Metal-saturated sulfide assemblages in NWA 2737: Evidence for impact-related sulfur devolatilization in Martian meteorites

International audienceNWA 2737, a Martian meteorite from the Chassignite subclass, contains minute amounts (0.010 ± 0.005 vol%) of metal-saturated Fe-Ni sulfides. These latter bear evidence of the strong shock effects documented by abundant Fe nanoparticles and planar defects in Northwest Africa (NWA) 2737 olivine. A Ni-poor troilite (Fe/S = 1.0 ± 0.01), sometimes Cr-bearing (up to 1 wt%), coexists with micrometer-sized taenite/tetrataenite-type native Ni-Fe alloys (Ni/Fe = 1) and Fe-Os-Ir-(Ru) alloys a few hundreds of nanometers across. The troilite has exsolved flame-like pentlandite (Fe/Fe + Ni = 0.5-0.6). Chalcopyrite is almost lacking, and no pyrite has been found. As a hot desert find, NWA 2737 shows astonishingly fresh sulfides. The composition of troilite coexisting with Ni-Fe alloys is completely at odds with Chassigny and Nahkla sulfides (pyrite + metal-deficient monoclinic-type pyrrhotite). It indicates strongly reducing crystallization conditions (close to IW), several log units below the fO2 conditions inferred from chromites compositions and accepted for Chassignites (FMQ-1 log unit). It is proposed that reduction in sulfides into base and precious metal alloys is operated via sulfur degassing, which is supported by the highly resorbed and denticulated shape of sulfide blebs and their spongy textures. Shock-related S degassing may be responsible for considerable damages in magmatic sulfide structures and sulfide assemblages, with concomitant loss of magnetic properties as documented in some other Martian meteorites

Redox-freezing and nucleation of diamond via magnetite formation in the Earth’s mantle

Diamonds and their inclusions are unique probes into the deep Earth, tracking the deep carbon cycle to >800 km. Understanding the mechanisms of carbon mobilization and freezing is a prerequisite for quantifying the fluxes of carbon in the deep Earth. Here we show direct evidence for the formation of diamond by redox reactions involving FeNi sulfides. Transmission Kikuchi Diffraction identifies an arrested redox reaction from pyrrhotite to magnetite included in diamond. The magnetite corona shows coherent epitaxy with relict pyrrhotite and diamond, indicating that diamond nucleated on magnetite. Furthermore, structures inherited from h-Fe3O4 define a phase transformation at depths of 320–330 km, the base of the Kaapvaal lithosphere. The oxidation of pyrrhotite to magnetite is an important trigger of diamond precipitation in the upper mantle, explaining the presence of these phases in diamonds

Growth of Transition-Metal Dichalcogenides by Solvent Evaporation Technique

Due to their physical properties and potential applications in energy conversion and storage, transition-metal dichalcogenides (TMDs) have garnered substantial interest in recent years. Among this class of materials, TMDs based on molybdenum, tungsten, sulfur, and selenium are particularly attractive due to their semiconducting properties and the availability of bottom-up synthesis techniques. Here we report a method which yields high-quality crystals of transition-metal diselenide and ditelluride compounds (PtTe2, PdTe2, NiTe2, TaTe2, TiTe2, RuTe2, PtSe2, PdSe2, NbSe2, TiSe2, VSe2, ReSe2) from their solid solutions, via vapor deposition from a metal-saturated chalcogen melt. Additionally, we show the synthesis of rare-earth-metal polychalcogenides and NbS2 crystals using the aforementioned process. Most of the crystals obtained have a layered CdI2 structure. We have investigated the physical properties of selected crystals and compared them to state of the art findings reported in the literature. Remarkably, the charge density wave transition in 1T-TiSe2 and 2H-NbSe2 crystals is well-defined at TCDW ≈ 200 and 33 K, respectively. Angle-resolved photoelectron spectroscopy and electron diffraction are used to directly access the electronic and crystal structures of PtTe2 single crystals and yield state of the art measurements. © 2020 American Chemical Society.M.A.-H. acknowledges support from the VR starting grant 2018-05339 and KL1824/6. The crystal growth experiments were supported by the Russian Science Foundation, Project 19-12-00414. The work has been supported by the program 211 of the Russian Federation Government agreements 02.A03.21.0006 and 02.A03.21.0011, by the Russian Government Program of Competitive Growth of Kazan Federal University. We acknowledge MAX IV Laboratory for time on Beamline Bloch under Proposal 20190335. Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research council under contract 2018-07152 the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496. We acknowledge ARPES experiment support from Craig Polley (MAX IV), Maciej Dendzik (KTH) Antonija Grubisic-Cabo (KTH) and Oscar Tjernberg (KTH). H.R., D.P. and G.J.M. acknowledge the Swedish Research Council (2018-06465, 2018-04330) and the Swedish Energy Agency (P43549-1) for financial support

The mineralogy and petrology of I-type cosmic spherules: Implications for their sources, origins and identification in sedimentary rocks

I-type cosmic spherules are micrometeorites that formed by melting during atmospheric entry and consist mainly of iron oxides and FeNi metal. I-types are important because they can readily be recovered from sedimentary rocks allowing study of solar system events over geological time. We report the results of a study of the mineralogy and petrology of 88 I-type cosmic spherules recovered from Antarctica in order to evaluate how they formed and evolved during atmospheric entry, to constrain the nature of their precursors and to establish rigorous criteria by which they may be conclusively identified within sediments and sedimentary rocks. Two textural types of I-type cosmic spherule are recognised: (1) metal bead-bearing (MET) spherules dominated by Ni-poor (100 and suggest that metal from H-group ordinary, CM, CR and iron meteorites may form the majority of particles. Oxidation during entry heating increases in the series MET 80 wt% Ni comprising a particle mass fraction of <0.2. Non-equilibrium effects in the exchange of Ni between wüstite and metal, and magnetite and wüstite are suggested as proxies for the rate of oxidation and cooling rate respectively. Variations in magnetite and wüstite crystal sizes are also suggested to relate to cooling rate allowing relative entry angle of particles to be evaluated. The formation of secondary metal in the form of sub-micron Ni-rich or Pt-group nuggets and as symplectite with magnetite was also identified and suggested to occur largely due to the exsolution of metallic alloys during decomposition of non-stoichiometric wüstite. Weathering is restricted to replacement of metal by iron hydroxides. The following criteria are recommended for the conclusive identification of I-type spherules within sediments and sedimentary rocks: (i) spherical particle morphologies, (ii) dendritic crystal morphologies, (iii) the presence of wüstite and magnetite, (iv) Ni-bearing wüstite and magnetite, and (v) the presence of relict FeNi metal

Monoclinic Pyrrhotite

Recent literature has produced much information on the thermal and compositional stability and on the natural occurrences and mineral associations of monoclinic pyrrhotite. These data together with those obtained from new field and laboratory studies have made it possible to derive the phase relations from 600° to about 200°C in the portion of the Fe-S-O system which involves the minerals hexagonal pyrrhotite, pyrite, monoclinic pyrrhotite and magnetite. Monoclinic pyrrhotite is stable below 310 ± 5°C. Near the upper limit of its thermal stability range it can only be synthesized at rather low, closely controlled oxygen pressures, but over a fairly large variation in Fe/S ratios. Monoclinic pyrrhotite has an Fe/S + O ratio of, or near, 7/8. Monoclinic pyrrhotite in the ternary system is stable with hexagonal pyrrhotite and pyrite below 310 ± 5°C. At about 220°C an invariant reaction involving hexagonal pyrrhotite, pyrite, monoclinic pyrrhotite, magnetite and vapor, takes place. Below this temperature monoclinic pyrrhotite and magnetite form a stable mineral pair. The maximum concentration of oxygen in solid solution in monoclinic pyrrhotite occurs at this invariant temperature. The monoclinic pyrrhotite solid solution composition may reach its closest proximity to the Fe-S boundary at the temperature where smythite becomes stable (about 75°C). Hexagonal pyrrhotite takes a small amount of oxygen in solid solution. This oxygen may be responsible for the formation of hexagonal superstructures and may be the cause of the metastable behavior of supersaturated hexagonal pyrrhotite