Liquid Fe-S Research Articles

Density measurement of liquid FeS at high pressures using synchrotron X-ray absorption

The density of liquid iron sulfide (FeS) was measured up to 3.8 GPa and 1800 K using an X-ray absorption method. The compression curve of liquid FeS was fitted using the Vinet equation of state. The isothermal bulk modulus and its temperature and pressure derivatives were determined using a nonlinear least-squares fit. The parameter sets determined were: K 0T = 2.5 ± 0.3 GPa at T = 1500 K, (dK 0 /dT) P = -0.0036 ± 0.0003 GPa/K, and (dK 0 /dP) T = 24 ± 2. These results suggest that liquid FeS is more compressible than Fe-rich liquid Fe-S.

Deformation of a crystalline aggregate with a small percentage of high-dihedral-angle liquid: Implications for core–mantle differentiation during planetary formation

Core–mantle differentiation of small planetesimals has been suggested to be initiated by deformation-assisted segregation of molten Fe–S through a crystalline peridotite matrix. In this study we used two different experimental approaches to investigate the effect of varying the strain rate and of varying the dihedral angle of the liquid phase on liquid interconnectivity. (i) High pressure experiments were performed on simplified systems consisting of olivine plus a small percentage of liquid FeS (dihedral angle ~ 75°) or gold (dihedral angle ~ 150°) at a temperature above the FeS/Au solidus but below the silicate solidus. Samples were deformed at high pressure in a deformation-DIA multianvil press at strain rates between 10 − 3 and 10 − 6 s − 1 . (ii) Optical in situ analogue experiments on norcamphor plus H 2O (dihedral angle ~ 86°) were performed at room temperature to complement the d-DIA experiments. The crystalline matrix in both the d-DIA and analogue experiments deforms by grain boundary migration assisted dislocation creep under the experimental conditions. Furthermore, the liquid pockets display a similar strain rate dependent behaviour in both sets of experiments: At high strain rates deformation is strongly localised by the liquid phase, and liquid interconnection and segregation occurs. At low strain rates the geometry of liquid pockets is controlled predominantly by surface tension and little elongation and interconnection occurs. We propose a new empirical scaling approach using the d-DIA results, which indicates that some degree of high dihedral angle liquid interconnection and segregation may be possible down to a few percent residual liquid under natural conditions for moderate dihedral angles (60° < Θ < 90°). This, however, would not suffice for efficient core–mantle segregation and an additional mechanism, such as a magma ocean, would be required.

In situ viscometry of high-pressure melts in the Paris–Edinburgh cell: application to liquid FeS

Knowledge of the viscosity of high-pressure melts is important in various domains of research, such as geosciences or materials science. Experimentally, the viscosity of liquids can be determined using the falling sphere technique. This method has been developed at high pressures and high temperatures at Beamline ID27 of the European Synchrotron Radiation Facility (ESRF), combining a Paris–Edinburgh cell with in situ X-ray radiography. The press is interfaced to Soller slits and imaging systems to measure high-quality diffraction patterns and high-resolution X-ray images of the sample. The viscosity of the liquid is derived from the velocity of a dense sphere falling through the pressurized melt and the Stokes law. An optimized two-circle diffractometer allows for moving the press upside down in order to perform a series of measurements on a single sample. The simultaneous collection of X-ray diffraction data on liquids offers the unique opportunity of investigating the relations between viscosity and the structure of melts. The potential of this new equipment is illustrated on the example of FeS melt viscosity and its implications for the Earth's core.

Structural asymmetry in liquid Fe–Si alloys

The thermodynamic properties and microscopic structure of liquid Fe–Si alloys at 1873 K were studied by using the regular associated solution model. The model was utilized to determine the complex concentration in a regular associated solution of Fe, Si and Fe2Si. The complex concentration was then used to calculate the integral excess free energy of mixing, activity, concentration fluctuations in the long-wavelength limit, SCC (0), and the Warren–Cowley short-range parameter α 1. The analysis suggests that heterocoordination leading to the formation of complex Fe2Si is likely to exist in the liquid and is of a strongly interacting nature. The theoretical analysis reveals that the alloy is more ordered towards the Fe-rich region. The observed asymmetry in the properties of mixing of Fe–Si alloys in the molten state is successfully explained on the basis of the regular associated solution model.

Solution Growth of Silicon Carbide Using Fe–Si Solvent

In this paper, we describe the use of liquid Fe–Si alloy as the solvent for the solution growth of silicon carbide (SiC). Iron was chosen owing to the high solubility of carbon in molten iron. The crystal growth of SiC from Fe–40 mol % Si solvent on a seed crystal wafer of 6H- or 4H-SiC was carried out at 1,623–1,723 K using induction heating. The growth rate of SiC was obtained to be 90–260 µm/h. Furthermore, Raman spectroscopy revealed homo epitaxial growth on both 6H- and 4H-SiC under the experimental conditions used. Thus, the Fe–Si solvent was found to be effective for the rapid solution growth of SiC.

Solubility of Au in Cl- and S-bearing hydrous silicate melts

The solubility of Au in Cl- and S-bearing hydrous rhyodacitic and andesitic melts has been experimentally investigated at 1050 °C, 200 MPa and log fO 2 close to the Ni/NiO solid oxygen buffer (NNO). The concentrations of Au in the experimental glasses have been determined using Laser Ablation ICP-MS (LA) with special efforts to avoid incorporation of Au micronuggets in the analysis. It is concluded that metal micronuggets are an experimental artefact and produced by Au partitioning into the fluids during heating with consequent precipitation on fluid dissolution in the melting glass powder. Hence, the micronuggets do not represent quench phases and must be excluded from the analysis. The micro-analytical data obtained by LA show that Au concentrations vary from ∼0.2 to ∼2.5 ppm by weight, generally consistent with the literature data for other melt compositions. The measured Au concentrations increase with increasing amounts of Cl and S dissolved in the silicate melt and show a correlation with the apparent activities of Cl and S in the system. The apparent activities of Cl and S are defined by the simplified linear relationship between volatile concentrations in the melt and activity of volatiles. The maximum activity ( a ∗ = 1) is assumed to be reached at the saturation of the systems in respect of Cl-rich brine or FeS liquid for Cl and S, respectively. The dependence of Au solubility on the concentrations/activities of Cl and S at the fixed redox conditions shows that Au may form not only oxide- but also Cl- and S-bearing complexes in silicate melts. Furthermore, it indicates that exsolution of S and Cl from the melt by degassing/segregation/crystallization processes may lead to mobilization and extraction of Au into the fluid, liquid and/or mineral phase(s).

Highly reducing conditions during core formation on Mercury: Implications for internal structure and the origin of a magnetic field

Abstract The high average density and low surface FeO content of the planet Mercury are shown to be consistent with very low oxygen fugacity during core segregation, in the range 3–6 log units below the iron–wustite buffer. These low oxygen fugacities, and associated high metal content, are characteristic of high-iron enstatite (EH) and Bencubbinite (CB) chondrites, raising the possibility that such materials may have been important building blocks for this planet. With this idea in mind we have explored the internal structure of a Mercury sized planet of EH or CB bulk composition. Phase equilibria in the silicate mantle have been modeled using the thermodynamic calculator p-MELTS, and these simulations suggest that orthopyroxene will be the dominant mantle phase for both EH and CB compositions, with crystalline SiO 2 being an important minor phase at all pressures. Simulations for both compositions predict a plagioclase-bearing “crust” at low pressure, significant clinopyroxene also being calculated for the CB bulk composition. Concerning the core, comparison with recent high pressure and high temperature experiments relevant to the formation of enstatite meteorites, suggest that the core of Mercury may contain several wt.% silicon, in addition to sulfur. In light of the pressure of the core–mantle boundary on Mercury (∼7 GPa) and the pressure at which the immiscibility gap in the system Fe–S–Si closes (∼15 GPa) we suggest that Mercury’s core may have a complex shell structure comprising: (i) an outer layer of Fe–S liquid, poor in Si; (ii) a middle layer of Fe–Si liquid, poor in S; and (iii) an inner core of solid metal. The distribution of heat-producing elements between mantle and core, and within a layered core have been quantified. Available data for Th and K suggest that these elements will not enter the core in significant amounts. On the other hand, for the case of U both recently published metal/silicate partitioning data, as well as observations of U distribution in enstatite chondrites, suggest that this element behaves as a chalcophile element at low oxygen fugacity. Using these new data we predict that U will be concentrated in the outer layer of the mercurian core. Heat from the decay of U could thus act to maintain this part of Mercury’s core molten, potentially contributing to the origin of Mercury’s magnetic field. This result contrasts with the Earth where the radioactive decay of U represents a negligible contribution to core heating.

Specific heat, enthalpy, and density of undercooled liquid Fe–Si–Sn alloy

The specific heat of liquid Fe90Si6.95Sn3.05 alloy has been investigated by electromagnetic levitation drop calorimetery in the temperature range of 1390–2140 K. The enthalpy of this liquid alloy increases linearly with the rise of temperature. Furthermore, the enthalpy obtained from molecular dynamics calculation shows a similar trend to the experimental results in a broader temperature range of 1000–2200 K. The calculated specific heat is 39.7 J mol−1 K−1, which agrees well with the experimental result of 39.9 J mol−1 K−1. The density of this liquid alloy decreases as a quadratic function of temperature.

The wetting ability of Si-bearing liquid Fe-alloys in a solid silicate matrix—percolation during core formation under reducing conditions?

The wetting characteristics of liquid Fe–Si alloys in a matrix of the respective predominating stable silicate mantle mineral (forsterite or silicate perovskite) at pressures of 2–5 and 25 GPa and temperatures of 1600–2000 °C were studied by determining the liquid metal–solid silicate contact angles. The median angle values from texturally equilibrated samples were found to be independent of pressure, temperature, silicate mineralogy and the Si content in the metal fraction and range between 130° and 140° which is far above the critical wetting boundary of 60°. This shows that within the studied range of conditions dissolved Si does not lower the surface energies between Fe-rich liquids and silicate mantle grains. As a consequence, under reducing conditions the presence of Si in the metal phase of planetary bodies would not have enhanced percolative flow as an effective metal–silicate separation process.

Application of the embedded-atom method to liquid Fe-S solutions

A procedure (proposed earlier to describe liquid Fe) for calculating the embedded-atom method (EAM) potential with the use of structural and thermodynamic data for the metal near its melting point has been applied to liquid Fe-S solutions at temperatures of up to 5000 K and pressures of up to 360 GPa. Molecular dynamics simulations have been carried out for solutions with 0–18 at % S with the EAM potential at temperatures from 1820 to 5000 K and densities from 10.69 to 12.28 g/cm3. We have calculated the thermodynamic, structural, and diffusion properties and estimated the viscosity of noncrystalline phases. At a pressure near 360 GPa and a temperature of 5000 K, pure iron crystallizes spontaneously, while the solutions of sulfur in iron behave as high-viscosity liquids or amorphous phases with a self-diffusion coefficient on the order of 10−7 cm2/s and a viscosity of up to tens of Pa s. As the sulfur content increases from 0 to 6.2 at %, the viscosity first decreases and then rises. During long-term relaxation, the model of an Fe-S solution may turn into a crystalline-like state. The origin of marked discrepancies between ab initio and EAM simulation results is discussed.

Critical thermodynamic assessment and modeling of the Fe-Ni-S system

A thorough review and critical evaluation of phase equilibria and thermodynamic data for phases in the iron-nickel-sulfur ternary system at 1 bar pressure has been made over the entire composition range for temperatures from 25°C to above the liquidus. The Gibbs energies of system phases have been modeled, and optimized model parameters have been obtained which reproduce all data simultaneously within experimental error limits. The modeling is based on the recent evaluations by the authors of the Fe-S and Ni-S binary subsystems. For the liquid phase, the recently extended modified quasichemical model for short-range ordering is applied for the first time to a liquid metal-sulfur phase. Although the binary Fe-S and Ni-S liquid solutions were modeled with the extended modified quasichemical model, while the binary Fe-Ni liquid solution was modeled with a simple random-mixing model, it is shown, for the first time, that these can be combined into a consistent model of the ternary liquid phase. Two-sublattice models, within the framework of the compound-energy formalism, are used for the solid solutions: pyrrhotite, pentlandite, pyrite, vaesite, and high-temperature heazlewoodite. The ternary-solution model for iron-nickel pyrrhotite permits very good prediction of thermodynamic properties solely on the basis of binary-model parameters.

THERMODYNAMICS OF LIQUID Fe-Si AND Fe-Ge ALLOYS

The enthalpies of mixing of liquid binary Fe-Ge (1765±5 K) and Fe-Si (1750±5 K) alloys were determined using a high-temperature isoperibolic calorimeter. The thermodynamic properties of Fe-Ge melts were also studied by electromotive force method in the temperature range of 1250-1580 K. The comparison of our measurement results with literature data has been performed. The extreme negative values of integral enthalpy of mixing and alternating-sign deviations from Raoult's low for germanium can be explained by the influence of binary clusters formation.

Potassium in the Earth’s core?

The partitioning of K and Na between liquid Fe–S–O alloys and silicate melt has been determined over the pressure and temperature range 2.5–24 GPa and 1500–1900°C. In experiments with S-free Fe alloys, the alkali elements show completely lithophile behaviour. When S is added, however, K and Na begin to enter the Fe-liquid phase and their distribution coefficients D I (=[I] metal/[I] silicate) correlate strongly with O content (and FeO activity) of the Fe–S–O liquid and with the composition of the silicate melt. For potassium, D K is ∼1.0 for Fe–sulphide liquid containing 30% S and 8% O. Increasing temperature leads to increasing O solubility in the Fe–sulphide liquid and correspondingly higher values of D K. Increasing pressure on the other hand slightly reduces D K values. Given a planetary core containing 10 wt% S and 4–8 wt% O, then several hundred ppm K would be present in the Fe–sulphide liquid if it segregated at low pressures, e.g. in a small planetary body such as Mars. If, as has recently been suggested, the Earth’s core separated at the base of a deep magma ocean, then its highest possible K content is about 250 ppm. The latter would generate approximately 20% of the total heat production of the core. K can only be present in the core, however, if, at some time during its formation, a discrete O-rich FeS liquid separated from the silicate mantle. Finally, the sulphide compositions produced in our experiments imply that a combination of S and O could contribute significantly to the light element content of the Earth’s core.

Structural properties of liquid Fe–Si alloys

The atomic structure of liquid Fe–Si alloys within the whole concentration range (including pure components) was investigated at the temperature of 1550 °C by means of X-ray diffraction. Analysis of the reported data on physical properties of Fe–Si melts has been carried out. Interrelation between liquid-state structure and solid-state properties was considered. Fe–Si alloys in the liquid state are shown to be micro-inhomogeneous and they contain atomic micro-formations (clusters) that differ by atomic composition and packing. There are five types of clusters: two of them consist of atoms of one kind (Fe or Al); the composition of the other clusters depends on the stoichiometry of solid Fe 3Si, FeSi and Fe 2Si 5 phases. The entire concentration range of the Fe–Si system consists of four concentration intervals. Within each interval melts are constituted of the clusters of two kinds. The variation of the component concentrations in Fe–Si alloys results in changing of volume fraction of each type clusters, whereas the atomic composition and arrangement inside the clusters remains constant.

Smyer H-chondrite impact-melt breccia and evidence for sulfur vaporization

Smyer is an H-chondrite impact-melt breccia containing ∼20 vol% 0.5- to 13-mm-thick silicate-rich melt veins surrounding unmelted subrounded chondritic clasts up to 7 cm in maximum dimension. At the interface between some of the melt veins and chondritic clasts, there are troilite-rich regions consisting of unmelted, crushed 0.2- to 140-μm-size angular silicate grains and chondrule fragments surrounded by troilite and transected by thin troilite veins. Troilite fills every available fracture in the silicates, including some as thin as 0.1 μm. Little metallic Fe-Ni is present in these regions: the FeS/Fe modal ratio ranges from ∼25:1 to ∼500:1, far higher than the eutectic weight ratio of 7.5:1. The texture of these regions indicates that the sulfide formed from a fluid of very low viscosity. The moderately high viscosity (0.2 poise) and large surface tension of liquid FeS, its inability to wet silicate grain surfaces at low oxygen fugacities, and the supereutectic FeS/Fe ratios in the troilite-rich regions indicate that the fluid was a vapor. It seems likely that during the shock event that melted Smyer, many silicates adjacent to the melt veins were crushed. Upon release of shock pressure, some of the troilite evaporated and dissociated. Molecules of S2 were transported and condensed into fractures and around tiny silicate grains; there, they combined with Fe from small adjacent metallic Fe-Ni grains to form troilite. The Ni content at the edges of some of these metal grains increased significantly; Co from these Ni-rich grains diffused into nearby kamacite. Impact-induced S volatilization may have played a major role in depleting the surface of 433 Eros (and other chondritic asteroids) in S.

Thermodynamic Study of Liquid Fe–Si–B Alloys: The Effect of Ternary Associated Groups on Transformation of the Alloy into the Amorphous State

Thermodynamic Study of Liquid Fe–Si–B Alloys: The Effect of Ternary Associated Groups on Transformation of the Alloy into the Amorphous State

Evaluation of Surface Tension and Adsorption for Liquid Fe-S Alloys.

Surface tension of liquid iron is strongly influenced by the adsorption of sulfur. In this study, surface tensions of liquid Fe-S alloys at 1823 K were measured by the sessile drop technique in a purified argon atmosphere. Experimental results were compared with the model based on Butler's equations considering the effect of size and interactions of the adsorbed elements assuming that the system was composed of binary. The model could evaluate the surface tension and the adsorption of sulfur more reasonably rather than a simple ideal adsorption model. In the calculations, excess free energy expanded by Margules' series over the entire composition range of Fe-FeS system was used.

Experimental verification of the Stokes-Einstein relation in liquid Fe—FeS at 5 GPa

Experiments have been performed at 5 GPa on liquid Fe-FeS in order to determine Fe and S self-diffusivity as a function of temperature. The viscosity of the sample was then obtained using the Stokes-Einstein relation. The results are in excellent agreement with previous experiments where the viscosity of a material of the same composition under similar conditions was measured directly. These results support high, near-metallic, values of diffusivity and low viscosity in liquid Fe-S up to a few hundred K above the eutectic temperature, in contrast with some previous studies. Moreover, these results fully confirm the validity of the Stokes-Einstein relation between viscosity and diffusion coefficients for Fe0.61S0.39.

Spontaneous Infiltration of Iron Silicides into Silicon Carbide Powder Preforms

The infiltration of liquid Fe3Si (mp of ∼1300°C), Fe5Si3 (mp of ∼1210°C), and FeSi (mp of ∼1410°C) into SiC powder preforms was performed at various infiltration temperatures for 60 min under either argon flow or dynamic vacuum. The amount of infiltration under various infiltration conditions was studied as a function of infiltration temperature. For the preforms as‐pressed from raw SiC powder, the amount of infiltration of the three silicides under argon flow was independent of their melting points, but suddenly increased within a common temperature range from 1450° to 1550°C. Thermodynamic analyses indicated that the common temperature range corresponded to the temperature at which the SiO2 on the surface of the SiC particles was decreased under argon flow. Infrared spectroscopy showed SiO2 on the surfaces of as‐received SiC powder particles, but not on the surfaces of the SiC powder particles fired under argon at 1600°C. The amount of infiltration of the as‐pressed SiC under vacuum and of fired SiC under argon and vacuum exhibited an obvious dependence on the silicide melting points. This was attributed to the SiO2 reduction taking place at temperatures lower than the melting points of the silicides. The amount of infiltration was then controlled by the melt viscosity.

In situ measurement of viscosity of liquids in the Fe-FeS system at high pressures and temperatures

The viscosity of liquid FeS and Fe-FeS eutectic was measured at pressures between 0.5 and 5.0 GPa using a synchrotron-based falling sphere technique. We obtain viscosities of 2 x 10(-2) to 4 x 10(-3) Pa-s in FeS at 1450 to 1700 degreesC and 2 x 10(-2) to 8 x 10(-3) Pa-s in Fe-S-eut at 1150 to 1380 degreesC. These results are consistent with recent viscosity measurements in Fe-S-eut at 5 to 7 GPa (Urakawa, in preparation), measured diffusivities (Dobson 2000) and ab initio simulated viscosity (Vocadlo et al. 2000). The results are also similar to the values for pure iron at low pressure (Shimoji and Itami 1986). A systematic increase in viscosity and activation energy is seen with increasing sulfur content. Interpolation between the data presented yields a viscosity of 1.4 x 10(-2) Pa-s for an outer core composition with similar to 10 wt% S at the melting temperature. There is good evidence of homologous behavior for Fe-S liquids which implies that the liquid alloy at the inner core boundary may have a similar viscosity.