained material with primitive chemistry (second category). The pre-impact configuration of the incident material deduced from this study seems comparable to the matrix of the most primitive chondrites (3.0) or to chondritic porous interplanetary dust particles.
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.
Due to its extremely high porosity and the nanoscale filaments that make up its structure, aerogel is an excellent material for the capture of hypervelocity, micron-sized particles. A great deal of the kinetic energy of a particle is converted to thermal energy during the capture process, altering or even destroying components of the particle. The studies described here were conducted using aggregate projectiles made up of magnetic sub-micron hematite particles in an attempt to directly measure the temperatures experienced by fine particles during hypervelocity capture in aerogels. When these particles are heated to a temperature above their Curie temperature (675°C) during the capture, they lose their magnetization. Thus, by impact testing these particles in aerogels at different velocities, we were able to determine if individual components of these aggregate particles were heated to a temperature greater than their Curie temperature by observing their magnetization. After impact testing, the particles were extracted from the aerogel, thin sectioned, and observed using atomic and magnetic force microscopy, as well as, electron paramagnetic resonance. Terminal particles for impacts at or above 4.5 km/sec were still magnetic, while those from the track walls were not. Even terminal particles captured at 6.6 km/sec were still magnetic. Iron oxide particles coated with silica, to mimic extraterrestrial materials, from track walls captured at 5.47 km/sec were still magnetic. The study also demonstrated that aggregate projectiles can survive the forces they are subjected to during hypervelocity launch in a light gas gun.
Ir and Os are excellent markers of extraterrestrial impact events, due
to their high abundance in ET objects (Alvarez et al., 1980 Science;
Turekian, 1982 Geol. Bull. Am. Spec. Pap.). Os has the advantage over
Ir, in that the 187Os/188Os ratio also greatly differs between
meteorites and upper continental crust (UCC). The combination of [Os]
and 187Os/188Os analyses would be superior in detecting any ET
contribution. Firestone et al (2007 PNAS) attributed a widespread 12.9
ka Ir containing black carbon layer to a potential extraterrestrial
impact at the Allerød-Younger Dryas (A-YD) boundary. In order to
test this inference, we measured [Os] and 187Os/188Os on a radiocarbon
dated A-YD record (13.210 to 12.788 cal years BP) from the Netherlands.
This location is close to Lommel, a Belgian site studied by Firestone et
al.(2007). The organic-rich sequence was sampled continuously over a 12
cm interval at 2 cm resolution (~70 years). About 10 g samples were
freeze-dried, ground and homogenized in a zirconia ball-mill. The
samples mixed with 190Os tracer solutions were dissolved in carius tubes
and Os extracted in liquid bromine. Os was further purified using
micro-distillation. Os isotopes were measured using N-TIMS on Dartmouth
Triton. The procedural blank was 7 fg/g Os with an isotopic composition
of 0.41±0.01 The Allerød samples have an order of
magnitude higher abundance than UCC (200 vs. 30 pg/g), but similar
187Os/188Os ratios, >1.1. The sample at the base of the YD
(12.893±75 cal years BP) contains a similar amount of Os, but has
a distinctly lower isotopic signature, 0.53±0.002. The high [Os]
in the A-YD section possibly reflects enrichment by preferential
partitioning into organic matter. The Os isotope composition of 0.53,
sandwiched between values >1.1, implies contribution of a significant
amount of non-radiogenic Os. Since the pollen spectra show no reworking,
the non-radiogenic Os could only have been delivered as a discrete pulse
at 12.893 cal yr BP. The observation of the non-radiogenic Os isotope
composition would therefore be consistent with a meteorite impact.
However, it is intriguing to note there was a small volcanic eruption of
Laacher See tephra (LST) at this time (surface immediately below LST =
12.979±147 cal years BP, Bittmann, 2007 Veget Hist Archeobot)
approximately 150 km away. The timing of this eruption has been
traditionally placed in upper Allerød, about 200 years before the
onset of YD. More work is needed to resolve the issue of ET Os at the YD
boundary.
Abstract– We have experimentally produced nanophase sulfide compounds and magnetite embedded in Si-rich amorphous materials by flash-cooling of a gas stream. Similar assemblages are ubiquitous, and often dominant components of samples of impact-processed silica aerogel tiles and submicron grains from comet 81P/Wild 2 were retrieved by NASA’s Stardust mission. Although the texture and compositions of nanosulfide compounds have been reproduced experimentally, the mechanisms of formation of these minerals and their relationship with the surrounding amorphous materials have not been established. In this study, we present evidence that both of these materials may not only be produced through cooling of a superheated liquid but they may have also been formed simultaneously by flash-cooling and subsequent deposition of a gas dominated by Fe-S-SiO-O2. In a dust generator at the Goddard Space Flight Center, samples are produced by direct gas-phase condensation from gaseous precursors followed by deposition, which effectively isolates the effects of gas-phase reactions from the effects of melting and condensation. High-resolution transmission electron microscopy images and energy-dispersive spectroscopy analysis show that these experiments replicate key features of materials from type B and type C Stardust tracks, including textures, distribution of inclusions, nanophase size, and compositional diversity. We argue that gas-phase reactions may have played a significant role in the capture environment for nanophase materials. Our results are consistent with a potential progenitor assemblage of micron and submicron-sized sulfides and submicron silica-bearing phases, which are commonly observed in chondritic interplanetary dust particles and in the matrices of the most pristine chondritic meteorites.
Abstract— Three-dimensional structures and elemental abundances of four impact tracks in silica aerogel keystones of Stardust samples from comet 81P/Wild 2 (bulbous track 67 and carrot-type tracks 46, 47, and 68) were examined non-destructively by synchrotron radiation-based microtomography and X-ray fluorescence analysis. Track features, such as lengths, volumes and width as a function of track depth, were obtained quantitatively by tomography. A bulbous portion was present near the track entrance even in carrot-type tracks. Each impact of a cometary dust particle results in the particle disaggregated into small pieces that were widely distributed on the track walls as well as at its terminal. Fe, S, Ca, Ni, and eight minor elements are concentrated in the bulbous portion of track 68 as well as in terminal grains. It was confirmed that bulbous portions and thin tracks were formed by disaggregation of very fine fragile materials and relatively coarse crystalline particles, respectively. The almost constant ratio of whole Fe mass to track volume indicates that the track volume is almost proportional to the impact kinetic energy. The size of the original impactor was estimated from the absolute Fe mass by assuming its Fe content (CI) and bulk density. Relations between the track sizes normalized by the impactor size and impact conditions are roughly consistent with those of previous hypervelocity impact experiments.
The discovery of nickel-, copper-, and zinc-bearing iron sulfides from comet 81P/Wild 2 (Wild 2) represents the strongest evidence, in the Stardust collection, of grains that formed in an aqueous environment. We investigated three microtomed TEM sections which contain crystalline sulfide assemblages from Wild 2 and twelve thin sections of the hydrothermally altered CI chondrite Orgueil. Detailed structural and compositional characterizations of the sulfide grains from both collections reveal striking similarities. The Stardust samples include a cubanite (CuFe2S3) grain, a pyrrhotite [(Fe,Ni)1−xS]/pentlandite [(Fe,Ni)9S8] assemblage, and a pyrrhotite/sphalerite [(Fe,Zn)S] assemblage. Similarly, the CI-chondrite sulfides include individual cubanite and pyrrhotite grains, cubanite/pyrrhotite assemblages, pyrrhotite/pentlandite assemblages, as well as possible sphalerite inclusions within pyrrhotite grains. The cubanite is the low temperature orthorhombic form, which constrains temperature to a maximum of 210 °C. The Stardust and Orgueil pyrrhotites are the 4C monoclinic polytype, which is not stable above ∼250 °C. The combinations of cubanite and pyrrhotite, as well as pyrrhotite and pentlandite signify even lower temperatures. The crystal structures, compositions, and petrographic relationships of these sulfides constrain formation and alteration conditions. Taken together, these constraints attest to low-temperature hydrothermal processing.
It has been proposed that fragments of an asteroid or comet impacted Earth, deposited silica- and iron-rich microspherules and other proxies across several continents, and triggered the Younger Dryas cooling episode 12,900 years ago. Although many independent groups have confirmed the impact evidence, the hypothesis remains controversial because some groups have failed to do so. We examined sediment sequences from 18 dated Younger Dryas boundary (YDB) sites across three continents (North America, Europe, and Asia), spanning 12,000 km around nearly one-third of the planet. All sites display abundant microspherules in the YDB with none or few above and below. In addition, three sites (Abu Hureyra, Syria; Melrose, Pennsylvania; and Blackville, South Carolina) display vesicular, high-temperature, siliceous scoria-like objects, or SLOs, that match the spherules geochemically. We compared YDB objects with melt products from a known cosmic impact (Meteor Crater, Arizona) and from the 1945 Trinity nuclear airburst in Socorro, New Mexico, and found that all of these high-energy events produced material that is geochemically and morphologically comparable, including: (i) high-temperature, rapidly quenched microspherules and SLOs; (ii) corundum, mullite, and suessite (Fe(3)Si), a rare meteoritic mineral that forms under high temperatures; (iii) melted SiO(2) glass, or lechatelierite, with flow textures (or schlieren) that form at > 2,200 °C; and (iv) particles with features indicative of high-energy interparticle collisions. These results are inconsistent with anthropogenic, volcanic, authigenic, and cosmic materials, yet consistent with cosmic ejecta, supporting the hypothesis of extraterrestrial airbursts/impacts 12,900 years ago. The wide geographic distribution of SLOs is consistent with multiple impactors.
Two representative thermally modified Stardust samples were investigated by analytical transmission electron microscopy in order to decipher their iron oxidation state after the strong thermal episode due to the capture in aerogel. Their dominant microstructure consists of evenly distributed rounded Fe–Ni–S nano-droplets within a silica-rich glassy matrix. The mineralogy and associated redox state of iron is assessed using a Fe–Mg–S ternary diagram on which ferromagnesian silicates, sulfides and metal can be represented and potentially compared with any other extraterrestrial material. In this diagram, all the data (bulk and local analysis of silicates, sulfide + metal) scatter along a mixing line between the Mg corner and the average composition of the iron-sulfide. There is an obvious genetic relationship between the different phases observed in such samples, further supported by the very low concentration of iron in the glassy matrix. Silicate glasses contain a significant concentration of dissolved sulfur probably present as MgS complexes. This chemical signature is typical of highly reduced environments. These secondary microstructures were established during the high temperature stage of the capture. A significant part of the Fe-droplets formed in situ by reduction at high temperature of ferromagnesian silicates (olivine and pyroxenes) during the impact. At this stage, the indigenous sulfides destabilized and sulfur readily volatilized as S2, diffused into molten materials and condensed later onto the Fe-precipitates that formed in the silicate melt. This scenario is supported by the structure of Fe–Ni–S beads with a metal core and a sulfide rim. It will be difficult to derive reliable information on the redox state of 81P/Wild 2 particles based on bulk analyses of whole tracks because particles found along the walls of tracks suffered strong reduction reactions, contrary to terminal particles that may have preserved their pristine redox state. The capture effect must be taken into account for comparison of Wild 2 particles with other chondritic material.
The NASA Stardust mission has provided for laboratory study an extensive data set of cometary dust of known provenance (from comet 81P/Wild 2) yielding detailed insights into the composition of the comet. Combined with the results of data from other missions to short-period Jupiter family comets (JFC), this has greatly deepened the understanding of such objects. If depressions on the surface of comet 81P/Wild 2 are all taken as evidence of impact cratering, their number suggests a long occupancy in the outer region of the Solar System. The dust from comet 81P/Wild 2 has been shown to be heavily deficient in pre-Solar grains and rich in materials formed at high temperatures in the inner Solar System. Although it is too early to know if this is typical of JFC, it does argue for rapid and thorough mixing of materials in the disk on timescales related to comet formation, and may also suggest outward migration of small icy bodies after their formation. Thus, instead of providing mainly new knowledge of the pre-Solar materials expected to be rich in comets, Stardust and comet 81P/Wild 2 have instead focussed attention on large-scale transport processes during the critical period when cometary parent bodies were forming in the early Solar System.
Grains entering our solar system at the heliopause or encountered by an interstellar probe will be dominated by materials formed in carbon-rich and oxygen-rich outflows from high mass-loss-rate AGB stars with moderate contributions from novae and supernovae. Laboratory studies of the condensation and thermal evolution of silicate grains have greatly increased our understanding of grain formation processes and may provide the basis for theoretical prediction of chemical speciation and spectral evolution of grains produced in specific stellar outflows. These grains will be modified by long-term exposure to high-energy cosmic rays, thus rendering them amorphous, and will potentially be coated by one or more layers of refractory carbonaceous material. This coating results when radiation-damaged organic-rich water ices are allowed to sublime in vacuum during the transition from the interior of a dense molecular cloud to the warm interstellar medium. Measurement of the chemical compositions of large numbers of individual grains in the local interstellar medium could lead to a better understanding of the life cycle of grains in the general interstellar medium. Such measurements might also serve as a diagnostic indicator of the primary source of interstellar grains: origin in stellar outflows or in the interstellar medium itself. These measurements would be possible using an impact-ionization, time-of-flight, mass spectrometer during the cruise phase of an interstellar probe mission.
Natural Metastable Equilibrium 2 AbstractChemical ordering at metastable eutectics was recognized in non-equilibrium gas-to-solid condensation experiments to constrain 'silicate' dust formation in O-rich circumstellar environments. The predictable metastable eutecti¢ behavior successfully predicted the observed ferromagnesiosilica compositions of circumstellar dust, presolar and solar nebula grains in the matrix of the collected aggregate IDPs. Many of the experimentally determined metastable eutecti¢ solids match the fundamental building blocks of common rock-forming layer silicates: this could have implications for the origin of Life. The physical conditions conducive to metastable eutecti¢ behavior, i.e. high temperature and (ultra)fast quenching, lead to unique amorphous, typically nano-to micrometer-sized, materials. The new paradigm of metastable eutectie behavior opens the door to new and exciting research opportunities in uncovering the many implications ef these unique amorphous, and typically nano-to micrometer-sized, metastable eutectic materials.
The North Haig olivine pigeonite achondrite (ureilite) is a polymict breccia consisting of major olivine, low-Ca pyroxene and an intergranular carbonaceous matrix. Olivine and low-Ca pyroxene vary widely in composition, covering the ranges observed in all known ureilites. Minor granular enstatite clasts with diopside exsolution blebs, akin to enstatite achondrites, were also observed. Native metal in ureilites is normally kamacite of variable Ni content, in some cases with up to 2% Si in solid solution. However, kamacite with trace amounts of Si is extremely rare in North Haig, only a few 1-2 micro m grains within silicates were observed. Instead, the common metallic phase is approximately Fe3Si, a new mineral which we have named suessite in honor of Professor Hans E. Suess. Suessite occurs as minor anhedral vein fillings in interstitial cracks, in silicates, and in the intergranular carbonaceous material and ranges in size from 1 micro m blebs to elongated grains about 30 x 150 micro m in size. Suessite is cream white in reflected light, isotropic, ferromagnetic, and shows no cleavage. Reflectance in percent (determined by G. A. Desborough) at the 4 standard wavelengths of 470, 546, 589 and 650 nm for 2 grains is 48.5(5), 51.6(3), 53.5(7), 50(2), and 49.7(5), 53.4(4), 54.5(6), 52(l), respectively. Analyses (by EMX) indicate presence of dominating low-Ni and less common high-Ni varieties of suessite (in wt.%, mean in parentheses): Fe 84.7, 83.1 (84.2); Ni 1.6, 4.5 (2.5); Co 0.21, 0.27 (0.23); Cr 0.10, 0.04 (0.08); Si 15.3, 13.7 (14.7); P 0.06, 0.17 (O.10). Thus, suessite is (Fe, Ni, Co, Cr)(sub 2.84-3.14) (Si, P)(sub 1.0), mean (Fe, Ni, Co, Cr)(sub 2.96)(Si, P)(sub 1.0), very close to Fe3Si. X-ray powder diffraction shows that suessite possesses a similar structure to alpha-Fe (kamacite) and the solid solution alloy (Fe3Si)ss in displaying three lines (relative intensities in parentheses): 2.005(10), 1.42(1), 1.160(3) (in A). The calculated cell size is 2.841 +/- 0.002 A (V = 22.93 A(sup 3). Thus, suessite is the Si-rich end member of the alpha solid solution region of the Fe-Si phase diagram with composition close to Fe3Si. Silicates, particularly olivine, in ureilites have core compositions of Fo(sub 76-92) and thin (less than 100 microns) rims of essentially Fo(sub 100) formed by reduction and reaction with the carbonaceous matrix material. We suggest that Si and Fe liberated in this reduction process formed suessite, possibly also by reaction with preexisting kamacite.
The high temperature processing of silicates often results in very reduced products, such as Si-bearing Fe metal in type-1 chondrules and in lunar regolith agglutinates. Previous work on fulgurites (the glassy products of the lightning strike fusion of sand, soil, or rock) found silicon metal and iron-silicon alloys inside the silicate glass. In this work, we present a new fulgurite that contains many reduced phases, and we model condensation of the Al-Fe-Si-O system.
Impact experiments using silica aerogel as a deceleration and capture medium for interplanetary dust are reported. A rough correlation is noted between increasing particle track lengths and decreasing aerogel density, and there is a poor correlation of track lengths with impact velocity at laboratory attainable velocities of 5-7 km/s. It is concluded that aerogel track lengths should not be used as velocity indicators. Chemical analyses are also reported of aerogel samples used in this study in order to assess the risks concerning contamination of interplanetary dust particles by the silica aerogel capture medium. It is demonstrated that this material is impressively clean.
Physical and chemical reactions occurring as a result of the high-velocity impacts of meteorites and micrometeorites and of cosmic rays and solar-wind particles are major causes of space weathering on airless planetary bodies, such as the Moon, Mercury, and asteroids. These weathering processes are responsible for the formation of their regolith and soil. We report here the natural occurrence of the mineral hapkeite, a Fe2Si phase, and other associated Fe-Si phases (iron-silicides) in a regolith breccia clast of a lunar highland meteorite. These Fe-Si phases are considered to be a direct product of impact-induced, vapor-phase deposition in the lunar soil, all part of space weathering. We have used an in situ synchrotron energy-dispersive, single-crystal x-ray diffraction technique to confirm the crystal structure of hapkeite as similar to the structure of synthetic Fe2Si. This mineral, hapkeite, is named after Bruce Hapke of the University of Pittsburgh, who predicted the presence and importance of vapor-deposited coatings on lunar soil grains some 30 years ago. We propose that this mineral and other Fe-Si phases are probably more common in the lunar regolith than previously thought and are directly related to the formation of vapor-deposited, nanophase elemental iron in the lunar soils.
The bulk of the comet 81P/Wild 2 (hereafter Wild 2) samples returned to Earth by the Stardust spacecraft appear to be weakly constructed mixtures of nanometer-scale grains, with occasional much larger (over 1 micrometer) ferromagnesian silicates, Fe-Ni sulfides, Fe-Ni metal, and accessory phases. The very wide range of olivine and low-Ca pyroxene compositions in comet Wild 2 requires a wide range of formation conditions, probably reflecting very different formation locations in the protoplanetary disk. The restricted compositional ranges of Fe-Ni sulfides, the wide range for silicates, and the absence of hydrous phases indicate that comet Wild 2 experienced little or no aqueous alteration. Less abundant Wild 2 materials include a refractory particle, whose presence appears to require radial transport in the early protoplanetary disk.
Organics found in comet 81P/Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition.
Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class
of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics.
Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary
dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/protostellar
heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained,
a diverse suite of organic compounds is present and identifiable within the returned samples.
Fulgurites result from transient high temperature processes, and some have extremely reduced phases. We performed both modeling and a microprobe analysis of natural fulgurites. The modeling suggests vapor phase C causes reduction of silicate liquid. Additional information is contained in the original extended abstract.
Klassische und atomistische Metallkunde. Aufbau der Mischkristalle und der Metallverbindungen; Strukturanalogien bei den Kupferlegierungen. Der Polymorphismus des Eisens als Grundlage für die Systematik seiner Legierungen; die vier Typen der Eisenlegierungen. Unmöglichkeit einer Erklärung für das Verhalten der Zusatzelemente aus ihrer Kristallstruktur; Beziehungen zwischen Verhalten und Atomvolumen oder Atomradius. Die Stellung der Elemente mit kleinen Atomradien: Bor und Beryllium. Beziehung zwischen Verhalten und Ordnungszahl im periodischen System der Elemente.
We describe the nanometer-sized phases in millimeter-sized spheres produced in a triggered lightning-strike experiment characterized by ultra-high heating of ~ 108 degrees s-1 followed by similarly rapid quenching. The compositions of the aluminosilica glass spheres define a metastable Al2O3-SiO2 eutectic at ~ 40 wt. % Al2O3. The other phases are defined by the liquidus in the SiO2-Al2O3-Fe3O4 (wt %) phase diagram whereby hercynite formed at ~ 1750°C and domains of Fe3+-rich cordierite glass that represent a ternary minimum melt at ~ 1400°C with cotectic glasses linking this glass and spinel. Metastable eutectics are potentially important to understand the phase relationships due to flash heating events.
The mineralogy of interstellar grains is established in the fiery death throes of the stars and characterized to varying degrees by telescopic observations of absorption bands against the background of other stars, by studies of the infrared emission of dusty circumstellar shells, and by analyses of materials separated from meteorites and Interplanetary Dust Particles (IDPs) captured in the lower stratosphere of the Earth. The mineralogy of interplanetary dust was established at the birth of the solar system. These topics have been reviewed on many occasions from the perspective of the analysis of IDPs (Sandford, 1986; Bradley, 1988; Rietmeijer, 1998) or the theoretical interpretation of Interstellar Extinction Spectra (Dorschner and Henning, 1995). We intend to take a very different perspective: that of an experimental chemist using the full range of laboratory studies of IDPs, presolar grains and analog materials to set limits on the processing history of solid materials in astrophysical environments. We warn the reader that some of the conclusions drawn from this perspective are at odds with currently accepted models of grains in the interstellar medium or with interpretations of specific features in IDPs and meteorites. These areas are fertile ground for experimental studies, and analytical or observational programs that will look for the subtle details by which one might distinguish between these alternate hypotheses.
X-ray micro-tomography system has been developed at BL47XU in SPring-8. A Fresnel zone plate (FZP) was used as an objective to achieve the 100-nm-order spatial resolution. The system consists of a light source, a Si(111) double crystal monochromator, a beam diffuser made by graphite powder, a condenser zone plate (CZP), high precision stages for sample rotation, FZP objective and an X-ray image detector. The X-ray energy was selected between 7keV and 10keV to obtain fine images for various samples. The FZP and CZP were fabricated by the electron-beam lithography technique. The material of the zone structure is tantalum with thickness of 1mum. The outermost zone width of FZP is 100 nm, and periodic zone width of the CZP is 200 nm (400 nm in pitch). The measured spatial resolution of the optical system was about 160 nm, which had a good agreement with the theoretical value derived from Hopkins' imaging theory. From a tomographic measurement of a concentric resolution test pattern, cross-sectional image with a spatial resolution of better than 300 nm was successfully reconstructed. A small meteorite and some kinds of minerals were successfully observed.
Kinetically controlled gas-to-solid condensation in Mg–SiO–H2–O2 vapors resulted in the formation of amorphous, chemically ordered, solid compositions at 26±7, 46.5±2 and 87±3.5 wt.% MgO. The first two solid compositions match those of metastable eutectics in existing MgO–SiO2 phase diagrams. An explanation for the highest-MgO metastable solid compositions requires a revision of this phase diagram to include a eutectic at 95 wt.% MgO.
Impact melts and other products of rapid heating and cooling of silicates often contain very reduced species. This work models a simple system, silica liquid condensing from a vapor, to investigate oxygen depletion in more complicated systems.
Non-equilibrium gas to solid condensation produced three distinct groupings of ferrosilica solids with compositions ∽15, 30 and 87.5 wt.% FeO (or, ∽17, 33 and 97 wt.% Fe2O3). These solid compositions define metastable eutectic points in the FeO/Fe2O3–SiO2 binary system. The position of a silica-rich metastable eutectic is sensitive to the ratio FeO:Fe2O3 during gas-to-solid quenching. The Fe-rich metastable eutectic indicates a two-liquid field in this revised pseudo-binary equilibrium phase diagram.
In this study triggered-lightning induced fulgurites were formed in 99.9% pure binary oxides of manganese (MnO) and nickel (NiO) in order to study oxide reduction mechanisms. The fulgurite formation process involved packing the oxide in PVC holders and using the standard rocket-and-wire technique to trigger a lightning strike through the oxide at the International Center for Lightning Research and Testing in Camp Blanding, Florida. These two oxides were chosen from the thermodynamic extrapolation of the oxide stability using the Ellingham Diagram. This diagram indicates that NiO is significantly less stable than MnO. Fulgurites from the pure oxides were analyzed in a scanning electron microscope (SEM); secondary electron images, backscattered images and energy dispersive spectroscopy (EDS) were used to determine the microstructure and composition of the fulgurites. SEM/EDS analysis of the NiO and MnO prior to fulgurite formation confirmed they were pure binary oxides with no metallic contamination. After fulgurite formation, it was found that the nickel oxide fulgurite contained metallic nickel particles; the manganese oxide fulgurite showed no metallic phase formation. Transmission electron microscopy (TEM) examination confirmed that the MnO was a pure oxide with no sign of metallic phase formation. However, TEM results of the NiO showed that approximately 50% of the NiO was reduced to metallic face-centered cubic Ni. The Ni and NiO were observed to be coherent with the [1 0 0]Ni//[1 0 0]NiO and [1 1 0]Ni//[1 1 0]NiO. These results are consistent with the aforementioned thermodynamic stability calculations and show that the presence of carbonaceous material or mixtures of oxides is not necessary for oxide reduction during fulgurite formation. These studies do not rule out the possibility that electrolysis plays a role in oxide reduction. However, these fulgurites were made simultaneously during the same lightning strike and therefore were subjected to the same electrical current, and thus it is proposed the thermodynamic stability of the oxide must play a role in oxide reduction.
An analytical and transmission electron microscope study showed that nonequilibrium gas-to-solid condensation in an AlO–SiO vapor yielded solid units with well defined compositional peaks at 11.5 wt % Al2O3 and 41–47 wt % Al2O3. These peaks coincide with metastable eutectic points in the Al2O3–SiO2 phase diagram. The compositions of thermally annealed units support the presence of a (metastable) miscibility gap between ∼15–∼60 wt % Al2O3. The important finding in this analysis is the determinative role of metastable eutectics in nonequilibrium gas-to-solid condensation. © 1999 American Institute of Physics.
We simulate hypervelocity capturing (6 km/s) analog particles by aerogel
(0.03 g/cc) for STARDUST mission. The extracted particles (lizardite and
cronstedtite) lost their volumes and original surface morphologies and
had vesiculated textures.
Aerogel is an ultra-low-density material that can be used to capture small particles incident upon it at speeds in excess of 1 km s−1. This permits capture of cosmic dust in space where the high speeds usually result in destructive impact events. The performance of aerogel in laboratory impact tests is described. Completely intact capture is rare; most studies show that between 10% to 100% of the incident particle's mass is captured. However, in all cases unaltered domains were found in the particles captured in the laboratory at speeds up to 6 or 7 km s−1. Several analytic techniques can be applied in situ to particles captured in aerogel, yielding data on the preimpact composition of the particle. Extraction techniques for removing small particles from aerogel are described, and after extraction, handling and analysis in the laboratory can proceed as for any small-sized particle. Coupled with the survival of intact regions in the captured particles, this allows detailed identification of the com...
Abstract— During preliminary examination of particles released from 81P/Wild 2 short-period comet, we analyzed 28 particles by nondestructive means, high-sensitive X-ray diffraction and high-resolution X-ray tomography, in order to characterize bulk mineralogy and three-dimensional structures of individual particles. The analyses were performed at synchrotron facilities, KEK and SPring-8 in Japan. Twenty-eight particles from 5 to 25 μm in size, including 25 particles from Track 35 and 3 particles from Track 44, were first analyzed by X-ray diffraction and then 4 out of 28 particles were analyzed by X-ray tomography.
All particles are classified into two groups based on silicate crystallinity: crystalline type and amorphous-rich type. The abundance of the former is approximately 10% of the particles investigated. Crystalline type shows very sharp reflections of olivine and low-Ca pyroxene, while amorphous-rich type shows no or very weak silicate reflections, suggesting that silicates are mostly amorphous. Broad reflections of Fe sulfides and Fe silicides are detected from most of amorphous-rich type particles. Subsequent tomography analysis revealed that the crystalline type is non-porous material consisting of coarse silicate crystals larger than 1 μm in size, while the amorphous-rich type is very porous aggregates with amorphous silicates and small Fe sulfide and Fe metallic grains. All characteristics of amorphous-rich type particles indicate that most of them are melted and rapidly solidified during capture in the silica aerogel. On the other hand, the crystalline type is indigenous cometary particle formed through high-temperature heating episodes that have taken place prior to formation of comet Wild 2. One of the crystalline-type particles (C2054,0,35,6,0) consists of Mg-rich olivine, pyroxene, and kamacite and exhibits porphyritic or poikilitic texture very similar to chondrules.
Abstract— We review the results of our recent experimental studies of astrophysical dust analogs. We discuss the condensation of amorphous silicates from mixed metal vapors, including evidence that such condensates form with metastable eutectic compositions. We consider the spectral evolution of amorphous magnesium silicate condensates as a function of time and temperature. Magnesium silicate smokes anneal readily at temperatures of about 1000–1100 K. In contrast we find that iron silicates require much higher temperatures (˜1300 K) to bring about similar changes on the same timescale (days to months). We first apply these results to infrared space observatory observations of crystalline magnesium silicate grains around high-mass-outflow asymptotic giant branch stars in order to demonstrate their general utility in a rather simple environment. Finally, we apply these experimental results to infrared observations of comets and protostars in order to derive some interesting conclusions regarding large-scale nebular dynamics, the natural production of organic molecules in protostellar nebulae, and the use of crystalline magnesium silicates as a relative indicator of a comet's formation age.
The mineralogical composition of grains produced in supernova ejecta is explored via chemical equilibrium condensation computations. These calculations are carried out for chemical compositions characteristic of each of several supernova zones, taking into account the pressure decrease due to adiabatic expansion and condensation. The distributions of the major elements among the various gaseous species and solid phases are graphically displayed. These computations reveal that many of the major condensates from supernova ejecta are also stable against evaporation in a gas of solar composition at high temperatures. This is especially true for minerals containing the elements O, Mg, Al, Si, Ca, Fe and Ti. Grains which form in supernova ejecta are less likely to become homogenized with solar nebular gas than SN gas and are thus potential sources of exotic isotopic compositions in the early solar system. The calculated elemental distributions of supernova condensates are applied to problems concerning isotopic anomalies and large mass-dependent isotopic fractionations discovered in the meteorite Allende. The order in which the major elements become totally condensed is found to be nearly independent of the supernova zone considered, being the same as that for a solar gas. The consequence of this may be that some of the observed depletions of heavy elements in the interstellar gas are due to supernova-produced dust.
AbstractThe astrophysical aspects of the evolutionary path from dust to planets have been known for a long time but it was only recently that it could be followed by astronomical observations (Hubble Space Telescope). It was also recently that the mineral and chemical properties of dust around young starsqu in different stage of stellar evolution could be determined (the Infrared Space Observatory). Dust around these stars, including Vega-type stars that serve as an analog for the solar nebula, was identified as pure Mg-silicates (forsteriteenstatite), Fe-bearing and pure-Fe amorphous ‘silicates’, silica, Fe-metal, Fe-oxides (possibly Fe-sulfides) amorphous carbon and polycyclic aromatic hydrocarbons. They are the same phases that make up the aggregate and cluster interplanetary dust particles (IDPs) collected in the lower stratosphere. Recent vapor phase condensation experiments showed that the original condensates were mostly amorphous, chemically ordered, metastable eutectic ‘FeSiO’, ‘MgSiO’and ferromagnesiosilica ‘silicate’dust from which the observed non-carbon mineralogy could have evolved during hierarchical dust accretion in the solar nebula. The hypothesis of hierarchical dust accretion uses the size distributions for the surprisingly limited number of non-chondritic dust types in aggregate and cluster IDPs as a measure of relative time. It predicts the accretion of gradually larger, relatively younger, dust aggregates with increasingly diverse chemical and mineral properties of increasingly larger crystalline grains that evolved from initially mostly amorphous dust. This early chemical and mineral dust evolution can be traced in the collected aggregate and in larger cluster IDPs and in even larger aggregate meteoroids that burn up during atmospheric entry flash-heating but whereof the resulting meteors contain information on the chemistry, grain size and texture of the original dust. These aggregate particles were protected against post-accretion, thermal or aqueous dust modification of the original presolar dust and the evolved mineralogy and chemistry during cold-storage inside icy protoplanets such as comet nuclei. The interplanetary dust particles provide ground truth to the properties and modification of the presolar dust in dense molecular clouds wherein stars, such as our sun, were born.
Observations of molten mid-ocean ridge basalt (MORB)-molybdenum (Mo) interactions produced by shock experiments provide insight into impact and differentiation processes involving metal-silicate partitioning. Analysis of fragments recovered from experiments (achieving MORB liquid shock pressures from 0.8 to 6 GPa) revealed significant changes in the composition of the MORB and Mo due to reaction of the silicate and metal liquids on a short time scale ( < 13 s). The FeO concentration of the shocked liquid de creases systematically with increasing pressure. In fact, the most highly shocked liquid (6 GPa) contains only 0.1 wt% FeO compared to an initial concentration of 9 wt% in the MORB. We infer from the presence of micrometer-sized Fe-, Si- and Mo-rich metallic spheres in the shocked glass that the Fe and Si oxides in the MORB were reduced in an estimated oxygen fugacity of 10^(−17) bar and subsequently alloyed with the Mo.
The in-situ reduction of FeO in the shocked molten basalt implies that shock-induced reduction of impact melt should be considered a viable mechanism for the formation of metallic phases. Similar metallic phases may form during impact accretion of planets and in impacted material found on the lunar surface and near terrestrial impact craters. In particular, the minute, isolated Fe particles found in lunar soils may have formed by such a process. Furthermore, the metallic spheres within the shocked glass have a globular texture similar to the textures of metallic spheroids from lunar samples and the estimated, slow cooling rate of ⩽ 140°C/s for our spheres is consistent with the interpretation that the lunar spheroids formed by slow cooling within a melted target.
Silica aerogel has been used as a capturing medium for micrometeoroids and space debris. Several previous investigations suggest that aerogel could capture hypervelocity particles macroscopically intact. However, it has not been fully evaluated whether retrieved grains retain their pristine mineralogy. This study attempts to evaluate the intact survivability of high-speed projectiles in aerogel using impact experiments. Such experiments are essential for rigorous examination or further scientific discussion on the samples of on-going and future sample return missions in which aerogels are/will be used as capturing media. We fired two kinds of micrometeoroid analog materials into aerogel with a two-stage light gas gun (2–4 km/s), serpentine and cronstedtite, which are commonly found in CM/CI, and CM chondrites, respectively. As these hydrated minerals are broken down into anhydrous ones at relatively low temperatures, it is suitable for the evaluation of thermal alteration during the capturing process. The retrieved residues were examined with SEM/EDS, Synchrotron Radiation-XRD, and TEM/EDS. The SR-XRD analysis revealed that most of the volumes of residues are mineralogically unaltered. TEM observations show that one serpentine grain shot at 4 km/s has an unaltered crystalline part inside, an amorphous layer, and the outermost molten aerogel layer. One cronstedtite grain shot at 3 km/s, also examined by TEM, was found to have an unaltered interior as well as a vesiculated silicate melt layer. Image analysis revealed both mineral grains reduced their volume down to 10% of the original on average. These results suggest that it is possible to capture serpentine and cronstedtite particles mineralogically intact with the aerogel, at least in the interior of each particle, below 4 km/s, in spite of their large volume loss.
We report the results of high-resolution, analytical and scanning transmission electron microscopy (STEM), including intensive element mapping, of severely thermally modified dust from comet 81P/Wild 2 caught in the silica aerogel capture cells of the Stardust mission. Thermal interactions during capture caused widespread melting of cometary silicates, Fe-Ni-S phases, and the aerogel. The characteristic assemblage of thermally modified material consists of a vesicular, silica-rich glass matrix with abundant Fe-Ni-S droplets, the latter of which exhibit a distinct core-mantle structure with a metallic Fe,Ni core and a iron-sulfide rim. Within the glassy matrix, the elemental distribution is highly heterogeneous. Localized amorphous "dust-rich" patches contain Mg, Al, and Ca in higher abundances and suggest incomplete mixing of silicate progenitors with molten aerogel. In some cases, the element distribution within these patches seems to depict the outlines of ghost mineral assemblages, allowing the reconstruction of the original mineralogy. A few crystalline silicates survived with alteration limited to the grain rims. The Fe- and CI-normalized bulk composition derived from several sections show CI-chondrite relative abundances for Mg, Al, S, Ca, Cr, Mn, Fe, and Ni. The data indicate a 5 to 15% admixture of fine-grained chondritic comet dust with the silica glass matrix. These strongly thermally modified samples could have originated from a finegrained primitive material, loosely bound Wild 2 dust aggregates, which were heated and melted more efficiently than the relatively coarse-grained material of the crystalline particles found elsewhere in many of the same Stardust aerogel tracks (Zolensky et al. 2006).
The critical partial pressure of SiO necessary to initiate avalanche nucleation in the SiO-H2 system is measured as a function of the ambient temperature in the range 750-1000 K. Results show that the condensate produced at low temperatures is Si2O3, while a mixture of Si2O3 and amorphous SiO2 is produced at high temperatures. A surface energy of approximately 500 ergs/sq cm for the particles is found by analyzing the critical partial pressure vs temperature using classical nucleation theory. It is concluded that classical nucleation theory is not applicable to this system, because several inconsistencies in the thermodynamic analysis are demonstrated, and it is suggested that a kinetic theory of nucleation may be the preferential way to describe the condensation process.
A glassy fulgurite, formed recently on a morainal ridge in southeastern Michigan, contains micrometer- to centimeter-sized metallic globules rich in native silicon, which unmixed from a silica-rich liquid. The unusual character of these globules and their potential for elucidating conditions of fulgurite formation prompted further study. Thermodynamic calculations indicate that temperatures in excess of 2000 K and reducing conditions approaching those of the SiO(2)-Si buffer were needed to form the coexisting metallic and silicate liquids. The phases produced are among the most highly reduced naturally occurring materials known. Some occurrences of other highly reduced minerals may also be due to lightning strike reduction. Extreme reduction and volatilization may also occur during high-temperature events such as lightning strikes in presolar nebulae and impacts of extraterrestrial bodies. As a result of scavenging of platinum-group elements by highly reduced metallic liquids, geochemical anomalies associated with the Cretaceous-Tertiary boundary may have a significant terrestrial component even if produced through bolide impact.
Nonstoichiometric grains with depletions of magnesium and silicon (relative to oxygen) and inclusions of iron-nickel metal and iron-rich sulfides have been identified in interplanetary dust particles from comets. These chemical anomalies accumulate in grains exposed to ionizing radiation. The grains, known as GEMS (glass with embedded metal and sulfides), were irradiated before the accretion of comets, and their inferred exposure ages, submicrometer sizes, and "amorphous" silicate structures are consistent with those of interstellar silicate grains. The measured compositional trends suggest that chemical (as well as isotopic) anomalies can be used to identify presolar interstellar components in primitive meteoritic materials.
The physics of hypervelocity impacts into foams is of interest because of the possible application to interplanetary dust particle (IDP) capture by spacecraft. We present a model for the phenomena occurring in such impacts into low-density organic polymer foams. Particles smaller than foam cells behave as if the foam is a series of solid slabs and are fragmented and, at higher velocities, thermally altered. Particles much larger than the foam cells behave as if the foam were a continuum, allowing the use of a continuum mechanics model to describe the effects of drag and ablation. Fragmentation is expected to be a major process, especially for aggregates of small grains. Calculations based on these arguments accurately predict experimental data and, for hypothetical IDPs, indicate that recovery of organic materials will be low for encounter velocities greater than 5 km/s. For an organic particle 100 micrometers in diameter, approx. 35% of the original mass would be collected in an impact at 5 km/s, dropping to approx. 10% at 10 km/s and approx. 0% at 15 km/s. For the same velocities the recovery ratios for troilite (FeS) are approx. 95%, 65%, and 50%, and for olivine (Mg2SiO4) they are approx. 98%, 80%, and 65%, demonstrating that inorganic materials are much more easily collected. The density of the collector material has only a second-order effect, changing the recovered mass by less than 10% of the original mass.
Phase relations in the Cu-Fe-S, Cu-Ni-S, and Fe-Ni-S systems
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Mineralogical study of binary iron silicides (Fe-Si system) in a fulgurite from Hidalgo, Mexico
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Chemical reduction of impact processed materials (abstract #2037). 32nd Lunar and Planetary Science Conference
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Shock-induced interaction between metal iron and silicates (abstract). 22nd Lunar and Planetary Science Conference
- D D Badjukov
- T L Petrova
Interstellar and interplanetary grains—Recent developments and new opportunities for experimental chemistry
- J A Nuth
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- S L Hallenbeck
Fulgurites: A look at transient high-temperature processes in silicates (abstract #1308). 33rd Lunar and Planetary Science Conference
- A A Wasserman
- H J Melosh
Economic Geology Monograph 4
- G Kullerud
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Mineralogical study of binary iron silicides (Fe-Si system) in a fulgurite from Hidalgo, Mexico
- Ramírez-Cardona
Shock-induced interaction between metal iron and silicates(abstract). 22nd Lunar and Planetary Science Conference
- . D L Badjukovd
Phase relations in the
- Kullerudg
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Are they really intact?-Evaluation of captured micrometeoroid analogs by aerogel at the fly-by speed of Stardust(abstract #1832)
- Okudairak
- Yanoh
- Noguchit
- Nakamurat
- J Burchellm
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Chemical reduction of impact processed materials(abstract #2037). 32nd Lunar and Planetary Science Conference
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