Spin state of ferric iron in MgSiO3 perovskite and its effect on elastic properties

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⁺.     

Tetrahedral occupancy of ferric iron in (Mg,Fe)O: Implications for point defects in the Earth's lower mantle

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 (Mg_0.8Fe_0.2)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.     

The location of Fe3+ ions in forsterite (Mg2SiO4)

The electron spin resonance spectrum of Fe³⁺ in a single crystal of forsterite was studied. Two distinct patterns of about equal intensities were observed which are due to Fe³⁺ at two distinct positions with 4a (M1)and4c (M2or Si) symmetry of Pbnm. The assignment of the 4c pattern to Fe³⁺ ions at the Si position cannot be excluded by symmetry but it is unlikely. The Hamiltonian parameters A and E/D are consistent with the conclusion that Fe³⁺ in this crystal is disordered over two distinct octahedral positions.     

Fe3+ in shocked olivine crystals of the ALHA 77005 meteorite

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.     

Chrome spinel in normal MORB‐type greenstones from the Paleozoic–Mesozoic Mino terrane,East Takayama area,central Japan: Crystallization course with a U‐turn

Phenocrystic chrome spinel crystallized in normal MORB‐type greenstones in the East Takayama area. Associated phenocryst minerals show a crystallization sequence that was olivine first, followed by plagioclase, and finally clinopyroxene. Chrome spinel ranges from 0.54 to 0.77 in Mg/(Mg+Fe²⁺) and 0.21 to 0.53 in Cr/(Cr+Al); the Fe³⁺ content varies from 0.07 to 0.22 p.f.u. (O = 4). Significant compositional differences of spinel were observed among the phenocryst mineral assemblages. Chrome spinel in the olivine–spinel assemblage shows a wide range in Cr/(Cr+Al), and is depleted in Fe²⁺ and Fe³⁺. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage is Fe²⁺‐ and Fe³⁺‐rich at relatively high Cr/(Cr+Al) ratios. Basalt with the olivine–plagioclase–spinel assemblage contains both aluminous spinel and Fe²⁺‐ and Fe³⁺‐rich spinel. The assumed olivine–spinel equilibrium suggests that chrome spinel in the olivine–spinel assemblage changed in composition from Cr‐ and Fe²⁺‐rich to Al‐ and Mg‐rich with the progress of fractional crystallization. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage, on the other hand, exhibits the reversed variations in Mg/(Mg+Fe²⁺) and in Cr/(Cr+Al) ratios that decrease and increase with the fractional crystallization, respectively. The entire crystallization course of chrome spinel, projected onto the Mg/(Mg+Fe²⁺)–Cr/(Cr+Al) diagram, exhibits a U‐turn, and appears to be set on a double‐lane route. The U‐turn point lies in the compositional field of chrome spinel in the olivine–plagioclase–spinel assemblage, and may be explained by plagioclase fractionation that began during the formation of the olivine–plagioclase–spinel assemblage.     

A M?ssbauer effect study of Fe3+ bearing γ-Fe2SiO4

Three synthetic Fe³⁺ bearing λ-Fe₂SiO₄ were analyzed using electron probe method, and the M?ssbauer spectra of the samples at 298 K, 150 K, and 95 K were measured. Each spectrum at three temperatures is composed of two doublets. These two doublets are assigned to Fe²⁺ in the octahedral sites and Fe³⁺ in the tetrahedral sites, respectively. Site occupancies were determined. The results show that Fe³⁺ and a small amount of Si⁴⁺ are in the tetrahedral and octahedral sites, respectively. The average bond lengths of the octahedral and tetrahedral sites were calculated according to the equations primarily given by Hill et al., O’Neill and Navrotsky and modified by the authors. Furthermore, the octahedral and tetrahedral bond lengths were used to calculate cell parameters and oxygen parameters. In addition, Fe³⁺ line broadening in the M?ssbauer spectra of Fe³⁺ bearing λ-Fe₂SiO₄ were interpreted by using the next nearest neighbor effects     

A new method to estimate the oxidation state of basaltic series from microprobe analyses

The oxygen fugacity and therefore the iron redox state of a melt is known to have a strong influence on the liquid line of descent of magmas and thus on the composition of the coexisting melts and crystals. We present a new method to estimate this critical parameter from electron probe microanalyses of two of the most common minerals of basaltic series, plagioclase and clinopyroxene. This method is not based on stoichiometric calculations, but on the different partitioning behaviour of Fe³⁺ and Fe²⁺ between both minerals and a melt phase: plagioclase can incorporate more Fe³⁺ than Fe²⁺, while clinopyroxene can incorporate more Fe²⁺ than Fe³⁺. For example, the effect of oxidizing a partly molten basaltic system (Fe³⁺ is stabilized with respect to Fe²⁺) results in an increase of FeO_total in plagioclase, but a decrease in the associated clinopyroxene. We propose an equation, based on published partition coefficients, that allows estimating the redox state of a melt from these considerations. An application to a set of experimental and natural data attests the validity of the proposed model. The associated error can be calculated and is on average < 1 log unit of the prevailing oxygen fugacity.In order to reduce the number of different variables influencing the Fe²⁺/Fe³⁺ mineral/melt equilibrium, our model is restricted to basaltic series with SiO₂ < 60% that have crystallized at intermediate to low pressure (< 0.5 GPa) and under relatively oxidizing conditions (?FMQ > 0; where FMQ is the fayalite–magnetite–quartz oxygen buffer equilibrium), but it may be parameterized for other conditions. A spreadsheet is provided to assist the use of equations, and to perform the error propagation analysis.     

Iron oxidation state in lower mantle mineral assemblages: I. Empirical relations derived from high-pressure experiments

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.     

Iron oxidation state in lower mantle mineral assemblages: II. Inclusions in diamonds from Kankan, Guinea

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 Na_0.5M_0.5³⁺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.     

Equilibrium approaches to natural water systems—Part 3

The concentrational control of Fe²⁺ has been studied in the anoxic hypolimnion of an ice-covered lake with special reference to redox equilibria with the suspended phase. The anoxic bottom water formed during ice conditions differed substantially in terms of vertical distributions from that reported from thermally stratified waters. This was particularly evident for the pH and pe gradients. The main process explaining the Fe²⁺ concentration profile was a diffusional model in which upwards moving Fe²⁺ met an opposing O₂ flux at the redox cline and precipitated as ferric hydroxide. When the anoxic water column is divided into thin stratums an equilibrium reaction between Fe²⁺ and amorphous FeOOH explained the Fe²⁺ concentrations in the upper hypolimnion. Increasing supersaturation was observed at greater depths. Similarly equilibration control of the sulfide concentration by Fe²⁺ could be shown in the lower hypolimnion. Apparent equilibrium constants for FeS and FeOOH have been calculated together with the eddy diffusion coefficient of Fe²⁺.     

Co2+ as a chemical analogue for Fe2+ in high-temperature experiments in basaltic systems

High-temperature experiments on ferromagnesian compositions have been hampered by the rapid absorption of up to 95% of the original iron by platinum and 40% by silver-palladium capsules. Molybdenum or iron capsule materials can decrease or alleviate iron loss, but restrict oxygen fugacities to values near the iron-wustite buffer. Because Co²⁺ is stable at f_O2 =HM and because the solubility of Co in platinum in this range of f_O2 is ～0.05% at temperatures to 1350°C, its use as an analogue for Fe²⁺ is possible. In addition, experiments simulating various Fe²⁺ ratios can be easily performed by choosing appropriate Co²⁺/Fe³⁺ ratios. The cobalt phases produced possess brilliant and distinctive colors which are valuable aids in optical identification of minute phases. The cobalt analogue hypothesis was tested with atmospheric pressure experiments in air on the cobalt analogue of the 1921 Kilauea basalt at three simulated Fe²⁺/Fe³⁺ ratios. The results were compared with those of R.E.T. Hill (1969) for the natural 1921 basalt. The phase relations were the same, with the cobalt system stability fields systematically shifted by about +50°C. Microprobe analysis of olivines and the coexisting glasses indicate that the distribution of Co²⁺ between olivine and melt is independent of temperature and liquid composition. Although the analogue liquid composition differs from the equilibrium composition of the natural system, it may be corrected be employing distribution coefficients (K_D = 0.61 for the Co system; K_D = 0.33 for the Fe system) to closely approximate what the natural system would yield if iron loss did not occur.     

Phase relations and equation-of-state of aluminous Mg-silicate perovskite and implications for Earth's lower mantle

We have investigated the effect of Al³⁺ on the room-temperature compressibility of perovskite for stoichiometric compositions along the MgSiO₃-AlO_1.5 join with up to 25 mol% AlO_1.5. Aluminous Mg-perovskite was synthesized from glass starting materials, and was observed to remain a stable phase in the range of ∼30-100 GPa at temperatures of ∼2000 to 2600 K. Lattice parameters for orthorhombic (Pbnm) perovskite were determined using in situ X-ray diffraction at SPring8, Japan. Addition of Al³⁺ into the perovskite structure increases orthorhombic distortion and unit cell volume at ambient conditions (V₀). Compression causes anisotropic decreases in axial length, with the a axis more compressive than the b and c axes by about 25% and 3%, respectively. The magnitude of orthorhombic distortion increases with pressure, but aluminous perovskite remains stable to pressures of at least 100 GPa. Our results show that substitution of Al³⁺ causes a mild increase in compressibility, with the bulk modulus (K₀) decreasing at a rate of −67±35 GPa/X_Al. This decrease in K₀ is consistent with recent theoretical calculations if essentially all Al³⁺ substitutes equally into the six- and eight-fold sites by charge-coupled substitution with Mg²⁺ and Si⁴⁺. In contrast, the large increase in compressibility reported in some studies with addition of even minor amounts of Al is consistent with substitution of Al³⁺ into six-fold sites via an oxygen-vacancy forming substitution reaction. Schematic phase relations within the ternary MgSiO₃-AlO_1.5-SiO₂ indicate that a stability field of ternary defect Mg-perovskite should be stable at uppermost lower mantle conditions. Extension of phase relations into the quaternary MgSiO₃-AlO_1.5-FeO_1.5-SiO₂ based on recent experimental results indicates the existence of a complex polyhedral volume of Mg-perovskite solid solutions comprised of a mixture of charge-coupled and oxygen-vacancy Al³⁺ and Fe³⁺ substitutions. Primitive mantle with about 5 mol% AlO_1.5 and an Fe³⁺/(Fe³⁺+Fe²⁺) ratio of ∼0.5 is expected to be comprised of ferropericlase coexisiting with Mg-perovskite that has a considerable component of Al³⁺ and Fe³⁺ defect substitutions at conditions of the uppermost lower mantle. Increased pressure may favor charge-coupled substitution reactions over vacancy forming reactions, such that a region could exist in the lower mantle with a gradient in substitution mechanisms. In this case, we expect the physical and transport properties of Mg-perovskite to change with depth, with a softer, probably more hydrated, defect dominated Mg-perovskite at the top of the lower mantle, grading into a stiffer, dehydrated, charge-coupled substitution dominated Mg-perovskite at greater depth.     

Elasticity of (Mg, Fe)(Si, Al)O3-perovskite at high pressure

The most abundant mineral on Earth has a perovskite crystal structure and a chemistry that is dominated by MgSiO₃ with the next most abundant cations probably being aluminum and ferric iron. The dearth of experimental elasticity data for this chemically complex mineral limits our ability to calculate model seismic velocities for the lower mantle. We have calculated the single crystal elastic moduli (c_ij) for (Mg, Fe^3 +)(Si, Al)O₃ perovskite using density functional theory in order to investigate the effect of chemical variations and spin state transitions of the Fe³⁺ ions. Considering the favored coupled substitution of Mg²⁺-Si^4 + by Fe³⁺-Al³⁺, we find that the effect of ferric iron on seismic properties is comparable with the same amount of ferrous iron. Ferric iron lowers the elastic moduli relative to the Al charge-coupled substitution. Substitution of Fe³⁺ for Al³⁺, giving rise to an Fe/Mg ratio of 6%, causes 1.8% lower longitudinal velocity and 2.5% lower shear velocity at ambient pressure and 1.1% lower longitudinal velocity and 1.8% lower shear velocity at 142 GPa. The spin state of the iron for this composition has a relatively small effect (< 0.5% variation) on both bulk modulus and shear modulus.     

Pickling Waste as an Inexpensive Iron Source for Fenton Oxidation of Synthetic Dye Bath Waste

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.     

Morphology and composition of spinel in Pu’u ’O’o lava (1996–1998), Kilauea volcano, Hawaii

The morphology and composition of spinel in rapidly quenched Pu’u ’O’o vent and lava tube samples are described. These samples contain glass, olivine phenocrysts (3–5 vol.%) and microphenocrysts of spinel (0.05 vol.%). The spinel surrounded by glass occurs as idiomorphic octahedra 5–50 μm in diameter and as chains of octahedra that are oriented with respect to each other. Spinel enclosed by olivine phenocrysts is sometimes rounded and does not generally form chains. The temperature before quenching was calculated from the MgO content of the glass and ranges from 1150°C to 1180°C. The oxygen fugacity before quenching was calculated by two independent methods and the log f_O2 ranged from −9.2 to −9.9 (delta QFM=−1). The spinel in the Pu’u ’O’o samples has a narrow range in composition with Cr/(Cr+Al)=0.61 to 0.73 and Fe²⁺/(Fe²⁺+Mg)=0.46 to 0.56. The lower the calculated temperature for the samples, the higher the average Fe²⁺/(Fe²⁺+Mg), Fe³⁺ and Ti in the spinel. Most zoned spinel crystals decrease in Cr/(Cr+Al) from core to rim and, in the chains, the Cr/(Cr+Al) is greater in the core of larger crystals than in the core of smaller crystals. The occurrence of chains and hopper crystals and the presence of Cr/(Cr+Al) zoning from core to rim of the spinel suggest diffusion-controlled growth of the crystals. Some of the spinel crystals may have grown rapidly under the turbulent conditions of the summit reservoir and in the flowing lava, and the crystals may have remained in suspension for a considerable period. The rapid growth may have caused very local (μm) gradients of Cr in the melt ahead of the spinel crystal faces. The crystals seem to have retained the Cr/(Cr+Al) ratio that developed during the original growth of the crystal, but the Fe²⁺/(Fe²⁺+Mg) ratio may have equilibrated fairly rapidly with the changing melt composition due to olivine crystallization. Six of the samples were collected on the same day at various locations along a 10-km lava tube and the calculated pre-collection temperatures of the samples show a 5°C drop with distance from the vent. The average Fe²⁺/(Fe²⁺+Mg) of the spinel in these samples shows a weak positive correlation with decreasing MgO in the glass of these samples. The range in Cr₂O₃ (0.041–0.045 wt.%) of the glass for these six samples is too small to distinguish a consistent change along the lava tube. The spinel in the Pu’u ’O’o samples shows a zoning trend in a Cr–Al–Fe³⁺ diagram almost directly away from the Cr apex. This compares with a zoning trend in rapidly quenched MORB samples away from Cr coupled with decreasing Fe³⁺. The trend away from Cr displayed by spinel in rapidly quenched samples is in marked contrast to the trend of increasing Fe³⁺ shown by spinel in slowly cooled lava.     

Ab initio study of the composition dependence of the pressure-induced spin crossover in perovskite (Mg1 − x,Fex)SiO3

We present ab initio calculations of the zero-temperature iron high- to low-spin crossover in (Mg_1 ? xFe_x)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.     

Copper- and iron-induced injuries in roots and rhizomes of reed (Phragmites australis)

About 1 mg/g dw Cu²⁺ and 8 mg/g dw Fe²⁺ were found in roots of reed plants when fed with heavy metal concentrations of 100 μM Cu²⁺ and 10 mM Fe²⁺ under hypoxia. Roots seemed to act as a kind of filter since the amounts in rhizomes were only 0.06 mg Cu²⁺/g dw and 2 mg Fe²⁺/g dw. Increased contents of both ions reduced posthypoxic respiration capacity by 40–50% and also the sum of adenylates (ATP, ADP, AMP) by the same order of magnitude, although energy charge values remained above 0.85 in Cu²⁺ and 0.79 in Fe²⁺ treatments. Energy metabolism of rhizomes was not affected. Copper and iron contents of roots as well as of rhizomes were high enough to induce oxidative stress when roots were fed with 40 μM Cu²⁺ and 1 mM Fe²⁺, respectively.From our results we conclude that increased, but environmentally attainable, amounts of copper and reduced iron ions disturb root energy metabolism, and therefore root functioning and development. Latent injuries, based on oxidative stress, may be harmful for roots and rhizomes under long term exposure.     

Transport processes of copper and zinc in a highly eutrophic and meromictic lake

Lake Baldegg was chosen to study the influence of a permanent anaerobic hypolimnion on the transport of copper and zinc. The evaluation of concentration variations in function of time and depth and the sedimentation rates lead to the conclusion that copper, in contrast to zinc, is sorbed additionally on newly formed iron particles at the Fe³⁺/Fe²⁺ interface and transported on the ‘iron wheel’. A flux scheme for the hypolimnic transport of copper is quantified in a one-dimensional diffusion model. By this the observed variations of concentration profiles can be explained.     

Chemical variations in assemblages including epidote and/or chlorite in the fossil hydrothermal system of Saint Martin (Lesser Antilles)

Epidote and/or chlorite are common minerals in the roots of the fossil geothermal system of Saint Martin (Lesser Antilles). They appear in four distinct assemblages: (1) epidote+actinolite+quartz±magnetite near the contact between the tuffaceous host rocks (andesitic modal composition) and the quartz-diorite intrusion of Philipsburg; (2) epidote+chlorite+quartz in host rocks as far as a lateral distance of about 3 km from the intrusion; (3) epidote+chlorite+haematite+quartz locally in iron and manganese rich host rocks; (4) chlorite±phengite±magnetite appearing as late sealing of porosity in fracture-controlled quartz veins with strongly phengitized wall rocks. All these assemblages constitute a large alteration grading from propylitic alteration to thermal metamorphism (actinolite-bearing assemblage).Detailed microprobe studies of epidotes replacing plagioclases and of chlorites replacing glass and mafic minerals reveal notable compositional variations which have been studied with respect to temperature paleogradients (estimated from fluid inclusions study), distance from the thermal source and f_o2 conditions. The mean Ps+Pm [100 × (Fe³⁺ + Mn³⁺)/(Al³⁺ + Fe³⁺ + Mn³⁺)] of epidotes vary from 21 in the presence of magnetite near the intrusion to 32 in haematite-bearing iron and/or manganese volcanic and sedimentary formations. The intra-grain chemical scattering of epidotes increases with increasing distance of the pluton and decreasing temperature of crystallization. All the chlorites coexisting with epidote are Mg-rich (X_Fe<0.50). Their main compositional variation consists in a significant enrichment in magnesium (toward the chlinochlore end member) in presence of haematite. The intra-grain chemical scattering of chlorite (expressed by the aluminium content in the structural formula) increases with increasing distance of the pluton and decreasing temperature of crystallization. Chlorites associated with phengite and magnetite in vein alteration are Fe- and Al-rich. The Mössbauer spectra indicate that the Fe³⁺ content of chlorite varies between 25 and 32% of total Fe in the presence of epidote; the higher content being attained in the presence of haematite. The Fe³⁺ content of chlorite associated with magnetite and phengite is 16% of total Fe. The compositional variations of epidote and/or chlorite of the four distinct assemblages observed at Saint Martin result from the combined effects of f_O2, temperature, and time of heating. The effect of f_O2 is particularly perceptible in the control of the epidote Ps content, of the chlorite X_Fe ratio of Fe³⁺ distribution between coexisting epidotes and chlorites. Despite the fact that it may be partially canceled out by the effect of f_O2, the variation of compositional ranges of both epidotes and chlorites, which increases toward the outer part of the geothermal system in response to the combination of decreasing temperatures and decreasing time of heating of the rocks, suggests that chemical equilibrium has not been attained in the assemblages bearing epidotes and chlorites.     

Self-reversal of magnetization in titanomagnetite: Effective field theory

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.