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A novel experimental set-up to investigate the sequestration of non-pure carbon dioxide (CO2) streams in geological formations through mineral trapping is discussed. The system includes a gas manifold system, a gas booster, a fluid reservoir, a high pressure-high temperature reactor, and auxiliary elements to ensure safe and reliable operation under the high temperatures and pressures required for the experiments. The apparatus described here is inspired by previously reported batch equipment, but has the versatility of co-injecting gas mixtures. The capacity of the vessel is 600 mL and the system is capable of applying pressures from ambient to 5000 psi (345 bar), and temperatures from ambient to 350 °C under batch continuous stirring conditions. These ranges of pressure and temperature can cover and even exceed the conditions relevant for the majority of subsurface sequestration studies. The interior wetted parts of the vessel are all in Hastelloy C276 (a Ni-Cr-Mo alloy) to be resistant to the corrosiveness of gases such as SO2. The system was tested by studying the sequestration of CO2-SO2 mixtures in ferric (FeIII) iron-containing sediments, as SO2 has been shown to reduce FeIII to FeII and permanently trap CO2 as the mineral siderite (FeCO3). A series of batch trials were conducted to demonstrate the experimental apparatus design efficacy along with the system´s behavior. The apparatus functioned well during the course of the experiments proving that this set-up is a useful research tool to investigate the effect of impurities present in the flue gas from fossil fuel combustion in the geological storage of CO2. [ABSTRACT FROM AUTHOR]

Highlights: [•] An EC/Fe3+/PMS process was employed to degrade clofibric acid in water. [•] The effect of operating parameters on the degradation was investigated. [•] The main intermediate products were determined by GC–MS technology. [•] A plausible degradation pathway of clofibric acid was proposed. [ABSTRACT FROM AUTHOR]

We have investigated the effects of different Fe2O3 bulk contents on the calculated phase equilibria of low- T/intermediate- P metasedimentary rocks. Thermodynamic modelling within the MnO-Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O (Mn NKFMASHTO) chemical system of chloritoid-bearing hematite-rich metasedimentary rocks from the Variscan basement of the Pisani Mountains (Northern Apennines, Italy) fails to reproduce the observed mineral compositions when the bulk Fe2O3 is determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P-XFe2O3 diagrams, obtaining equilibration conditions of 475 °C and 9-10 kbar related to a post-compressional phase of the Alpine collision. The introduction of ferric iron affects the stability of the main rock-forming silicates that often yield important thermobaric information. In Fe2O3-rich compositions, garnet- and carpholite-in curves shift towards higher temperatures with respect to the Fe2O3-free systems. The presence of a ferric-iron oxide (hematite) prevents the formation of biotite in the mineral assemblage even at temperatures approaching 550 °C. The use of P-T-XFe2O3 phase diagrams may also provide P-T information in common greenschist facies metasedimentary rocks. [ABSTRACT FROM AUTHOR]

Crystals of ferrous and ferric iron-bearing hydrous wadsleyite have been synthesized at 1400 °C and 12-13.5 GPa in a multi-anvil press. Crystal structures (atom positions, occupancies, and cell parameters) have been refined by single-crystal X-ray diffraction at ambient conditions. Assuming cation vacancies to be in the M3 site only, their concentration has been estimated from the unit-cell parameter b/a ratio. Total refined site Fe contents are consistent with microprobe chemical analysis. There appears to be up to 11% iron (presumably ferric) in the tetrahedral site, consistent with reduced silica content (<1 Si per 4 O atoms) in the chemical analysis. Also the volume of the tetrahedron increases with increasing ferric iron content. Strong ordering of Fe in the octahedral sites is apparent in the order FeM3 ≥ FeM1 >> FeM2. The presence of ferric iron in the mantle transition zone is expected to partition preferentially into wadsleyite and may expand the stability region of wadsleyite relative to olivine and ringwoodite. Also, the observation of tetrahedral ferric iron in these samples increases the likelihood that there is compositional continuity between wadsleyite and the spinelloid III phase field observed in the Mg-free system fayalite-magnetite. [ABSTRACT FROM AUTHOR]

Applying Fe-Mg exchange geothermometers to natural samples may lead to incorrect temperature estimates if significant Fe is present. In order to quantify this effect, high-pressure experiments were carried out in a belt apparatus in a natural system close to CFMAS at 5 GPa and 1,100-1,400 °C. The oxygen fugacity in the experiments was at or below the Re-ReO buffer. This is at significantly more oxidized conditions than in previous experiments, and, as consequence, higher Fe/Fe ratios were generated. The Fe content of garnet in the experiments was quantified by electron microprobe using the flank method. Making the usual assumption that Fe = Fe, the two-pyroxene thermometer of Brey and Köhler (J Pet 31:1353-1378, ) reproduced the experimental temperature to ±35 °C and the garnet-clinopyroxene Fe-Mg exchange thermometer of Krogh (Contrib Miner Pet 99:44-48, ) overestimated the temperatures on average by only 25 °C. On the other hand, application of the garnet-olivine (O'Neill and Wood in Contrib Miner Pet 70:59-70, ) and garnet-orthopyroxene (Harley in Contrib Miner Pet 86:359-373, ) exchange geothermometers yielded an underestimation in calculated temperatures of >200 °C. However, making explicit accounting for Fe in garnet (i.e. using only measured Fe) leads to a vast improvement in the agreement between calculated and experimental temperatures, generally to within ±70 °C for the garnet-orthopyroxene geothermometer as well as noticeable improvement of calculated temperatures for the garnet-olivine geothermometer. Our results demonstrate that the two-pyroxene and garnet-clinopyroxene thermometers are rather insensitive to the presence of Fe whilst direct accounting of Fe in garnet is essential when applying the garnet-olivine and garnet-orthopyroxene thermometers. [ABSTRACT FROM AUTHOR]

[Display omitted] • This is the first time to apply red P and Fe3+ to establish an efficient Fenton system. • The decomposition rate of H 2 O 2 reaches a high-level value of 92.4% within 15 min. • Pollutants could be degraded efficiently with a wide initial pH range of 2.0–8.0. • Solid waste irons can directly activate H 2 O 2 and degrade pollutants with the assist of red P. Traditional Fenton reaction (Fe2+/H 2 O 2) is limited by the instability and the sluggish regeneration of ferrous iron (Fe2+). Compared with Fe2+, ferric iron (Fe3+) is cheaper, easier to obtain and more stable. However, Fe3+-induced Fenton reaction activity is low. Here, a high-efficiency Fe3+-induced Fenton system is achieved by introduction of red phosphorus (P). The red P can donate electrons to Fe3+ for Fe2+ regeneration. As a result, the red P/H 2 O 2 /Fe3+ system exhibits three obvious advantages: i) The degradation rate of pollutant is 25.6 times and 18.3 times that of the Fe3+/H 2 O 2 and red P/H 2 O 2 systems. ii) The decomposition rate of H 2 O 2 reaches an amazing value of 92.4%, which is much higher than that of the traditional Fenton system (51.6%). iii) Pollutants could be degraded efficiently with a wide initial pH range of 2.0–8.0, indicating that the red P/H 2 O 2 /Fe3+ system gets rid of pH constraints. Importantly, this study realizes the transformation of waste Fe3+ into treasure and resource reuse and further reduces the cost of Fenton reaction. Therefore, this study opens up a new strategy for utilization of widely-spread Fe3+ for environmental remediation and also provides a new application of red P. [ABSTRACT FROM AUTHOR]

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Highlights: [•] A novel process named “the electro/Fe3+/peroxydisulfate” process is proposed. [•] The effect of operating parameters on the degradation was investigated. [•] The changes of TOC during the treatment of bisphenol A were monitored. [ABSTRACT FROM AUTHOR]

Aims: Inflammatory diseases associated with iron overload are characterized by a changed coagulation profile, where there is a persistent presence of fibrin-like material of dense-matted deposits (DMDs). It is believed that one source of such material is a result of the activation of blood coagulation without the generation of thrombin, causing clots to become resistant to fibrinolytic dissolution. The aim of the current manuscript therefore is to apply a novel scanning electron microscopy method for assessing the role of functional chelation in the prevention or reversal of iron-induced fibrin formation. Methods and results: Purified fibrinogen and platelet-rich plasma were exposed to chelating agents followed by iron, to determine the chelating effects. We show that there is another, pathological pathway of fibrin formation initiated by free iron (initially as Fe (III)), leading to the formation of highly reactive oxygen species such as the hydroxyl radical that can oxidize and insolubilize proteins, a process that might be inhibited by iron-chelating compounds. The final product of such a pathway is a fibrin-like material, termed DMDs that are remarkably resistant to proteolytic degradation. Conclusions: Scanning electron microscopy shows that iron-chelating agents are effective inhibitors of DMD formation. The most active inhibitors of DMD formation proved to be Desferal, Clioquinol and Curcumin, whereas Epigallocatechin gallate and Deferiprone were less effective. The functional model we describe may point the clinical utility of various substances in iron-mediated degenerative diseases. [ABSTRACT FROM AUTHOR]

Abstract: Fe(III) supplemented into microbial fuel cells (MFC) at initial start-up increased electricity generation by Shewanella oneidensis MR-1. The maximum power density reached 158.1mW/m2 with 6mM Fe(III), compared with 73.9mW/m2 without Fe(III). A significant increase in power output was observed when MFC with established biofilm in the absence of Fe(III) was supplemented with Fe(III) afterward. The cell biomass of the anode biofilm of MFCs operated in the presence of Fe(III) was higher than that obtained without Fe(III). An optimum concentration of Fe(III) obviously improved electrochemical activity and the flavins secretion of strain MR-1. The results demonstrated that Fe(III) addition can enhance power generation by strain MR-1 due to the increase of cell biomass and flavins secretion. [Copyright &y& Elsevier]

Abstract: This study explored a new approach to the pretreatment of lignocellulosic biomass using FeCl3 combined with a fuel cell system to generate electricity. After pretreatment, ferric iron (Fe3+), a strong catalyst in the hydrolysis of carbohydrate, was found to be reduced to ferrous iron (Fe2+) by means of the oxidation of xylose and lignin. Ferrous iron, as a fuel, was employed to the anode part of a fuel cell, generating power of 1110mW/m2. During the fuel cell operation, ferrous iron was completely removed through oxidation to ferric iron and precipitated out. The optimal conditions for the operation of the fuel cell were found to be a pH of 7.0 and ferrous iron concentration of above 0.008M. These results clearly show that a fuel cell system could be used not only to remove ferrous iron from liquid hydrolysate, but also to produce electricity. [Copyright &y& Elsevier]

Samples with eclogitic composition in the system CaO-FeO-FeO-MgO-AlO-SiO were produced from various kinds of starting materials held in graphite-lined Pt capsules at a pressure of 2.5-3.0 GPa and temperatures of 800-1,300 °C using a piston-cylinder or Belt apparatus. Garnets and clinopyroxenes were characterized by analytical transmission electron microscopy and electron probe micro-analysis (EPMA). Fe/ΣFe ratios determined by electron energy-loss spectroscopy (EELS) decrease in clinopyroxene from 22.2 ± 3.4 % at 800 °C to 13.3 ± 5.4 % at 1,300 °C, while in garnet, they vary between 10.8 ± 1.5 and 15.4 ± 4.7 %, respectively. Temperature estimates according to Krogh (Contrib Mineral Petrol 99:44-48, ) reproduce the experimental temperature to ±60 °C without systematic deviations if total iron is used in the calculation. If only the Fe content is used, which was obtained by combining EPMA and EELS results, the experimental temperature is underestimated by 33 °C on average at 800-1,200 °C and overestimated by 77 °C on average at 1,300 °C. These systematic deviations can be explained by the temperature-dependent ratio of Fe/ΣFe in garnet divided by that in clinopyroxene. Since the difference between the calculated and experimental temperature is relatively small, a Fe-based recalibration of the thermometer appears not to be necessary for the investigated system in the range of pressure, temperature and composition covered by the experiments of this study. [ABSTRACT FROM AUTHOR]

Abstract: In order to evaluate the leaching of cinnabar, a chemobiological reactor system with Acidithiobacillus ferrooxidans for cinnabar dissolution was investigated. The results demonstrated cinnabar dissolution had relation to bioprocess of A. ferrooxidans and iron concentration tightly. The optimal dilution rate and iron concentration were 0.4/h and 2–3g/L in chemobiological reactor. The process may be contributed to the indirect catalyzing of ferric iron generated with A. ferrooxidans and direct adherence oxidation function. This research shows the new microbiological technique may be a feasible and economical method in application. [Copyright &y& Elsevier]

Abstract: 4n-Nonylphenol (4n-NP) and triclosan (TCS) are endocrine disrupters that are used widely in personal care products. They are mainly transported in domestic sewage, which can eventually reach natural waters (rivers, lakes or reservoirs). In this work, the photodegradation of 4n-NP and TCS using a solar simulator (300–800nm) at two different pH levels, two different concentrations of ferric ion, Fe(III), and in the absence/presence of humic acids (HAs) were investigated. A Xe lamp (250Wm−2) was employed as the light source. The conditions were chosen to simulate a natural aquatic environment. The experiments were performed using a factorial design for three factors with eight combinations of the factors studied. Experiments performed in ultrapure water showed that the degradation of 4n-NP was faster than that of TCS. The data showed that first-order reaction kinetics describe the degradation of both compounds well; the half-lives of TCS and 4n-NP were 5.4h and 2.3h, respectively. For TCS, the results indicated that the oxidation rate increases in the presence of Fe(III) and at basic pH levels; however, the rate it decreased in the presence HAs. In the case of 4n-NP, synergistic effect was observed for presence of Fe(III) and HAs. The best conditions were a basic medium (pH 9) and an Fe(III) concentration of 2mgL−1 without HAs for TCS and a pH of 9, an Fe(III) concentration of 2mgL−1 and a HAs concentration of 1mgL−1 for 4n-NP. The results of photodegradation of both compounds in water from two reservoirs showed that the photodegradation was highly dependent on the conditions of the aquatic environment. [Copyright &y& Elsevier]

Abstract: The reaction of FeS with Fe(III)-bearing acidic solutions (Fe(III)BAS) was probed at 25°C and pH between 2 and 3. Initial dissolved Fe3+ ([Fe3+]init) was varied from 0.1mM to 1mM, and the length of the experiments was 240min. Except for the experiment at initial pH 2, total dissolved iron ([Fe]total) decreased immediately (within 1min) after contact of FeS and Fe(III). Afterwards, [Fe]total increased smoothly. A progressive increase in pH values and an Eh decrease within 240min of reaction time were also observed. The reaction order of FeS dissolution in Fe(III)BAS with respect to [H+] is estimated to 0.65 at initial pH 3.0, and increases up to 1.0 with decreasing initial pH, indicating that [H+] is an important parameter of FeS dissolution in Fe(III)BAS. In contrast, changes in [Fe3+]init have only a limited effect on the rate of FeS dissolution in Fe(III)BAS. Raman spectra of initial and reacted FeS samples reveal the accumulation of α-S8-like material on FeS surface. These results support a mechanism of FeS dissolution in Fe(III)BAS starting with the protonation of mineral surface and Fe3+ (aq) adsorption. Adsorbed protons subsequently accelerate Fe2+ release from FeS matrix into solution. The adsorbed Fe3+ may oxidize sulfur moieties and generate insoluble species, presumably polysulfide and elemental sulfur. The subsequent migration of Fe2+ into solution is controlled by the formed sulfur rich layer. [Copyright &y& Elsevier]

Metapelitic granulites from the Anosyen domain of southeastern Madagascar are exposed in three intercalated formations: the Amparihy, Bakika and Ihosy formations. Although mineralogically distinct from each other, the rocks from these formations show very similar bulk-rock compositions when measured on a FeT basis. The preserved mineral assemblages thus do not reflect differences in the ratios of the main rock-forming oxides (i.e. Al2O3:FeT:MgO), but instead reflect variations in the pre-metamorphic oxidation state of the protolith rocks. These differences in oxidation state are manifested via differences in iron speciation - either Fe+2 or Fe+3. The relatively reduced rocks of the Amparihy Formation preserve the assemblage bi-sp-sill-g-cd, which contrasts markedly with the mostly garnet and spinel-absent bi-cd-sill-mt assemblages preserved in the strongly oxidized rocks of the Ihosy Formation. Compositionally intermediate rocks of the Bakika Formation are garnet bearing, but sillimanite-absent, and contain the assemblage sp-g-cd-mag. Modelling of these rocks in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O system suggests that they evolved along a heating and cooling P-T path with only limited decompression accompanying cooling on the retrograde path. Peak temperatures and pressures of ∼880-920 °C and 6-6.5 kbar are inferred for the majority of the Anosyen domain, with slightly lower peak temperatures (∼840 °C) estimated in the extreme northwest of the area. The high-temperature and relatively low-pressure nature of metamorphism suggests high geothermal gradients existed during orogenesis, which in southern Madagascar is related to the amalgamation of Gondwana (580-520 Ma). Although metamorphic temperatures may have been augmented via thermal advection from the emplacement of the syn- to post-tectonic Ambalavao suite, the high geothermal gradients nevertheless suggest thin and consequently hot lithosphere existed prior to orogenesis. [ABSTRACT FROM AUTHOR]

We have synthesized magnesium-iron silicate perovskites with the general formula Mg1-xFex+y3+Si1-yO3, in which the iron cation is exclusively trivalent. To investigate the crystal chemistry of Fe3+-bearing perovskite, six samples (both with and without Al) were analyzed using scanning electron microscopy, electron microprobe, X-ray diffraction, and Mössbauer spectroscopy. Results indicate that Fe3+ substitutes significantly into both the octahedral and dodecahedral sites in the orthorhombic perovskite structure, but prefers the octahedral site at Fe3+ concentrations between 0.04 and 0.05 Fe per formula unit, and the dodecahedral site at higher Fe3+ concentrations. We propose a model in which Fe3+ in the A/B site (in excess of that produced by charge coupled substitution) is accommodated by Mg/O vacancies. Hyperfine parameters refined from the Mössbauer spectra also indicate that a portion of dodecahedral sites undergo significant structural distortion. The presence of Fe3+ in the perovskite structure increases the unit-cell volume substantially compared to either the Mg end-member, or Fe2+-bearing perovskite, and the addition of Al did not significantly alter the volume. Implications for increased compressibility and a partially suppressed spin transition of Fe3+ in lower mantle perovskite are also discussed. [ABSTRACT FROM AUTHOR]

Measurements of Fe3+/ΣFe in geological materials have been intractable because of lack of access to appropriate facilities, the time-consuming nature of most analyses, and the lack of precision and reproducibility in most techniques. Accurate use of bulk Mössbauer spectroscopy is limited by largely unconstrained recoilless fraction (f), which is used to convert spectral peak area ratios into valid estimates of species concentrations and is unique to different mineral groups and compositions. Use of petrographic-scale synchrotron micro-XANES has been handicapped by the lack of a consistent model to relate spectral features to Fe3+/ΣFe. This paper addresses these two deficiencies, focusing specifically on a set of garnet group minerals. Variable-temperature Mössbauer spectra of the Fe2+-bearing almandine and Fe3+-bearing andradite end-members are used to characterize f in garnets, allowing Fe3+/ΣFe to be measured accurately. Mössbauer spectra of 19 garnets with varying composition were acquired and fit, producing a set of garnet-specific standards for Fe3+ analyses. High-resolution XANES data were then acquired from these and 15 additional previously studied samples to create a calibration suite representing a broad range of Fe3+ and garnet composition. Several previously proposed techniques for using simple linear regression methods to predict Fe3+/ΣFe were evaluated, along with the multivariate analysis technique of partial least-squares regression (PLS). Results show that PLS analysis of the entire XANES spectral region yields the most accurate predictions of Fe3+ in garnets with both robustness and generalizability. Together, these two techniques present reliable choices for bulk and microanalysis of garnet group minerals. [ABSTRACT FROM AUTHOR]

Wadsleyites with various iron contents were synthesized at ~12-14 GPa and 1400 °C under oxidizing and hydrous conditions in coexistence with enstatite. The samples were studied using micro-X-ray absorption near edge structure (XANES) and micro-Mössbauer spectroscopy to determine the ferric iron contents in polyphasic samples and secondary ion mass spectrometry (SIMS) to determine the water concentrations. XANES and Mössbauer analyses show that ferric iron content increases with increasing total iron content, and reaches a maximum of ~30% Fe3+/Fetotal. Two XANES results were cross-checked by Mössbauer analysis and both methods are in reasonable agreement. The use of Fourier transform infrared spectroscopy reveals a new protonation scheme in wadsleyite, with a significant proportion of protons associated with the high-frequency band at 3611 cm-1 and a new band located at 3500 cm-1. The intensity of these two bands is higher for Fe3+-rich wadsleyite. SIMS analyses show that water contents in wadsleyite vary from 4500 to 9400 ppm H2O by weight. Pyroxene water contents range from 790 to 1600 ppm wt H2O. The concentration of water in both phases decreases with increasing iron content. The partition coefficient of water between wadsleyite and pyroxene varies between 5 and 9 and increases with increasing Fe-number of wadsleyite [i.e., XFe/(XFe + XMg) x 100 ratio]. The divalent cation concentrations (i.e., Mg2+ + Fe2+), the Si as well as the H content in wadsleyite decrease with increasing Fe3+ content, indicating an incorporation mechanism via substitution into the metal (Me = Mg2+ and Fe2+) and Si sites with a ratio of 5/3 for (Fe3++H+):Me and of 5/1 for (Fe3++H+):Si, similarly as in the dry system. Thus, coupled substitution of Fe3+ and H+ does not affect the incorporation mechanism of Fe3+, but does affect the location of H+, which is partly incorporated at tetrahedral edges forming [FeSi'-(OH)o⋅]X neutral defects substituting for Si. In this model 25% of the ferric iron occupies the tetrahedral sites. [ABSTRACT FROM AUTHOR]

Abstract: The reactivities of various types of iron mixtures to degrade chlorinated hydrocarbons (PCE, TCE and 1,1,1-TCA) in the form of non-aqueous phase liquids were investigated. The iron mixtures included a mixture of Fe(II) and Portland cement (Fe(II)-C), a mixture of Fe(II), Fe(III) and Ca(OH)2 (Fe(II/III)-L), and a mixture of Fe(II), Fe(III), Ca(OH)2, and Portland cement (Fe(II/III)-C). When the same amount of Fe(II) was used, Fe(II)-C was more reactive with chlorinated ethylenes (i.e. PCE and TCE) than Fe(II/III)-L. The reductive pathway for high concentrations of total PCE (i.e. above solubility) with Fe(II)-C was determined to be a combination of two-electron transfer, β-elimination and hydrogenolysis. Increasing the cement dose from 5% to 10% in Fe(II)-C did not affect PCE dechlorination rates, but it did favor the β-elimination pathway. In addition, when Fe(II/III)-C with 5%C was used, PCE dechlorination was similar to that by Fe(II)-C, but this mixture did not effectively degrade TCE. A modified second-order kinetic model was developed and shown to appropriately describe degradation of TCE at high concentrations. Fe(II/III)-L effectively degraded high concentrations of 1,1,1-TCA at rates that were similar to those obtained with Fe(II)-C using 10% C. Moreover, both increasing cement doses and the presence of Fe(III) increased dechlorination rates of 1,1,1-TCA, which was mainly through the hydrogenolysis pathway. The reactivity of Fe(II/III)-L was strongly dependent on the target compound (i.e. less reactivity with TCE, more with 1,1,1-TCA). Therefore, Fe(II/III)-L could be a potential mixture for degrading 1,1,1-TCA, but it should be modified to degrade TCE more effectively. [Copyright &y& Elsevier]

Abstract: Cr(VI) is a group A chemical based on the weight of evidence of carcinogenicity. Its transport and retention in soils and groundwater have been studied extensively. Zeolite is a major component in deposits originated from volcanic ash and tuff after alteration. In this study, zeolite aggregates with the particle size of 1.4–2.4mm were preloaded with Fe(III). The influence of present Fe(III) on Cr(VI) retention by and transport through zeolite was studied under batch and column experiments. The added Fe(III) resulted in an enhanced Cr(VI) retention by the zeolite with a capacity of 82mg/kg. The Cr(VI) adsorption on Fe(III)-zeolite followed a pseudo-second order kinetically and the Freundlich adsorption isotherm thermodynamically. Fitting the column experimental data to HYDRUS-1D resulted in a retardation factor of 3 in comparison to 5 calculated from batch tests at an initial Cr(VI) concentration of 3mg/L. The results from this study showed that enhanced adsorption and retention of Cr(VI) may happen in soils derived from volcanic ash and tuff that contains significant amounts of zeolite with extensive Fe(III) coating. [Copyright &y& Elsevier]

Abstract: We have performed partitioning experiments to assess the role of chromium-rich spinel in controlling the behavior of the platinum-group elements (PGEs) during igneous differentiation. Spinels were equilibrated with natural and synthetic iron-bearing basalt at 0.1MPa and 2GPa at 1400–1900°C over an fO2 range of IW+1.6 to IW+7. Results from relatively reduced, graphite-encapsulated experiments done at 2GPa indicate that Ru is compatible in Cr-spinel (mineral/melt partition coefficient, D, of ~4), followed by Rh and Ir, which are moderately incompatible (D range of 0.04 to ~1), with Pt and Pd the most incompatible (D<0.2). Partition coefficients for Ir, Ru and Rh measured at more oxidizing conditions in this and previous studies are 10 to 1000 times higher than results from experiments using graphite capsules. We account for the variation in spinel–melt partitioning with a model which considers both the affinity of the PGE cation for a particular spinel lattice site, and the change in site occupancy accompanying the increase in ferric iron component with fO2. Assuming that Ir and Rh are present as divalent species, with a strong affinity for VI-fold coordination, DIr and DRh are predicted to rise rapidly with the ferric iron component, explaining the large D-values for magnetite-rich spinels. Model results indicate that DIr ≤20 and DRh are ≤100 for ferric-iron poor, Cr-rich compositions, as would crystallize in komatiites, some layered intrusions, and ophiolites. The overall compatibility of Ru for chromite is consistent with the predominance of Ru3+ at experiment conditions and the similarity in the size of Ru3+ to Cr3+ and Fe3+. The increase in DRu with the ferric iron content of the spinel likely involves a strong effect of mineral composition superimposed on a change in melt speciation (Ru2+ to Ru3+) with increased fO2. The effect of mineral composition is a consequence of the difference in octahedral site preference energy (OSPE) between Ru3+, Fe3+ and Cr3+, with stronger partitioning of Ru into Fe3+-rich compositions due to the enhanced reduction in energy gained by the Ru3+ substitution. Ru partition coefficients for ferric-iron poor spinel are expected to be ~30, which is somewhat lower than values estimated from natural samples obtained from in situ chromite analyses. Results indicate that the ferric iron content of chromite exerts a strong control on the partitioning of some PGEs which should be taken into account in both future experimental work and in models of igneous differentiation. [Copyright &y& Elsevier]