Isotopic Exchange between Aqueous Fe(II) and Solid Fe(III) in Lake Sediment─A Kinetic Assemblage Approach.

The catalytic effect of aqueous Fe(II) (Fe) on the transformation of Fe(oxyhydr)oxides has been extensively studied in the laboratory. It involves the transfer of electrons between Fe and Fe-(oxyhydr)oxides, rapid atomic exchange of Fe between the two states, and recrystallization of the Fe-oxides into more stable Fe-(oxyhydr)oxides. The potential occurrence of these reactions in natural soils and sediments can have an important impact on biogeochemical cycling of iron, carbon, and phosphorus.

Crystal chemistry and compressibility of FeMgAlSiO and FeMgSiO silicate perovskites at pressures up to 95 GPa.

Silicate perovskite, with the mineral name bridgmanite, is the most abundant mineral in the Earth's lower mantle. We investigated crystal structures and equations of state of two perovskite-type Fe-rich phases, FeMgSiO and FeMgAlSiO, at high pressures, employing single-crystal X-ray diffraction and synchrotron Mössbauer spectroscopy. We solved their crystal structures at high pressures and found that the FeMgSiO phase adopts a novel monoclinic double-perovskite structure with the space group of at pressures above 12 GPa, whereas the FeMgAlSiO phase adopts an orthorhombic perovskite structure with the space group of at pressures above 8 GPa.

Cubic Fe-bearing majorite synthesized at 18-25 GPa and 1000 °C: implications for element transport, subducted slab rheology and diamond formation.

The chemistry and mineralogy of slabs subducted into lower mantle control slab rheology and impact the deep volatile cycle. It is known that the metamorphism of little-altered oceanic crust results in eclogite rocks with subequal proportions of garnet and clinopyroxene. With increasing pressure, these minerals react to stabilize pyrope-rich tetragonal majoritic garnet.

Extreme redox variations in a superdeep diamond from a subducted slab.

The introduction of volatile-rich subducting slabs to the mantle may locally generate large redox gradients, affecting phase stability, element partitioning and volatile speciation. Here we investigate the redox conditions of the deep mantle recorded in inclusions in a diamond from Kankan, Guinea. Enstatite (former bridgmanite), ferropericlase and a uniquely Mg-rich olivine (Mg# 99.

Subduction-related oxidation of the sublithospheric mantle evidenced by ferropericlase and magnesiowüstite diamond inclusions.

Ferropericlase (Mg,Fe)O is the second most abundant mineral in Earth's lower mantle and a common inclusion found in subcratonic diamonds. Pyrolitic mantle has Mg# (100 × Mg/(Mg+Fe)) ~89. However, ferropericlase inclusions in diamonds show a broad range of Mg# between 12 and 93.

The crystal structures of Fe-bearing MgCO - and -carbonates at 98 GPa from single-crystal X-ray diffraction using synchrotron radiation.

The crystal structure of MgCO-II has long been discussed in the literature where DFT-based model calculations predict a pressure-induced transition of the carbon atom from the to the type of bonding. We have now determined the crystal structure of iron-bearing MgCO-II based on single-crystal X-ray diffraction measurements using synchrotron radiation. We laser-heated a synthetic (MgFe)CO single crystal at 2500 K and 98 GPa and observed the formation of a monoclinic phase with composition (MgFe)CO in the space group 2/ that contains tetra-hedrally coordinated carbon, where CO tetra-hedra are linked by corner-sharing oxygen atoms to form three-membered CO ring anions.

Author Correction: Discovery of FeO: a new iron oxide with a complex monoclinic structure.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Saline aqueous fluid circulation in mantle wedge inferred from olivine wetting properties.

Recently, high electrical conductors have been detected beneath some fore-arcs and are believed to store voluminous slab-derived fluids. This implies that the for-arc mantle wedge is permeable for aqueous fluids. Here, we precisely determine the dihedral (wetting) angle in an olivine-NaCl-HO system at fore-arc mantle conditions to assess the effect of salinity of subduction-zone fluids on the fluid connectivity.

Deep magma ocean formation set the oxidation state of Earth's mantle.

The composition of Earth's atmosphere depends on the redox state of the mantle, which became more oxidizing at some stage after Earth's core started to form. Through high-pressure experiments, we found that Fe in a deep magma ocean would disproportionate to Fe plus metallic iron at high pressures. The separation of this metallic iron to the core raised the oxidation state of the upper mantle, changing the chemistry of degassing volatiles that formed the atmosphere to more oxidized species.

Pressure tuning of charge ordering in iron oxide.

A Verwey-type charge-ordering transition in magnetite at 120 K leads to the formation of linear units of three iron ions with one shared electron, called trimerons. The recently-discovered iron pentoxide (FeO) comprising mixed-valent iron cations at octahedral chains, demonstrates another unusual charge-ordering transition at 150 K involving competing formation of iron trimerons and dimerons. Here, we experimentally show that applied pressure can tune the charge-ordering pattern in FeO and strongly affect the ordering temperature.

Stability of iron-bearing carbonates in the deep Earth's interior.

The presence of carbonates in inclusions in diamonds coming from depths exceeding 670 km are obvious evidence that carbonates exist in the Earth's lower mantle. However, their range of stability, crystal structures and the thermodynamic conditions of the decarbonation processes remain poorly constrained. Here we investigate the behaviour of pure iron carbonate at pressures over 100 GPa and temperatures over 2,500 K using single-crystal X-ray diffraction and Mössbauer spectroscopy in laser-heated diamond anvil cells.

High-Pressure NiAs-Type Modification of FeN.

The combination of laser-heated diamond anvil cells and synchrotron Mössbauer source spectroscopy were used to investigate high-temperature high-pressure chemical reactions of iron and iron nitride Fe N with nitrogen. At pressures between 10 and 45 GPa, significant magnetic hyperfine splitting indicated compound formation after annealing at 1300 K. Subsequent in situ X-ray diffraction reveals a new modification of FeN with NiAs-type crystal structure, as also rationalized by first-principles total-energy and chemical-bonding studies.

Discovery of Fe7O9: a new iron oxide with a complex monoclinic structure.

Iron oxides are fundamentally important compounds for basic and applied sciences as well as in numerous industrial applications. In this work we report the synthesis and investigation of a new binary iron oxide with the hitherto unknown stoichiometry of Fe7O9. This new oxide was synthesized at high-pressure high-temperature (HP-HT) conditions, and its black single crystals were successfully recovered at ambient conditions.

Stability of Fe,Al-bearing bridgmanite in the lower mantle and synthesis of pure Fe-bridgmanite.

The physical and chemical properties of Earth's mantle, as well as its dynamics and evolution, heavily depend on the phase composition of the region. On the basis of experiments in laser-heated diamond anvil cells, we demonstrate that Fe,Al-bearing bridgmanite (magnesium silicate perovskite) is stable to pressures over 120 GPa and temperatures above 3000 K. Ferric iron stabilizes Fe-rich bridgmanite such that we were able to synthesize pure iron bridgmanite at pressures between ~45 and 110 GPa.

Charge-ordering transition in iron oxide Fe4O5 involving competing dimer and trimer formation.

Phase transitions that occur in materials, driven, for instance, by changes in temperature or pressure, can dramatically change the materials' properties. Discovering new types of transitions and understanding their mechanisms is important not only from a fundamental perspective, but also for practical applications. Here we investigate a recently discovered Fe4O5 that adopts an orthorhombic CaFe3O5-type crystal structure that features linear chains of Fe ions.

Effect of lone-electron-pair cations on the orientation of crystallographic shear planes in anion-deficient perovskites.

Factors affecting the structure and orientation of the crystallographic shear (CS) planes in anion-deficient perovskites were investigated using the (Pb(1-z)Sr(z))(1-x)Fe(1+x)O(3-y) perovskites as a model system. The isovalent substitution of Sr(2+) for Pb(2+) highlights the influence of the A cation electronic structure because these cations exhibit very close ionic radii. Two compositional ranges have been identified in the system: 0.

The oxidation state of the mantle and the extraction of carbon from Earth's interior.

Determining the oxygen fugacity of Earth's silicate mantle is of prime importance because it affects the speciation and mobility of volatile elements in the interior and has controlled the character of degassing species from the Earth since the planet's formation. Oxygen fugacities recorded by garnet-bearing peridotite xenoliths from Archaean lithosphere are of particular interest, because they provide constraints on the nature of volatile-bearing metasomatic fluids and melts active in the oldest mantle samples, including those in which diamonds are found. Here we report the results of experiments to test garnet oxythermobarometry equilibria under high-pressure conditions relevant to the deepest mantle xenoliths.

The 57Fe Synchrotron Mössbauer Source at the ESRF.

The design of a (57)Fe Synchrotron Mössbauer Source (SMS) for energy-domain Mössbauer spectroscopy using synchrotron radiation at the Nuclear Resonance beamline (ID18) at the European Synchrotron Radiation Facility is described. The SMS is based on a nuclear resonant monochromator employing pure nuclear reflections of an iron borate ((57)FeBO(3)) crystal. The source provides (57)Fe resonant radiation at 14.

Lattice thermal conductivity of lower mantle minerals and heat flux from Earth's core.

The amount of heat flowing from Earth's core critically determines the thermo-chemical evolution of both the core and the lower mantle. Consisting primarily of a polycrystalline aggregate of silicate perovskite and ferropericlase, the thermal boundary layer at the very base of Earth's lower mantle regulates the heat flow from the core, so that the thermal conductivity (k) of these mineral phases controls the amount of heat entering the lowermost mantle. Here we report measurements of the lattice thermal conductivity of pure, Al-, and Fe-bearing MgSiO(3) perovskite at 26 GPa up to 1,073 K, and of ferropericlase containing 0, 5, and 20% Fe, at 8 and 14 GPa up to 1,273 K.

Magnetic exchange bias of more than 1 Tesla in a natural mineral intergrowth.

Magnetic exchange bias is a phenomenon whereby the hysteresis loop of a 'soft' magnetic phase is shifted by an amount H(E) along the applied field axis owing to its interaction with a 'hard' magnetic phase. Since the discovery of exchange bias fifty years ago, the development of a general theory has been hampered by the uncertain nature of the interfaces between the hard and soft phases, commonly between an antiferromagnetic phase and a ferro- or ferrimagnetic phase. Exchange bias continues to be the subject of investigation because of its technological applications and because it is now possible to manipulate magnetic materials at the nanoscale.

Local structural properties of (Mn,Fe)Nb2O6 from Mössbauer and X-ray absorption spectroscopy.

The MnNb(2)O(6)-FeNb(2)O(6) solid solution has been investigated by Fe-K- and Mn-K-edge X-ray absorption (XANES and EXAFS), and Mossbauer spectroscopy. The first-shell M-O bond lengths deduced from EXAFS show a fairly small compositional dependence. A degree of static disorder, which increases with increasing manganese content, is clearly seen by the loss of correlation for the next-neighbour (NN) interaction.

Pressure-induced magnetization in FeO: evidence from elasticity and Mössbauer spectroscopy.

The complete elastic tensor of Fe0.94O (wüstite) has been determined to 10 GPa using acoustic interferometry at GHz frequencies inside a diamond-anvil cell. The soft mode (C44) elastic constant of FeO is reduced by 20% over the experimental pressure range.

Iron isotope fractionation and the oxygen fugacity of the mantle.

The oxygen fugacity of the mantle exerts a fundamental influence on mantle melting, volatile speciation, and the development of the atmosphere. However, its evolution through time is poorly understood. Changes in mantle oxidation state should be reflected in the Fe3+/Fe2+ of mantle minerals, and hence in stable iron isotope fractionation.