Ni-rich Metal Research Articles

Siderophile element and Hf-W isotope characteristics of the metal-rich chondrite NWA 12273 – Implication for its origin and chondrite metal formation

Metal phases in primitive chondrites contain important information for understanding early Solar System processes. Northwest Africa (NWA) 12273 is an ungrouped chondrite and, due to its preternatural high modal abundance of metal grains (>60%), provides a unique perspective on the formation and evolution of primitive planetesimals. To investigate the formation of metal phases in early solar system materials we report detailed petrography, siderophile geochemistry and Hf-W isotope geochronology for NWA 12273. The primitive textures and heterogeneous mineral chemistry of chondrules, as well as siderophile element compositions of Ni-rich metal, all indicate an affinity of NWA 12273 to unequilibrated (type 3.8) ordinary chondrites that experienced limited thermal metamorphism. Modeling shows that crucial inter-element fractionations among refractory siderophile elements for kamacite in NWA 12273, were most likely caused by solid metal-liquid metal partitioning, for example during partial melting of Fe-Ni metal containing ∼3.9 wt.% of S. Analysis of the metal fractions within NWA 12273 gives a Hf-W model age of ∼2.4 Ma after CAI formation (ε182W6/4=−3.29±0.11), older than metal formation in most of H chondrites and IIE irons. In comparison with published data for chondrites and ‘non-magmatic’ iron meteorites, siderophile element data suggest that the metal phases in NWA 12273 may have originated from similar precursor materials and/or accreted in adjacent nebular locations to IIE irons and H chondrites. These primary planetesimals experienced impact-related evaporation and mixing followed by rebuilding of the secondary body. This scenario is consistent with that proposed for IIE and other silicate-bearing iron meteorites, indicating that the building blocks of some chondrite parent bodies were not entirely primitive (undifferentiated) materials.

Formation of metallic-Cu-bearing mineral assemblages in type-3 ordinary and CO chondrites

Abstract Studies of the new growth and re-distribution of Cu-rich phases in chondrites of different petrologic subtypes can potentially provide insights into post-accretionary parent-body processes. We present a systematic study of the distribution of Cu-rich phases and metallic Cu in Ornans-like carbonaceous chondrites (CO3) that underwent little aqueous alteration or shock (most with shock stages of S1) but exhibit a range of thermal metamorphism (subtype 3.0–3.7). A comparison to ordinary chondrites (OCs), which have undergone a larger range of shock levels, allows us to constrain the relative roles of radiogenic and shock heating in the origin of Cu distribution in chondrites. We found that the Cu content of Ni-rich metal and calculated bulk Cu content of CO3 chondrites (based on mass-balance calculations) show an increase from CO3.0 to CO3.2 chondrites. We speculate that some unidentified phases in the matrix account for a significant portion (nearly ~100 ppm) of the Cu budget in bulk samples of CO3.0 chondrites, while Ni-rich metal is the main Cu-carrier for CO3.2–3.7 chondrites. Within CO3.2–3.7 chondrites, Cu and Ni contents of Ni-rich metal are positively correlated, showing a systematic decrease from lower to higher subtype (~0.41 wt% Cu and ~45.0 wt% Ni in CO3.2 Kainsaz; ~0.28 wt% Cu and ~38.8 wt% Ni in CO3.7 Isna). Metallic Cu grains were found in every sample of CO3.2–3.7 chondrites, but not in any CO3.0–3.1 chondrites. Metallic Cu is: (1) present at metallic-Fe-Ni-pyrrhotite interfaces; (2) associated with fine irregular pyrrhotite grains in Ni-rich-metal-pyrrhotite nodules; (3) associated with fizzed pyrrhotite (fine-grained mixtures of irregularly shaped metal grains surrounded by pyrrhotite); (4) present at the edges of metallic Fe-Ni grains; and (5) present as isolated grains. In some metallic-Cu-bearing mineral assemblages, pyrrhotite has higher Cu concentrations than adjacent Ni-rich metal and shows a drop in Cu concentration at the interface between metallic Cu and Cu-rich pyrrhotite. This implies that the precipitation of metallic Cu grains could be related to the local Cu enrichment of pyrrhotite. We consider that radiogenic heating is mainly responsible for the formation of opaque phases in CO chondrites based on the relatively slow metallographic cooling rate (~0.1–5 °C/Ma), the increasing uniformity of Ni contents in Ni-rich metal with increasing CO subtype (44.3 ± 17.3 wt% in CO3.00 to 38.8 ± 3.4 wt% in CO3.7 chondrite), and the relatively narrow range of pyrrhotite metal/sulfur ratios (~0.976–0.999). Metal/sulfur ratios of pyrrhotite grains in most CO3.2–3.7 chondrites (mean = ~0.986–0.997; except Lancé) are slightly higher than those in CO3.0–3.1 chondrites (mean = ~0.981–0.987; except Y-81020), possibly indicative of a release and re-mobilization of sulfur during progressive heating as previously reported for type-3 chondrites. In this regard, we suggest most metallic Cu grains in CO3 chondrites may have precipitated from Cu-rich pyrrhotite due to sulfidation of Fe-Ni metal during parent-body thermal metamorphism. Locally, a few metallic Cu grains associated with fizzed pyrrhotite could have formed during transient shock-heating. Both thermal and shock metamorphism could be responsible for the formation of metallic Cu. Although the systematic decrease in the Ni contents of Ni-rich metal from subtype-3.2 to subtype-3.8 also occurs in OCs, the average Cu contents of Ni-rich metal grains are indistinguishable among type-3 OCs of different subtypes. The paucity of metallic Cu in weakly shocked type-3 OCs could be related to: (1) the relatively low-bulk Cu contents of OCs, and/or (2) the relatively rapid metallographic cooling rates at <500–600 °C (~1–10 °C/Ma for LL chondrites), possibly resulting from early disturbance of OC parent bodies. The intergrowth of metallic Cu and irregular pyrrhotite more commonly occurs in shocked type-4 to type-6 OCs than in CO3 chondrites. This could be due to S in type-4 to type-6 OCs being more mobilized due to shock heating than in unshocked CO3 chondrites. We predict that some other groups of carbonaceous chondrites (e.g., CI and CM) are less likely to produce metallic Cu due to the: (1) relatively low amount of metallic Fe-Ni; (2) relatively low parent-body temperatures of ~100–300 °C; (3) high mobility of Cu in solution for aqueously altered samples; and (4) the short heating duration for metamorphosed samples.

The alteration history of the CY chondrites, investigated through analysis of a new member: Dhofar 1988

We provide the first detailed analysis of the carbonaceous chondrite Dhofar (Dho) 1988. This meteorite find was recovered in 2011 from the Zufar desert region of Oman and initially classified as a C2 ungrouped chondrite. Dho 1988 is a monomict breccia composed of millimetre-sized clasts, between which large (∼50–250 µm) intermixed sulphide-Ca-carbonate veins formed. It has high sulphide abundances (∼14 vol%), medium-sized chondrules (avg. 530 µm, N = 33), relatively low chondrule/CAI abundances (<20 area%), a heavy bulk O-isotope composition (δ17O = 9.12‰, δ18O = 19.46‰) and an aqueously altered and then dehydrated alteration history. These characteristics are consistent with the newly defined Yamato-type (CY) carbonaceous chondrite group, suggesting this meteorite should be reclassified as a CY chondrite.Dho 1988 experienced advanced aqueous alteration [petrologic subtype 1.3 in the scheme of Howard et al. (2015)]. Alteration style and extent are similar to the CM chondrite group, with the matrix having been replaced by tochilinite-cronstedtite intergrowths and chondrules progressively pseudomorphed by phyllosilicates, sulphides and in one instance Ca-carbonates. However, departures from CM-like alteration include the replacement of chondrule cores with Al-rich, Na-saponite and upon which Cr-spinel and Mg-ilmenite grains precipitated. These late-stage aqueous alteration features are common among the CY chondrites. Fractures in Dho 1988 that are infilled by phyllosilicates, sulphides and carbonates attest to post-brecciation aqueous alteration. However, whether aqueous alteration was also active prior to brecciation remains unclear. Veins are polymineralic with a layered structure, allowing their relative chronology to be reconstructed: intermixed phyllosilicate-sulphide growth transitioned to sulphide-carbonate deposition. We estimate temperatures during aqueous alteration to have been between 110 °C < T < 160 °C, based on the co-formation of Na-saponite and tochilinite.Dho 1988 was later overprinted by thermal metamorphism. Peak temperatures are estimated between 700 °C and 770 °C, based on the thermal decomposition of phyllosilicates (both serpentine and saponite) combined with the survival of calcite. As temperatures rose during metamorphism the thermal decomposition of pyrrhotite produced troilite. Sulphur gas was liberated in this reaction and flowed through the chondrite reacting with magnetite (previously formed during aqueous alteration) to form a second generation of troilite grains. The presence of both troilite and Ni-rich metal in Dho 1988 (and other CY chondrites) demonstrate that conditions were constrained at the iron-troilite buffer.

Degradation and Aging Routes of Ni-Rich Cathode Based Li-Ion Batteries

Driven by the increasing plea for greener transportation and efficient integration of renewable energy sources, Ni-rich metal layered oxides, namely NMC, Li [Ni1−x−yCoyMnz] O2 (x + y ≤ 0.4), and NCA, Li [Ni1−x−yCoxAly] O2, cathode materials have garnered huge attention for the development of Next-Generation lithium-ion batteries (LIBs). The impetus behind such huge celebrity includes their higher capacity and cost effectiveness when compared to the-state-of-the-art LiCoO2 (LCO) and other low Ni content NMC versions. However, despite all the beneficial attributes, the large-scale deployment of Ni-rich NMC based LIBs poses a technical challenge due to less stability of the cathode/electrolyte interphase (CEI) and diverse degradation processes that are associated with electrolyte decomposition, transition metal cation dissolution, cation–mixing, oxygen release reaction etc. Here, the potential degradation routes, recent efforts and enabling strategies for mitigating the core challenges of Ni-rich NMC cathode materials are presented and assessed. In the end, the review shed light on the perspectives for the future research directions of Ni-rich cathode materials.

Formation and destruction of magnetite in CO3 chondrites and other chondrite groups

Primitive CO3.00–3.1 chondrites contain ∼2-8 vol.% magnetite, minor troilite and accessory carbide and chromite; some CO3.1 chondrites have fayalite-rich veins, chondrule rims and euhedral matrix grains. All CO3.00–3.1 chondrites contain little metallic Fe-Ni (0.4–1.2 vol.%). CO3.2–3.7 chondrites contain 1–5 vol.% metallic Fe-Ni, minor troilite, accessory chromite and 0-0.6 vol.% magnetite. Magnetite is formed in primitive CO3 chondrites from metallic Fe by parent-body aqueous alteration, resulting in decreased metallic Fe-Ni and an increase in the proportion of high-Ni metal grains. The paucity or absence of magnetite in CO chondrites of subtype ≥3.2 suggests that magnetite is destroyed during thermal metamorphism; thermochemical calculations from the literature suggest that magnetite is reduced by H2 and reacts with SiO2 to form fayalite and secondary kamacite. Analogous processes of magnetite formation and destruction occur in other chondrite groups: (1) Primitive type-3 OC have opaque assemblages containing magnetite, carbide, Ni-rich metal and Ni-rich sulfide, but OC of subtype >3.4 contain little or no magnetite. (2) Primitive R3 chondrites and clasts (subtype ≲3.5) contain up to 6 vol.% magnetite, but most R chondrites contain no magnetite. The principal exception is magnetite with 9–20 wt.% Cr2O3 in a few R4-6 chondrites. Magnetite grains with high Cr2O3 behave like chromite and are more stable under reducing conditions. (3) CK chondrites average ∼4 vol.% magnetite with substantial Cr2O3 (up to ∼15 wt.%); these magnetite grains also are stable against reduction during metamorphism. (4) The modal abundance of magnetite decreases with metamorphic grade in CV3 chondrites. (5) Chromite occurs instead of magnetite in those rare samples classified CR6, CR7 and CV7.

Effect of Electrolyte Composition on Capacity Fading of Ni Rich Lithium Transition Metal Oxide Cathode for High Energy Density Lithium Ion Batteries

The lithium-ion battery has been widely employed in mobile applications because of its advantages, such as high energy density, insignificant self-discharge rate, and reasonable price. While it is also the most promising candidate power source for advanced automobiles, it needs further development to meet the specifications of electric vehicles1,2. For example, the driving distance of an electric vehicle on a single charge is becoming more and more important, which requires batteries with ever higher energy density. One of the main strategies to enhance the energy density is to raise the average potential and specific capacity of the cathode materials3. For this purpose, Ni rich lithium transition metal oxides have been developed for promising cathode materials4,5. Nevertheless, there are several limitations that need to be surmounted before the mass production of high energy density batteries containing Ni rich lithium transition metal oxides. In particular, significant capacity fading has been observed with lithium ion battery using Ni rich lithium transition metal oxides6,7. In this study, we found that cathode (Ni-rich LiNi0.8Co0.1Mn0.1O2) surface modification – phase transformation from a layered structure to a cubic phase – exerted a significant influence on battery performance. Such cubic phase hindered ionic and electronic conductivity of cathode particles. The kind of the cubic phase was different by changing the composition of the electrolyte. As a result, the resistance of the cylindrical cells after 300 cycles using different electrolyte showed a large gap, especially on cathode side. [1] B. Scrosati, J. Garche, J. Power Sources, 2010, 195, 2419[2] M. Armand, J.-M. Tarascon, Nature, 2008, 451, 652[3] J. B. Goodenough, Y. Kim, Chem. Mater., 2010, 22, 587[4] W. Liu et al, Angew. Chem. Int. Ed. Engl. 2015, 54, 4440[5] A. Manthiram et al, Adv. Energy Mater, 2016, 6, 1501010[6] D.P. Abraham et al, Electrochem. Comm., 2002, 4, 620[7] S. Watanabe et al, J. Power Sources, 2014, 258, 210

Long stable cycling of fluorine-doped nickel-rich layered cathodes for lithium batteries

Theoretically, layered Ni-rich metal oxides are capable of delivering 200 mA h g−1. Herein, the synthesized F doped Ni-rich of composition, LiNi0.7Co0.15Mn0.15O1.95F0.05 exhibited a capacity of 170 mA h g−1 even after 100 cycles when tested against lithium.

An atom probe tomography study of internal oxidation processes in Alloy 600

Internal oxidation in Alloy 600 (Ni–16Cr–9Fe) involves the inward diffusion of O, which preferentially reacts with solute elements (e.g. Cr, Fe) forming internal oxide precipitates. In this work, the internal oxidation process and its resultant metal-oxide heterostructures are systematically analyzed in 3D at the finest length scales, using atom probe tomography (APT). Internal oxidation is induced by exposure of Alloy 600 to 480 °C hydrogenated steam, with an oxygen partial pressure below the dissociation pressure of the solvent metal oxide, NiO. Following exposure, nodules of metallic Ni are observed on the sample surface, originating from material that is expelled to relieve compressive stresses generated by internal oxidation. Probing the material immediately below the surface reveals these nodules to be directly connected to their parent grains via Ni-rich metal channels. The matrix metal is intertwined with a continuous network of oxides, which are primarily FeCr2O4. The continuous interface between oxides and matrix provides the short-circuit diffusion pathways necessary for Ni expulsion. Deeper below the surface, oxides are observed as particles, rather than a continuous network, and are often aligned on matrix planes. The small oxide particles are Cr2O3, which is also found to be the composition at the centres of many larger FeCr2O4 oxides. This indicates that oxides first precipitate as Cr2O3, then later grow as FeCr2O4 when local depletion of Cr changes which phase is most energetically-favourable. Initial comparisons indicate that some fundamental findings in this work are applicable to known internal oxidation systems at lower and higher temperatures.

The mineralogy and space weathering of a regolith grain from 25143 Itokawa and the possibility of annealed solar wind damage

We report the results of detailed mineralogical investigations by analytical scanning and transmission electron microscopy of particle RA-QD02-0115 recovered from the surface of asteroid 25143 Itokawa. We divided the 65 μm × 50 μm small particle into eight individual subsample slices via the focused ion beam method. The particle dominantly consists of olivine and contains inclusions of merrillite, tetrataenite/taenite, troilite, chromite, kamacite, and Cl-bearing apatite (in approx. decreasing order of frequency). The composition of olivine (fayalite 29.8 ± 1.1 mol% and molar Fe/Mn ratio of 57 ± 2) as well as the Ni-rich metal assemblage indicates an LL-type affinity in accord with previous classifications. The particle shows effects of solar wind irradiation on one of its principal faces. Olivine developed an approximately 34 nm wide rim composed of low-angle misoriented, nanometer-sized crystallites accompanied by a small amount of amorphous material. Exposed troilite developed a 4 to 8 nm wide polycrystalline rim with large-angle misorientations of the iron sulfide nanocrystallites. Merrilite shows marginally discernable surface damage but was too unstable under the electron beam for a detailed study. Cl-bearing apatite was found fully crystalline with no discernable rim structure. We discuss the unusual polycrystalline nature of the olivine rim in terms of possible annealing and recrystallization effects, which may have occurred during periods of time when Itokawa’s surface temperature may have been warmer due to closer perihelion distances. Model calculations show that the dynamical orbital evolution of near-Earth asteroids could lead to complex space weathering processes, arising from the competing interplay between irradiation-induced damaging and thermally driven annealing.

Shock veins in the Sahara 02500 ordinary chondrite

Shock veins in the Sahara 02500 ordinary chondriteA specimen of the Sahara 02500 ordinary chondrite contains shock-produced veins consisting of recrystallised fine-grained pyroxenes that include small droplets of Ni-rich metal. Non-melted olivines and pyroxenes show planar deformations filled by shock-melted and -polluted metal and troilite. Shock-melted feldspathic glass is present close to the shock veins. Geothermometric estimations indicate that the meteorite locally experienced moderate shock metamorphism with a minimum local peak temperature above 1400°C, resulting in partial melting of Ca-poor pyroxene and full melting of feldspars, metal and sulphides. The mineral assemblage in the shock veins suggests a pressure during melt recrystallisation below 10 GPa.

Metal–silicate partitioning experiments in the diamond anvil cell: A comment on potential analytical errors

We have performed simulations of the electron microbeam analysis of metal–silicate partitioning experiments in which a small (∼50μm) ball of metal is embedded in a silicate matrix. This geometry approximates that found in many high pressure diamond-cell experiments aimed at determining the partition coefficients (Di) of elements between metal and silicate. The simulations involved using the (publicly available) Monte Carlo simulation package PENEPMA which is based on the general-purpose electron–photon transport code PENELOPE. The code allows complex geometrical structures to be defined by quadric surfaces, which enclose homogeneous materials specified by the user. The package simulates both electron and photon transport, keeps track of the X-ray production mechanism and includes the continuum contribution.Our principal results came from simulating the effects of secondary fluorescence on the apparent metal–silicate partition coefficient of Ni for the case of an Ni-rich metal (26.8% Ni, 73.2% Fe) embedded in a silicate matrix. If, as is frequently the case, analysis of the silicate is performed 5–10μm from the silicate–metal interface then secondary fluorescence of Ni in the metal will increase the apparent NiO concentration in the silicate by ∼1000ppm. In the case considered, this is 25% of the Ni content of the silicate and the fluorescence effect decreases the apparent DNi from 110 to 84. For this type of experiment, therefore, the fluorescence effect reduces the apparent partition coefficient.We performed similar simulations of the analysis of the centre of a 10μm diameter metal ball embedded in the silicate matrix to determine apparent Si and O concentrations in the metal. In this case, because the electrons do not penetrate so far into the metal, the fluorescence effect is very small and can be neglected.Finally, we show that focused ion beam (FIB) sections, while providing the potential physically to separate metal from silicate present additional, but not insurmountable, problems of standardisation. The best results would be obtained with a section >3μm in thickness.We recommend that, in general, experimentalists consider simulating analysis of a typical experimental product before publication of results which may be affected by secondary fluorescence.

Impact of Cr-poisoning on the conductivity of LaNi 0.6Fe 0.4O 3

This study demonstrates the significant impact of Cr on the electronic conductivity of a LaNi 0.6Fe 0.4O 3 (LNF) porous cathode layer at 800 °C. Vapor transport of Cr-species, originating from a porous metallic foam, and subsequent reaction with LNF, results in a decrease of the electronic conductivity of the LNF-layer. Cr has been detected throughout the entire cross-section of a 16 μm thick LNF layer, while Ni, besides its compositional distribution in the LNF layer, has also been found in enriched spots forming Ni-rich metal oxide crystals. Transmission electron microscopy revealed that Cr is gradually incorporated into the LNF-grains, while Ni is proportionally expelled. Electron diffraction performed in the center of a sliced grain showed the initial rhombohedral crystal structure of LNF, whereas diffraction performed close to the edge of the grain revealed the orthorhombic perovskite crystal structure, indicating a Cr-enriched perovskite phase. Progressive Cr deposition and penetration into the LNF grains and necks explains the electronic conductivity deterioration. The impact of Cr-poisoning on the electronic conductivity of the LNF porous layer is considerably smaller at 600 °C than at 800 °C.

Spherules from the Tunguska event site: Could they originate from the Tunguska Cosmic Body?

Soil layers at the Tunguska event site may have accumulated infalling extraterrestrial matter derived from the Tunguska Cosmic Body (TCB). Using mineralogical, textural, and chemical criteria, a set of metal and silicate spherules of probably cosmic origin was identified in the collection of spherules and rounded particles recovered from sites with high concentrations of magnetic spherules on a terrace above the floodplain of the Chunya River. The metal spherules consist of Ni(Cr)-bearing wustite and magnetite with Ni-rich metal inclusions. The silicate spherules are glassy, cryptocrystalline, barred, and porphyritic melted micrometeorites, some of which contain metal droplets. The number of spherules counted in our samples is higher than the background level, indicating the possible presence of the TCB material.

Corrosion of alumina-forming austenitic steel Fe–20Ni–14Cr–3Al–0.6Nb–0.1Ti in supercritical water

The oxidation behavior of alumina-forming austenitic (AFA) steel Fe–20Ni–14Cr–3Al–0.6Nb–0.1Ti has been examined after exposure to supercritical water (SCW) at 500 °C and 25 MPa pressure and with a dissolved oxygen content of 25 wppb, for exposure periods of 1300 h, 1700 h and 3000 h. A double layer oxide structure developed on all samples, consisting specifically of a Fe-rich outer hematite layer and an inner layer of Al–Cr–Fe-rich oxide containing a small fraction of FCC Ni-rich metal. Additionally, a thin Ni-rich layer in the bulk alloy close to the oxide/metal interface was also observed. Based on weight gain measurements, the oxidation resistance of the AFA steel was superior to other austenitic alloys, 800H, D9, and 316 stainless steel, tested under similar conditions. The formation of the protective Al–Cr–Fe-rich oxide layer may provide good long-term protection against corrosion in SCW environment.

Advanced TEM characterization of stress corrosion cracking of Alloy 600 in pressurized water reactor primary water environment

Advanced transmission electron microscopy techniques were carried out in order to investigate stress corrosion cracking in Alloy 600 U-bend samples exposed in simulated PWR primary water at 330 °C. Using high-resolution imaging and fine-probe chemical analysis methods, ultrafine size oxides present inside cracks and intergranular attacks were nanoscale characterized. Results revealed predominance of Cr 2O 3 oxide and Ni-rich metal zones at the majority of encountered crack tip areas and at leading edge of intergranular attacks. However, NiO-structure oxide was predominant far from crack tip zones and within cracks propagating along twin boundaries and inside grains. These observations permit to suggest a mechanism for intergranular stress corrosion cracking of Alloy 600 in PWR primary water. Indeed, the results suggest that stress corrosion cracking is depending on chromium oxide growth in the grain boundary. Oxide growth seems to be dependent on oxygen diffusion in porous oxide and chromium diffusion in strained alloy and in grain boundary beyond crack tip. Strain could promote transport kinetic and oxide formation by increasing defaults rate like dislocations.

Opaque phases in type-II chondrules from CR2 chondrites: Implications for CR parent body formation

We report the results of a detailed study of sulfide-bearing opaque assemblages from the MAC 87320, EET 92011, and Renazzo CR carbonaceous chondrites. The objectives of this study are to (1) characterize sulfide and associated phases within CR2 chondrites; (2) determine the petrographic relationship between sulfides, metals, and chondrules; (3) constrain the history of type-II chondrules; (4) ascertain the environments in which type-II chondrules formed and were altered; and (5) unravel the formation and alteration history of the CR parent body as recorded in sulfide-bearing assemblages. Sulfide-bearing opaque assemblages occur primarily within type-II (FeO-rich) chondrules. The sulfide assemblages are concentrated near the chondrule edges. Assemblages in MAC 87320 are composed of troilite, phosphate, and Ni-rich metal. EET 92011 contains assemblages composed of pentlandite, troilite, and Ni-rich metal. The assemblages in Renazzo contain tochilinite, magnetite, troilite, pentlandite, and phosphate. In all of the assemblages in Renazzo the tochilinite is fine grained and intimately mixed with troilite, pentlandite, or magnetite. Opaque assemblages in CR chondrites record a complex history that includes both high- and low-temperature processes. The morphology and composition of sulfides in CR2 chondrites suggests that the sulfide-bearing assemblages originally formed in gas–solid reactions in the nebula at temperatures above the Fe–FeS eutectic (988 °C). Many of the assemblages were subsequently aqueously altered on the CR-chondrite parent body to various degrees at temperatures from ∼50 to 200 °C. We combine these observations and interpretations to provide a detailed model of the history of the CR parent body.

Petrogenesis of opaque assemblages in the Ningqiang carbonaceous chondrite

Numerous round to oblate opaque assemblages (OAs) are found in chondrules and matrix of the Ningqiang carbonaceous chondrite. They are mainly composed of Ni-rich metal, magnetite, Fe, Ni-sulfides, with minor amounts of phosphate, phosphoran-olivine, pyroxene and trace amounts of nano-sized platinum-group metal alloys. The mineralogy of Ningqiang OAs is very similar to that of OAs previously reported in Ca, Al-rich inclusions of CV chondrites. Being a rare mineral phase in nature, phosphoran-olivine is thought to form by nonequilibrium reactions between P-bearing molten metal and olivine crystals during rapid cooling. Its occurrence in Ningqiang OAs indicates that the precursor of OAs was locally produced during chondrule formation, rather than directly condensed from the solar nebula as previously thought. The petrographic and mineralogical characteristics of Ningqiang OAs reveal that OAs formed by low temperature alterations of pre-existing homogeneous alloys within chondrules on a planetary body.

Aqueous alteration of opaque assemblages in the Ningqiang carbonaceous chondrite: Evidence from oxygen isotopes

Chondrules and matrix of the Ningqiang carbonaceous chondrite contain numerous opaque assemblages (OAs). These OAs are mainly composed of Ni-rich metal, magnetite, sulfides, phosphates with minor amounts of silicates and trace amounts of Pt-metal grains. Their mineral assemblage is very similar to that of OAs found in Allende refractory inclusions. In this study, we performed in situ analyses of O-isotopic compositions of magnetite and whitlockite in OAs and of olivine adjacent to OAs from Ningqiang. O-isotopic compositions of magnetite vary by 16‰ in δ 18O and fall on a regression line with a slope of 0.51 and a mean Δ 17O of − 2.9 ± 0.3 (1 σ)‰ in the three-O-isotope diagram. Magnetite grains have consistently higher Δ 17O values than their adjacent olivine grains. The mineral chemistry and O-isotopic compositions of OAs in Ningqiang chondrules and matrix are most consistent with the formation of magnetites and whitlockite by aqueous alteration of preexisting metal alloys within a planetary body. Formation of magnetites and phosphates in OAs thus do not constrain processes in the early solar nebula, but rather provide information on metamorphism in a planetary environment.

A menagerie of graphite morphologies in the Acapulco meteorite with diverse carbon and nitrogen isotopic signatures: Implications for the evolution history of acapulcoite meteorites

Morphologies, petrographic settings and carbon and nitrogen isotopic compositions of graphites in the Acapulco meteorite, the latter determined by secondary ionization mass spectrometry, are reported. Seven different graphite morphologies were recognized, the majority of which occur enclosed exclusively in kamacite. Individual graphite grains also rarely occur in the silicate matrix. Kamacite rims surrounding taenite cores of metal grains are separated from the Ni-rich metal cores by graphite veneers. These graphite veneers impeded or prevented Ni-Fe interdiffusion during cooling. In addition, matrix FeNi metal contains considerable amounts of phosphorous (≈ 700 ppm) and silicon (≈ 300 ppm) ( Pack et al., 2005 in preparation) thus indicating that results of laboratory cooling experiments in the Fe-Ni binary system are inapplicable to Acapulco metals. Graphites of different morphologies display a range of carbon and nitrogen isotopic compositions, indicating a diversity of source regions before accretion in the Acapulco parent body. The isotopic compositions point to at least three isotopic reservoirs from which the graphites originated: (1) A reservoir with heavy carbon, represented by graphite in silicates (δ 13C = 14.3 ± 2.4 ‰ and δ 15N = −103.4 ± 10.9 ‰), (2) A reservoir with isotopically light carbon and nitrogen, characteristic for the metals. Its C- and N-isotopic compositions are probably preserved in the graphite exsolutions that are isotopically light in carbon and lightest in nitrogen (δ 13C = −17 to −23 ‰ δ 15N = −141 to −159 ‰). (3) A reservoir with an assumed isotopic composition (δ 13C ∼ −5 ‰; δ 15N ∼ −50 ‰). A detailed three-dimensional tomography in reflected light microscopy of the decorations of metal-troilite spherules in the cores of orthopyroxenes and olivines and metal-troilite veins was conducted to clarify their origin. Metal and troilite veins are present only near the fusion crust. Hence, these veins are not pristine to Acapulco parent body but resulted during passage of Acapulco in Earth’s atmosphere. A thorough search for symplectite-type silicate-troilite liquid quench textures was conducted to determine the extent of closed-system partial silicate melting in Acapulco. Metal-troilite spherules in orthopyroxenes and olivines are not randomly distributed but decorate ferromagnesian silicate restite cores, indicating that the metal-spherule decoration around restite silicates took place in a silicate partial melt. Graphite inclusions in these spherules have C- and N- isotopic compositions (δ 13C = −2.9 ± 2.5 ‰ and δ 15N = −101.2 ± 32 ‰) close to the average values of graphite in metals and in the silicate matrix, thus strongly suggesting that they originated from a mixture of graphite inclusions in metals and silicate matrix graphite during a closed system crystallization process subsequent to silicate-metal-sulfide partial melting. Troilite-orthopyroxene quench symplectite textures in orthopyroxene rims are clear evidence that silicate-sulfide partial melting took place in Acapulco. Due to petrographic heterogeneity on a centimeter scale, bulk REE abundances of individual samples or of individual minerals provide only limited information and the REE abundances alone are not entirely adequate to unravel the formational processes that prevailed in the acapulcoite-lodranite parent body. The present investigations demonstrate the complexity of the evolutionary stages of acapulcoites from accretion to parent body processes.

TEM investigations of intergranular stress corrosion cracking in austenitic alloys in PWR environmental conditions

Analytical transmission microscopy has been used to investigate the initiation of stress corrosion cracking in Inconel 600 subjected to constant load testing under simulated pressured water reactor primary water conditions. The observations revealed that intergranular attack proceeded by the development of a zone of polycrystalline chromia along the boundary plane intersecting either the free surface or a blunted, open crack in contact with the free surface. Ni-rich metal particles were interspersed within the chromia. Conversely, open cracks were filled with nanocrystalline NiO and large compound particles of spinel and NiO, indicating a difference in potential between closed, attacked boundaries and open cracks. Open cracks appeared to have initiated by fracture of the chromia zones, such fracture being strongly dependent on boundary geometry with respect to loading direction. The observations suggest that stress corrosion crack initiation and propagation is dependent on diffusion of oxygen through the porous oxides. Dislocations and stress could enhance diffusion as chromia was observed along slip planes at the arrested tips of blunt cracks.