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1.
We investigated the concentration and site occupation of ferric iron (Fe3+) in (Mg,Fe)O to understand the influence of point defects on transport properties such as atomic diffusion, electrical conductivity and viscosity. We conducted Mössbauer spectroscopy of (Mg0.8Fe0.2)O single crystals synthesized at temperatures from 1673 to 2273 K and pressures from 5 to 15 GPa with Re–ReO2 and Mo–MoO2 oxygen fugacity buffers. The isomer shift of the Mössbauer spectra suggests that Fe3+ occupies mostly the tetrahedral site at reduced conditions and both the octahedral and tetrahedral sites at oxidized conditions. We formulate a thermodynamic model of point defect dissolution in (Mg,Fe)O which suggests that unassociated tetrahedral Fe3+ is more stable than unassociated octahedral Fe3+ at high-pressure and low oxygen fugacity due to the effect of configurational entropy. The pressure dependence of Fe3+ concentration indicates a change in the dominant site occupancy of Fe3+: (1) Fe3+ in the tetrahedral site, (2) Fe3+ in the octahedral site, and (3) defect clusters of Fe3+ and cation vacancy, in the order of increasing oxygen fugacity and decreasing pressure. This is in reasonable agreement with previously reported experiments on Fe3+ concentration, Mg–Fe interdiffusivity and electrical conductivity. We consider it plausible that (Mg,Fe)O accommodates Fe3+ in the tetrahedral site down to the lower mantle. Based on our results and available experimental data, we discuss the solubility competition between Fe3+ and protons (H+), and its implications for transport properties in the lower mantle.  相似文献   

2.
Four pyroxenes with compositions En48Fs48Wo4, En47·5Fs47·5Wo5, En45Fs45Wo10 and En40Fs40Wo20, synthesized at 1200°C at atmospheric pressure, were heat-treated at 500, 600, 700, and 800°C for various lengths of time. These pyroxenes are variously ordered with respect to Fe2+ and Mg2+ without unmixing. The Fe2+-Mg2+ distribution over the two nonequivalent sites M1 and M2, determined through Mössbauer spectroscopy, is found to be a function of both temperature and concentration of Ca2+ at the M2 site. The preference of Fe2+ for the M2 site increases with decreasing temperature and increasing Ca2+. These data can be used to determine cation equilibration temperatures of lunar and terrestrial pigeonites. The lunar pigeonites usually indicate equilibration temperatures of 700–860°C, except the pigeonite from rock 14053, which may have been subjected to shock heating due to meteoritic impact.  相似文献   

3.
The elastic moduli of a synthetic single crystal of pyrope (Mg3Al2Si3O12) have been determined using a technique based on Brillouin scattering. These results are used in an evaluation of the effect of composition on the elastic properties of silicate garnet solid solution series (Mg, Fe, Mn, Ca)3 (Al, Fe, Cr)2 Si3O12. In the pyralspites (Mg FeMn aluminum garnets), for which a large amount of data is available, this analysis indicates that the bulk modulus K is independent of the Fe2+/Mg2+ ratio, which is similar to the behavior observed in olivines and pyroxenes. However, the shear modulus μ of the garnets increases by 10% from the Mg to the Fe end member, in contrast to the decrease of μ with Fe content which is observed in olivines and pyroxenes. This contrasting behavior is most probably related to the oxygen coordination of the cation site occupied by Mg2+ and Fe2+ in these different minerals.  相似文献   

4.
In this paper, we analyze the self-reversal of magnetization in titanomagnetites as a function of the Ti content and the distribution of Fe3+ to Fe2+ ion transitions in sublattices (which is associated with the law of charge conservation). The dependence of the Curie point on the Ti concentration and the temperature dependence of the mean magnetic moment per iron atom at different Ti concentrations and different cation distributions in sublattices are calculated.  相似文献   

5.
We present ab initio calculations of the zero-temperature iron high- to low-spin crossover in (Mg1 ? xFex)SiO3 perovskite at pressures relevant to Earth's lower mantle. Equations of state are fit for a range of compositions and used to predict the Fe spin transition pressure and associated changes in volume and bulk modulus. We predict a dramatic decrease in transition pressure as Fe concentration increases. This trend is contrary to that seen in ferropericlase, and suggests the energetics for spin crossover is highly dependent on the structural environment of Fe. Both Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) exchange-correlation methods are used, and both methods reproduce the same compositional trends. However, GGA gives a significantly higher transition pressure than LDA. The spin transition is made easier by the decreasing spin-flip energy with pressure but is also driven by the change in volume from high to low spin. Volume trends show that high-spin Fe2+ is larger than Mg2+ even under pressure, but low-spin Fe2+ is smaller at ambient conditions and approximately the same size as Mg2+ under high pressure, indicating that low-spin Fe2+ is less compressible than high-spin Fe2+. We find large changes between high- and low-spin in the slope of volume with Fe concentration. Although these changes are small in absolute magnitude for small Fe content, they are still important when measured per Fe and could be relevant for calculating partitioning coefficients in the lower mantle.  相似文献   

6.
Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45–205 kbar and in the temperature range 1050–1200°C. Partition of elements, particularly Mg2+Fe2+, among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(α) to spinel(γ) through modified spinel(β) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg2SiO4Fe2SiO4. At pressures of 50–150 kbar, a continuous increase in the solid solubility of the pyroxene component in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45–75 kbar and 1200°C, the Fe2+/(Mg + Fe2+) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144–146 kbar and 1200°C, garnet exhibits the highest Fe2+/(Mg + Fe2+) value; modified spinel(β) and Ca-poor clinopyroxene follow it. When the modified spinel(β)-spinel(γ) transformation occurred, a higher concentration of Fe2+ was found in spinel(γ) rather than in garnet. As a result of the change in the Mg2+Fe2+ partition relation among coexisting minerals, an increase of about 1% in the Fe2SiO4 component in (Mg,Fe)2SiO4 modified spinel and spinel was observed compared with olivine.These experimental results strongly suggest that the olivine(α)-modified spinel(β) transformation is responsible for the seismic discontinuity at depths of 380–410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(β)-spinel(γ) transformation and the pyroxene-garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100–150, 150–380, 380–410, 410–500, 500–600 and 600–650 km.  相似文献   

7.
Equilibrium reversals of Fe2+Mg distribution between the M1 and M2 sites of an orthopyroxene from the Johnstown meteorite were achieved at several temperatures between 700 and 1000°C. One single crystal was used for the whole thermal treatment and for collecting all the X-ray data after quenching. The intracrystalline ion exchange for the bulk chemical composition: (Mg1.453Fe0.441Cr0.024Ca0.054Mn0.015Fe0.005Ti0.003Al0.005)(Si1.960Al0.040)O6 is given by: ln KD = −3027(±39)/T(K) + 0.872(±0.013)> where KD is the distribution coefficient for the reaction: FeM22+ + MgM1 = MgM2 + FeM12+.A transmission electron microscopy (TEM) study of part of the crystal showed the presence of very thin augite lamellae and Guinier-Preston zones indicating a relatively rapid cooling of the host rock at temperatures close to 1000°C. The new temperature scale yields a relatively high quenching temperature of 379 (±8)°C for the pyroxene which appears consistent with a rapid cooling (estimated few degrees per hundred years) of a magmatic cumulate excavated by an impact on its parental body.  相似文献   

8.
Inclusions of ferropericlase and former (Mg,Fe)(Si,Al)O3 perovskite in diamonds from Kankan, Guinea believed to originate in the lower mantle were studied using Mössbauer spectroscopy to determine Fe3+/ΣFe. Fe3+ concentration in the (Mg,Fe)(Si,Al)O3 inclusion is consistent with empirical relations relating Fe3+/ΣFe to Al concentration, supporting the inference that it crystallised in the perovskite structure at lower mantle conditions. In ferropericlase there is a nearly linear variation of trivalent cation abundance with monovalent cation abundance, suggesting a substitution of the form Na0.5M0.53+O (M=Fe3+, Cr3+, Al3+). Excess positive charge is likely balanced by cation vacancies, where their abundance is observed to increase with increasing iron concentration, consistent with high-pressure experiments. The abundance of cation vacancies is related to oxygen fugacity, where ferropericlase inclusions from Kankan and São Luiz (Brazil) are inferred to have formed at conditions more oxidising than Fe-(Mg,Fe)O equilibrium, but more reducing than Re-ReO2 equilibrium. Fe2+/Mg partition coefficients between (Mg,Fe)(Si,Al)O3 and ferropericlase were calculated for inclusions co-existing in the same diamond using Mössbauer data and empirical relations based on high-pressure experimental work. Most values are consistent with high-pressure experiments, suggesting that these inclusions equilibrated at lower mantle conditions. The measured ferropericlase Fe3+ concentrations are consistent with diamond formation in a region of redox gradients, possibly arising from the subduction of oxidised material into reduced lower mantle. Reduction of carbonate to form ferropericlase and diamond is consistent with a slight shift of Kankan δ13C values to isotopically heavy compositions compared to the worldwide dataset, and could supply the oxygen necessary to satisfy the high Fe3+ concentration in (Mg,Fe)(Si,Al)O3 perovskite, as well as account for the high proportion of ferropericlase in the lower mantle paragenesis. The heterogeneity of lower mantle diamond sources indicates that the composition of lower mantle diamonds do not necessarily reflect those of the bulk mantle.  相似文献   

9.
The Barda igncous complex of western Saurashtra (Gujarat) consists of a group of volcanic and subvolcanic igneous rocks, viz., pitchstone, rhyolite, felsite, granophyre and dioritic rocks. They are generally grouped under the Deccan Trap series and have been emplaced through the Deccan Trap basalt flows. Using different varieties of clinopyroxenes as characteristic phases several units of felsites and granophyres have been distinguished in the southeastern part of the Barda hills. The augites of the adjacent Deccan Trap basalt flows range between Ca33Mg61Fe6 and Ca41Mg44Fe15. (All compositions of minerals are derived from optical data). The clinopyroxene of a monzodiorite is Ca41Mg42.5Fe16.5. In the felsites and granophyres the clinopyroxenes range from salite (Ca46Mg31Fe23) through ferrosalite (Ca46Mg20Fe34) and ferroaugite (Ca43Mg17Fe46) to hedenbergite (Ca46Mg9Fe45). Beyond that a peralkaline trend is indicated by common occurrence of aegirine-augite granophyre in which the clinopyroxene is alkalic with 20 percent of NaFe3 Si2O6 (acmite) molecule. Several bodies of acmite-riebeckitebearing granophyres have been found; they also indicate a high degree of oxidation. A fayalitic olivine-bearing granophyre is of interest as it shows the strongest iron enrichment with 13 weight percent FeO + Fe2O3; its mineralogy indicates a very low oxygen fugacity of 1013 atmospheres during crystallization.  相似文献   

10.
黑臭现象在我国东部地区浅水湖泊频繁发生,已经严重影响环湖地区的社会经济发展.厌氧环境和高浓度Fe~(2+)、S~(2-)是引起黑臭现象的必要条件.本文解析巢湖南淝河口区黑臭水团范围内Fe~(2+)、S~(2-)与DO、流速的分布特性;基于空间计量模型重点探讨了流速、DO和Fe~(2+)、S~(2-)分布的空间关联性.结果发现,水体流动在黑臭水团中主要有两个作用:使Fe~(2+)、S~(2-)和DO彼此产生空间相关性以及通过分散作用改变局部Fe~(2+)、S~(2-)浓度分布;同时流速通过增强复氧间接影响Fe~(2+)及S~(2-)总体浓度的作用较小.流速与Fe~(2+)、S~(2-)之间均满足线性空间滞后模型;而线性回归模型中流速对Fe~(2+)的影响低估了约7%,对S~(2-)的影响则低估了12%.  相似文献   

11.
The aim of this work is to study the effects of a wildfire on water‐extractable elements in ash from a Pinus pinaster forest located in Portugal. The pH, electrical conductivity (EC), calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), sodium and potassium adsorption ratio (SPAR), aluminium (Al3+), manganese (Mn2+), iron (Fe2+), zinc (Zn2+), sulphur (S), silica (Si) and phosphorous (P) were analysed in ash sampled from a sloped area burned in a wildfire and from litter from a contiguous unburned area, with similar morphological conditions. The results showed that ash leachates had higher pH and EC, and were significantly richer in water‐extractable Ca2+, Mg2+, Na+, K+, SPAR, S and Si and significantly poorer in water‐extractable Al3+, Fe2+, Mn2+ and Zn2+ than litter solutions. No significant differences were observed in water‐extractable P. The fire changed the ash solute chemistry compared with the unburned litter and increased the sample variability of nutrient distribution with potential implications for plant recovery. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

12.
The present work focuses on the performance of Fenton, sono‐Fenton, and sono‐photo‐Fenton processes for the oxidation of phenol present in aqueous solution. The effects of H2O2 concentration, Fe2+ concentration, pH, and initial phenol concentration on the oxidation of phenol were studied. The optimum Fe2+ and H2O2 concentrations for the Fenton process were 45 and 800 mg/L, respectively. For the sono‐Fenton process, the optimum Fe2+ and H2O2 concentrations were 30 and 800 mg/L, respectively. The optimal conditions for the sono‐photo‐Fenton process were found to be 20 mg/L of Fe2+ and 700 mg/L of H2O2. The optimum pH was found to be 3 for the processes investigated in the present study. The analysis of results showed that the sono‐photo‐Fenton method reduced the Fe2+ concentration by 30–50% and the H2O2 concentration by 12.5%. It was found that the sono‐photo‐Fenton technique showed better performance than the Fenton and sono‐Fenton processes for the oxidation of phenol. A lumped kinetic model was used to predict the chemical oxygen demand reduction and the model was found to fit the data.  相似文献   

13.
Recent studies have indicated that a significant amount of iron in MgSiO3 perovskite (Pv) is Fe3+ (Fe3+/ΣFe = 10–60%) due to crystal chemistry effects at high pressure (P) and that Fe3+ is more likely than Fe2+ to undergo a high-spin (HS) to low-spin (LS) transition in Pv in the mantle. We have measured synchrotron Mössbauer spectroscopy (SMS), X-ray emission spectroscopy (XES), and X-ray diffraction (XRD) of Pv with all iron in Fe3+ in the laser-heated diamond-anvil cell to over 100 GPa. Fe3+ increases the anisotropy of the Pv unit cell, whereas Fe2+ decreases it. In Pv synthesized above 50 GPa, Fe3+ enters into both the dodecahedral (A) and octahedral (B) sites approximately equally, suggesting charge coupled substitution. Combining SMS and XES, we found that the LS population in the B site gradually increases with pressure up to 50–60 GPa where all Fe3+ in the B site becomes LS, while Fe3+ in the A site remains HS to at least 136 GPa. Fe3+ makes Pv more compressible than Mg-endmember below 50 GPa because of the gradual spin transition in the B site together with lattice compression. The completion of the spin transition at 50–60 GPa increases bulk modulus with no associated change in density. This elasticity change can be a useful seismic probe for investigating compositional heterogeneities associated with Fe3+.  相似文献   

14.
This paper aims to identify the spatial distribution of exchangeable base cations in soils on an acid hillslope and to investigate possible cation release processes from slope soils to the stream. The basic assumption underlying this research is that the amount of exchangeable cations in soils reflects the nutrient stores and cation leaching processes across the slope where vegetation and parent materials are similar. The distribution of exchangeable Ca2+, Mg2+, K+ and Na+ has been investigated on a three-dimensional hillslope on the Quantock Hills, Somerset, UK. A two-way ANOVA shows that soil depth is predominant in explaining the total variance of exchangeable bases, despite the steep slope gradient and clear podzolic catena development. Major nutrient base cations, such as Ca2+, Mg2+ and K+, display homogeneous topsoil storage right across the slope. This spatial pattern may indicate that the spatial distribution of major nutrient cations is tightly controlled by the soil–vegetation system in nutrient-poor heathland environments. Na+ is an exception to this vegetation-controlled spatial distribution, because of its small involvement in the soil–vegetation and soil exchangeable systems. In subsurface soils, cations liberated from the soil–vegetation system are subject to redistribution over the slope according to the hydrological flowpaths operating on the slope, with some eventually released into the stream. The saturated wedge developed at the base of the slope plays a key role in the storage and release processes of base cations from slope soils to the stream. Ca2+, Mg2+ and Na+ carried by throughflow are stored in the saturated wedge and gradually released into the stream at times of high flow. K+, however, shows an apparently different spatial behaviour, being deficient in the saturated wedge. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

15.
ALHA 77005, a shocked achondrite of the shergottite group, is unique in containing cumulus olivine crystals which display a brownish color in thin section. Mössbauer, EPR, and optical spectroscopic studies show the presence of Fe3+ in the olivine. Approximately 4.5 wt.% of the total iron in the olivine (24 wt.% as FeO) is in the trivalent state. The Fe3+ ions preferentially occupy M2 lattice positions. Charge transfer between Fe3+ and Fe2+ as well as between Fe3+ and oxygen ligands causes a broad absorption band in the optical spectrum at high energies which is the reason for the olivine color. It is concluded that the color of the ALHA 77005 olivine is preterrestrial and may be the result of shock-induced oxidation of the olivine crystals on the meteorite parent body.  相似文献   

16.
Experimental study of the phase boundary for the disproportionation of the inverse spinel Mg2SnO4 into its isochemical mixed oxides indicates a slope dP/dT = 40 ± 10bars/°K. This positive slope is consistent with the large entropies of inverse (relative to normal) spinels predicted from high-temperature entropy-molar volume systematics. Thermodynamic data do not support the existence of a distinctly negative slope for the proposed disproportionation of Mg2SiO4 normal spinel. Evidence from X-ray and phase equilibria studies suggests the possibility that Si4+, Mg2+, and Fe2+ share the octahedral sites in silicate spinels under mantle conditions. The consequences of this partial inverse character are a positive slope for the postulated spinel-mixed oxide phase boundary near 650 km depth, removal of a widely accepted constraint on mantle-wide convection, and anomalous elasticity-density behaviour within the transition zone.  相似文献   

17.
The oxidation state of iron can significantly influence the physical and chemical properties of lower mantle minerals. To improve methods for estimation of Fe3+/∑Fe, synthetic assemblages of (Mg,Fe)(Si,Al)O3 perovskite and (Mg,Fe)O ferropericlase were synthesised from oxide starting mixtures in Re or Fe capsules at 26 GPa and 1650-1850 °C using a multianvil press. (Mg,Fe)(Si,Al)O3 majorite was also present in some of the run products. Both electron energy loss spectra (EELS) and Mössbauer spectra were measured for each run product, and a robust fitting method was developed for Mössbauer spectra using EELS results as a standard that enabled Fe3+/∑Fe of (Mg,Fe)(Si,Al)O3 perovskite to be determined from Mössbauer spectra of multiphase assemblages. There is a close to linear variation between Fe3+/∑Fe and Al concentration in (Mg,Fe)(Si,Al)O3 perovskite, independent of oxygen fugacity. The concentration of Fe3+ in (Mg,Fe)O increases with increasing iron concentration along curves of constant oxygen fugacity, where higher oxygen fugacity stabilises greater Fe3+ concentrations. Fe2+/Mg partition coefficients calculated from chemical composition data corrected for measured Fe3+/∑Fe showed values nearly identical within experimental error for all samples, and independent of Al concentration and oxygen fugacity. Simple empirical relations were derived to calculate Fe3+/∑Fe in (Mg,Fe)(Si,Al)O3 perovskite and (Mg,Fe)O ferropericlase samples for which no Mössbauer or EELS data were available, and tested by applying them to calculation of Fe2+/Mg partition coefficients from literature data for (Mg,Fe)(Si,Al)O3 perovskite-(Mg,Fe)O assemblages where only total iron concentrations had been measured. Results showed Fe2+/Mg partition coefficients that were equal to existing values within experimental error, hence confirming the validity of the empirical relations.  相似文献   

18.
The sorption of reactive (textile) dyes onto cucurbituril, a cyclic polymer with hydrophobic cavity, was studied. Dye sorption is strongly enhanced by Ca2+ or Sr2+ concentrations up to 100 mmol/L for all studied dyes. Mg2+ and alkaline ions had similar effects for only one dye (Reactive Red 120), and only at higher concentrations. Concentrations above 100 mmol/L – depending on cation and dye – dissolve cucurbituril and prevent dye removal. As shown in previous studies by our group loadings obtained under suitable conditions (calcium concentration between 2 and 100 mmol/L, total salt concentration not exceeding 100 mmol/L) are 1 to 1.7 mol/mol or 0.9 to 1.8 g/g. The chemical mechanism responsible for the ionic influences is still under investigation. Generally, cucurbituril is a potent sorbent for reactive dyes. However, the technical application is still limited by the lack of a support material that would allow use in fixed bed filters.  相似文献   

19.
Bench-scale experiments were conducted to investigate the effect of hydraulic loadings and influent concentration on the migration and biotransformation behaviour of three groundwater pollutants: ammonium (NH4+), iron (Fe2+) and manganese (Mn2+). Columns packed with aquifer media collected from a riverbank filtration (RBF) site in Harbin City, NE China were introduced synthetic groundwater (SGW) or real groundwater (RGW) were at two different constant flow rates and initial contaminant concentrations to determine the impact of system conditions on the fate of the target pollutants biotransformation. The results showed that the biotransformation rate of Fe2+ Mn2+ and NH4+ decreased by 8%, 39% and 15% under high flow rate (50 L d−1) compared to low flow rate (25 L d−1), which was consistent with the residence-time effect. While the biotransformation rate of Fe2+ Mn2+ and NH4+ decreased by 7%, 14% and 9% under high influent concentration comparing with original groundwater. The 16S rRNA analysis of the aquifer media at different depths after experiments completion demonstrated that the relative abundance of major functional microbes iron-oxidizing bacteria and manganese-oxidizing bacteria under higher flow rate and higher influent concentration decreased 13%, 14% and 25%, 24%, respectively, whereas the ammonium-oxidizing bacteria and nitrite-oxidizing bacteria exhibited minimal change, compared to the lower flow rate. Above all results indicated that both high flow rate and high concentration inhibit the biotransformation of NH4+, Fe2+ and Mn2+. The biotransformation of Fe2+ and Mn2+ occurs primarily in the 0–40 cm and 20–60 cm depth intervals, respectively, whereas the NH4+ biotransformation appears to occur relatively uniformly throughout the whole 110 cm column. The biotransformation kinetics of NH4+ in RGW and SGW, Mn2+ in RGW at different depths accords with the first order kinetics model, while Fe2+ in RGW and SGW, Mn2+ in SGW presented more complicated biotransformation process. The results should improve understanding of the transport and fate of common groundwater pollutants in RBF and other groundwater recharge environments.  相似文献   

20.
A simple, low cost, highly effective, and useful Fenton oxidation treatment of synthetic dye bath waste with pickling liquor as a source of iron (Fe2+ catalyst) is reported. Optimizations of contact time, Fe2+ and H2O2 doses are carried out. Oxidative de‐colorization and degradation of Reactive Blue 4 and Reactive Orange 16 was measured in terms of decrease in absorbance at their wavelength of maximum absorption (RB4, 599 nm; and RO16, 493 nm) and also as reduction in chemical oxygen demand (COD). Approximately, 62% COD was removed in 2 h at optimized doses of Fe2+ (8.95 mM) and H2O2 (61.8 mM) by using pickling waste as a source of Fe2+ catalyst. Similar performance efficiency was observed when neat FeSO4 was used as a source of Fe2+, indicating that pickling liquor can be a low cost source of Fe2+ to treat synthetic dye bath waste by Fenton method.  相似文献   

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