Fetot Ratio Research Articles

Phase transformation and textural properties of an unpromoted iron Fischer–Tropsch catalyst

The phase transformations and textural properties were systematically investigated over an unpromoted iron Fischer–Tropsch synthesis (FTS) catalyst prepared from ferrous sulfate with different Fe 3+/Fe total ratios at different preparation stages (precipitation, dryness and calcination). The catalyst samples were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy and N 2 physisorption. The characterization studies show that after precipitation the samples are composed of α-FeOOH, Fe 3O 4 and α-Fe 2O 3. The content of Fe 3O 4 increases with the increase of Fe 3+/Fe total ratio ( R m) and reaches the maximum content at the R m of 0.72. There is not a significant phase transformation during the dryness of the catalyst samples. After being calcined at 500 °C for 5 h, α-FeOOH and Fe 3O 4 existed in the catalyst samples are transformed into α-Fe 2O 3. The characterization results from N 2 physisorption show that when the R m of sample is 0.72, the BET surface area and pore volume have the maximum values (26 m 2/g, 0.21 cm 3/g) after the samples being calcined at 500 °C.

Structural properties of Cr2O3–Fe2O3–P2O5 glasses, Part I

The structural properties of xCr2O3–(40−x)Fe2O3–60P2O5, 0⩽x⩽10 (mol%) glasses have been investigated by Raman and Mössbauer spectroscopies, X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The Raman spectra show that the addition of up to 5.3mol% Cr2O3 does not produce any changes in the glass structure, which consists predominantly of pyrophosphate, Q1, units. This is in accordance with O/P≈3.5 for these glasses. The increase in glass density and Tg that occurs with increasing Cr2O3 suggests the strengthening of glass network. The Mössbauer spectra indicate that the Fe2+/Fetot ratio increases from 0.13 to 0.28 with increasing Cr2O3 content up to 5.3mol%, which can be related to an increase in the melting temperature from 1423 to 1473K. After annealing, the 10Cr2O3–30Fe2O3–60P2O5 (mol%) sample was partially crystallized and contained crystalline β-CrPO4 and Fe3(P2O7)2. The SEM and AFM micrographs of the partially crystallized sample revealed randomly distributed crystals embedded in a homogeneous glass matrix. EDS analysis indicated that the glass matrix was rich in Fe2O3 (39.6mol%) and P2O5 (54.9mol%), but contained only 5.5mol% of Cr2O3. These results suggest that the maximum solubility of chromium in these iron phosphate melts is 5.5mol% Cr2O3.

Chemical compositions and XANES speciations of Fe, Mn and Zn from aerosols collected in China and Japan during dust events

Variations of the chemical composition and elemental speciation of aeolian dust transported from China to Japan were investigated in this study. During 6‐12 April 2002, a large-scale dust event was observed in China, Korea, and Japan. Aerosol samples collected in Beijing, Fukuoka, Nagoya, and Tsukuba at that time were used for this study. Variations of most elemental concentrations against the particle size showed similar trends to that of Al concentration. The particle size distributions indicate that myriad elements in the aerosols originated mainly from mineral aerosols. The metal/Al concentration ratios showed little difference in chemical compositions for aerosols sampled at Beijing and Japanese stations: the chemical composition of aeolian dust is conserved during transportation. Examination by X-ray absorption near-edge structure (XANES) spectroscopy revealed that the Fe(III)/Total Fe (T-Fe) ratios of the aerosols collected in Beijing and Fukuoka were mutually resemblant, indicating that the oxidation-reduction reaction of Fe in aeolian dust during its transportation is negligible. However, their ratios were much higher than those of China loess and simulated Asian mineral dust (Chinese geo-reference materials: CJ-1 and CJ-2). Because the aerosol grain size is much smaller than those of CJ-1 and CJ-2, the fine mineral was transported selectively by wind from top soils. It was easily oxidized and had a high Fe(III)/TFe ratio. The XANES spectra of Mn in the aerosol indicated that the Mn(III)/Total Mn (T-Mn) ratios of the aerosol collected in Fukuoka were smaller than those of the dust collected in Beijing. The ratios of the aerosols in Beijing resembled those of CJ-1 and CJ-2. Moreover, only medium grains of the aerosol in Fukuoka had high Mn(III). The other grains had an almost identical Mn(III)/T-Mn ratio as the aerosol collected in Fukuoka in the no event. Thereby, most medium grain aerosols collected in Fukuoka comprised aeolian dust coming from China with high Mn(III)/T-Mn ratios like those of CJ-1 and CJ-2, but the other grain aerosols mixed greatly with local aerosols having low Mn(III)/T-Mn ratios. On the other hand, some elements such as Zn and Pb had different Al concentration ratios from Mn and Fe. The Zn/Al and Pb/Al concentration ratios of fine grains increased tenfold to a hundredfold with the decrease of particle size. Therefore, the increasing trend for these elements indicates that the fraction of anthropogenic materials was greatly increased in fine grains. The XANES spectra of Zn suggested that the contribution of mineral aerosols decreased gradually with decreasing particle size and that the residual components were explained by Zn(NO 3 ) 2 in coarse-medium grains and ZnSO 4 in fine grains. However, Zn in aeolian dust, which is a main component of mineral aerosols, only rarely originated from anthropogenic materials when a large-scale dust event was observed.

Dielectric behavior and impedance spectroscopy of bismuth iron phosphate glasses

The electrical and dielectrical properties of Bi2O3–Fe2O3–P2O5 glasses were measured by impedance spectroscopy in the frequency range from 0.01Hz to 4MHz and over the temperature range from 303 to 473K. It was shown that the dc conductivity strongly depends on the Fe2O3 content and Fe(II)/Fetot ratio. With increasing Fe(II) ion content from 17% to 34% in the bismuth-free 39.4Fe2O3–59.6P2O5 and 9.8Bi2O3–31.7Fe2O3–58.5P2O5 glasses, the dc conductivity increases. On the other hand, the decrease in dc conductivity for the glasses with 18.9mol% Bi2O3 is attributed to the decrease in Fe2O3 content from 31.7 to 23.5mol%, which indicates that the conductivity for these glasses depends on Fe2O3 content. The conductivity for these glasses is independent of the Bi2O3 content and arises mainly from polaron hopping between Fe(II) and Fe(III) ions suggesting an electronic conduction. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency the dispersion was investigated in terms of dielectric loss. The thermal activated relaxation mechanism dominates the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron hopping between iron ions. The Raman spectra show that the addition of up to 18.9mol% of Bi2O3 does not produce any changes in the glass structure which consists predominantly of pyrophosphate units.

Studies of lead–iron phosphate glasses by Raman, Mössbauer and impedance spectroscopy

The effect of Fe2O3 content on electrical conductivity and glass stability against crystallization in the system PbO–Fe2O3–P2O5 has been investigated using Raman, XRD, Mössbauer and impedance spectroscopy. Glasses of the molar composition (43.3−x)PbO–(13.7+x)Fe2O3–43P2O5 (0⩽x⩽30), were prepared by quenching melts in the air. With increasing Fe2O3 content and molar O/P ratio there is corresponding reduction in the length of phosphate units and an increase in the Fe(II) ion concentration, which causes a higher tendency for crystallization. Raman spectra of the glasses show that the interaction between Fe sites, which is essential for electron hopping, strongly depends on the cross-linking of the glass network. The electronic conduction of these glasses depends not only on the Fe(II)/Fetot ratio, but also on easy pathways for electron hopping in a non-disrupted pyrophosphate network. The Raman spectra of crystallized glasses indicate a much lower degree of cross-linking since more non-bridging oxygen atoms are present in the network. Despite the significant increase in the Fe2O3 content and Fe(II) ion concentration, there is a considerable weakening in the interactions between Fe sites in crystalline glasses. The impedance spectra reveal a decrease in conductivity, caused by poorly defined conduction pathways, which are result of the disruption and inhomogeneity of the crystalline phases that are formed during melting.

Ferrous hydroxy carbonate is a stable transformation product of biogenic magnetite

An ~1:1 mixture of ferrihydrite and nanocrystalline akaganeite (β-FeOOH; 10.15 nm) was incubated with Shewanella putrefaciens (strain CN32) under anoxic conditions with lactate as an electron donor and anthraquinone-2,6-disulfonate (AQDS) as an electron shuttle. The incubation was carried out in a 1,4-piperazinediethanesulfonic acid (PIPES)-buffered medium, without PO43- at circumneutral pH. Iron reduction was measured as a function of time (as determined by 0.5 N HCl extraction), and solids were characterized by X-ray diffraction (XRD), electron microscopy, and Mössbauer spectroscopy. The biogenic reduction of Fe3+was rapid; with 60% of the total Fe (FeTOT) reduced in one day. Only an additional 10% of FeTOT was reduced over the next three years. A fine-grained (~10 nm), cation-excess (CE) magnetite with an Fe2+/FeTOT ratio of 0.5-0.6 was the sole biogenic product after one day of incubation. The CE magnetite was unstable and partially transformed to micrometer-sized ferrous hydroxy carbonate [FHC; Fe2(OH)2CO3(s)], a rosasite-type mineral, with time. Ferrous hydroxy carbonate dominated the mineral composition of the three year incubated sample. The Fe2+/FeTOT ratio of the residual CE magnetite after three years of incubation was lower than the day 1 sample and was close to that of the stoichiometric magnetite (0.33). To the best of our knowledge, this is the first report of biogenic FHC, and was only reported twice in literature but in a very different context. Ferrous hydroxy carbonate appeared to form by slow reaction of microbially produced carbonate with Fe2+-excess magnetite. The FHC may be an overlooked mineral phase that explains the infrequent occurrence of fine-grained, biogenic magnetite in anoxic sediments.

The relationships between crystal-chemical and hyperfine parameters in members of the astrophyllite-group: A combined57Fe Mossbauer spectroscopy and single-crystal X-ray diffraction study

A suite of 22 members of the heterophyllosilicate astrophyllite-group from SiO 2 -over-and undersaturated igneous rocks were studied by single-crystal X-ray diffraction, wave-length dispersive electron microscopy and room temperature 57 Fe Mossbauer spectroscopy. This is the first study of its kind for this mineral group and presents the relationships between average and distribution-related crystallographic and hyperfine parameters. The room temperature, thin-limit, thickness-corrected 57 Fe Mossbauer spectra of astrophyllite-group minerals are characterized by two strong absorption peaks centered at ≈ −0.1 and 2.3 mm/s and a third, weaker shoulder at ≈ 0.9 mm/s, corresponding to (1) the sum of the low-energy lines from [6] Fe 2+ and [6] Fe 3+ doublets, (2) the high-energy lines from [6] Fe 2+ doublets, and (3) the high-energy lines from [6] Fe 3+ doublets, respectively. Average centre shift values (〈CS〉) for [6] Fe 2+ range from 1.124 to 1.154 mm/s and average quadrupole splitting (〈QS〉) values from 2.202 to 2.416 mm/s. No evidence for [4] Fe 3+ was found in the samples studied. Fe 3+ /Fe tot ratios range from 0.01 to 0.21, corresponding to 0.05 to 0.56 apfu Fe 3+ . A general trend in quadrupole splitting distributions (QSDs) is observed from narrow distributions with large 〈QS〉 and QS peak values in Fedominant samples, to broad distributions with small 〈QS〉 and QS peak values in Mn-dominant samples. The low edge of the distribution moves towards higher values as the width of the distribution decreases, while the high edge remains relatively constant. This behavior has been noted in experimental QSDs of many layered silicates, and has been shown by ab inito electronic structure calculations to be the result of a combination of a maximum in the QS versus octahedral flattening (ψ) curve and a maximum in the electric field gradient due to chemical disorder in the first nearest-neighbour octahedra. In both astrophyllite-group minerals and trioctahedral micas, the dominant octahedral structural parameter affecting the QSD is ψ. The presence of the D ϕ 6 bridging octahedron in the astrophyllite structure imposes an additional distortion effect on the sheet of octahedra, resulting in two distinct QSD populations based on the Zr content of the sample. Zr-rich samples (Zr > 0.40 atoms per formula unit, apfu ) have overall larger QS compared to Zr-deficient samples (Zr apfu ). This shift in QSDs of Zr-rich samples to higher average values corresponds to a higher maximum local distortion environment. The existence of such a variation in local distortion environments is further supported by the existence of a discontinuous relationship in chemical and structural parameters (Mossbauer, single-crystal X-ray diffraction and EMPA) between astrophyllite-group minerals which crystallized in hydrothermal (Zr-deficient) versus magmatic or post-magmatic (Zr-deficient/Nb-rich) environments.

Oxidation of mudstone in a tunnel (Tournemire, France): consequences for the mineralogy and crystal chemistry of clay minerals

AbstractThe excavation of a tunnel through a mudstone formation provides an opportunity to examine the effects of the modification of the physical and chemical environment on the rock. The mineralogical and chemical consequences of hydration-dehydration cycles and of oxidation have been evaluated in the case of the Toarcian mudstone formation at the Tournemire experimental site (France). Studies by X-ray diffraction and tansmission electron microscopy of both altered and preserved samples show that the introduction of air and condensed water causes the oxidation of pyrite and the subsequent generation of acid and sulphate-rich waters at the micron scale, in the local environment of pyrites. The fluid-clay particle interactions around the oxidized pyrites induce: (1) a statistical enrichment in Si of the I-S clay minerals; (2) an increase in the Fe(III)/Fe total ratio in some of the I-S particles; and (3) the dissolution of illite layers in mixed-layer I-S. These evolutions are consistent with the results of numerical modelling which reproduced the interaction between the clay particles and the acid water.

Seasonal and spatial distributions and dry deposition fluxes of atmospheric total and labile iron over the tropical and subtropical North Atlantic Ocean

The tropical and subtropical region of the North Atlantic Ocean is under the influence of mineral dust plumes transported from North Africa, resulting in high atmospheric iron (Fe) deposition. Atmospheric Fe, as a major Fe source to this open oceanic region, may influence the rate of nitrogen fixation since Fe is a critical nutrient cofactor for the nitrogenase enzyme in diazotrophic microorganisms. Field measurements of Fe concentrations and Fe speciation in aerosols provide crucial information to understanding the biological role of this atmospheric Fe flux to the ocean. In this study, 24‐hour aerosol samples were collected during winter (6 January 2001 to 18 February 2001) and summer (27 June 2001 to 15 August 2001) research cruises over this North Atlantic region. Three labile Fe species, that included ferrous iron (Fe(II)) and reducible ferric iron (Fe(III)) species, were measured in aerosol samples using a sequential aqueous extraction method. Microwave digestion of the aerosol samples followed by inductively coupled plasma‐mass spectrometry was used to quantify total elemental concentrations. A spatial gradient of over nearly 3 orders of magnitude in the total Fe concentrations (from 1.6 ng m−3 at 28.6°N to 1688 ng m−3 at 10.9°N) was observed during the winter, while this gradient was not as strong in the summer. The mean total Fe concentrations were approximately a factor of 2 higher during the winter (mean value of 670 ng m−3 between 5°N and 26°N) than in the summer (mean value of 324 ng m−3 between 6°N and 26°N). The highest percentage of labile Fe to total Fe was observed between 26°N and 30°N in the winter with a mean value of 32%, which corresponded to low concentrations of total Fe. At latitude 0°N to 10°N, where the lowest Fe concentrations were observed in the summer, the labile Fe fraction with a mean of 5.0% was similar to the region from 10°N to 20°N where there were much higher mineral aerosol concentrations. Air mass back trajectories showed that mineral dust transport from North Africa is a significant atmospheric Fe source in this Atlantic region. However, the highest labile Fe to total Fe ratios were observed in air masses that had been over the ocean for greater than 7 days and also corresponded to high ratios of non‐seasalt‐sulfate (NSS‐sulfate) to total Fe and oxalate to total Fe. The correlation with NSS‐sulfate and oxalate suggests that labile Fe concentrations may have been influenced by anthropogenic activities from North America or Europe.

Experimental evidence for the existence of iron-rich metal in the Earth's lower mantle.

The oxidation state recorded by rocks from the Earth's upper mantle can be calculated from measurements of the distribution of Fe3+ and Fe2+ between the constituent minerals. The capacity for minerals to incorporate Fe3+ may also be a significant factor controlling the oxidation state of the mantle, and high-pressure experimental measurements of this property might provide important insights into the redox state of the more inaccessible deeper mantle. Here we show experimentally that the Fe3+ content of aluminous silicate perovskite, the dominant lower-mantle mineral, is independent of oxygen fugacity. High levels of Fe3+ are present in perovskite even when it is in chemical equilibrium with metallic iron. Silicate perovskite in the lower mantle will, therefore, have an Fe3+/total Fe ratio of at least 0.6, resulting in a whole-rock ratio of over ten times that of the upper mantle. Consequently, the lower mantle must either be enriched in Fe3+ or Fe3+ must form by the disproportionation of Fe2+ to produce Fe3+ plus iron metal. We argue that the lower mantle contains approximately 1 wt% of a metallic iron-rich alloy. The mantle's oxidation state and siderophile element budget have probably been influenced by the presence of this alloy.

Study of electrical properties of MoO 3–Fe 2O 3–P 2O 5 and SrO–Fe 2O 3–P 2O 5 glasses by impedance spectroscopy. II

The electrical and dielectric properties for three series of MoO 3–Fe 2O 3–P 2O 5 and one series of SrO–Fe 2O 3–P 2O 5 glasses were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and over the temperature range from 303 to 473 K. It was shown in Part I that the MoO 3 is incorporated into phosphate network and the structure/properties are strongly influenced by the overall O/P ratio. The Fe 2O 3 content and Fe(II)/Fe tot ratio in these glasses have significant effects on the electrical conductivity and dielectric permittivity. With decreasing Fe 2O 3 content in MoO 3–Fe 2O 3–P 2O 5 glasses with O/P at 3.5 the dc conductivity, σ dc( ω) decreases for two orders of magnitude, which indicates that the conductivity for these glasses depends on Fe 2O 3 and is independent of the MoO 3 content. Also, the dielectric properties such as ε ′( ω), ε ″( ω) and σ ac( ω) and their variation with frequency and temperature indicates a decrease in relaxation intensity with increase in the concentration of MoO 3. On the other hand, the dc conductivity for MoO 3–Fe 2O 3–P 2O 5 glasses with O/P > 3.5 increases with the substitution of MoO 3 which has been explained by an increase in the number of non-bridging oxygens and formation of Fe–O–P bonds that are responsible for formation of small polarons. The increase in the dielectric permittivity, ε ′( ω) with increasing MoO 3 content is attributed to the increase in the deformation of glass network with increasing bonding defects. For SrO–Fe 2O 3–P 2O 5 glasses the conductivity and dielectric permittivity remained constant with increasing SrO.

Micro-XANES determination of ferric iron and its application in thermobarometry

Micro-X-ray absorption near-edge structure (XANES) analysis was employed to determine the content of ferric iron in minerals formed in ultrahigh-pressure (UHP) eclogites. It is observed that omphacite and phengite contain significant amounts of Fe 3+/Fe tot (0.2–0.6), whereas only very low contents are present in garnet (Fe 3+/Fe tot=0.0–0.03), the latter being consistent with results from stoichiometric charge-balance calculations. Furthermore, considerable variations in the Fe 3+/Fe tot ratios of omphacite and phengite are observed depending on the textural sites and local bulk chemistry (eclogite and calc-silicate matrix) within one thin section. The oxidation state of isofacial minerals is thus likely to depend on the local fluid composition, which, in the studied case, is controlled by calcareous and meta-basic mineral compositions. These first in-situ measurements of ferric iron in an eclogite sample from the Dabie Shan, E China, are used to recalculate geothermobarometric data. Calculations demonstrate that the temperature during UHP metamorphism was as high as 780 °C, about 80–100 °C higher than previously estimated. Temperatures based on charge balance calculations often give erroneous results. Pressure estimates are in good agreement with former results and confirm metamorphism in the stability field of diamond (43.7 kbar at 750 °C). These P– T data result in a geothermal gradient of ca. 6 °C/km during UHP metamorphism in the Dabie Shan. However, accounting for ferric iron contents in geothermobarometry creates new difficulties inasmuch as calibrations of geothermometers may not be correctable for Fe 3+ and the actual effect on Mg–Fe 2+ partitioning is unknown. The present study further shows that micro-XANES is a promising technique for the in situ determination of ferric iron contents without destroying the textural context of the sample: a clear advantage compared to bulk methods.

Phase transitions in the SrSnO 3–SrFeO 3 solid solutions: X-ray diffraction and Mössbauer studies

SrSn 1− x Fe x O y (0⩽ x⩽1) oxides were prepared by conventional solid-state reaction in air using high-purity (⩾99%) SrCO 3, SnO 2 and Fe(C 2O) 2·2H 2O at elevated temperatures of 1300°C for 24 h and furnace cooled. Samples obtained from 1300°C were annealed at 620°C for 2 days and quenched in liquid nitrogen (LN). Powder XRD analysis by Rietveld refinement and Fe Mössbauer spectroscopy measurements were employed to characterize synthesized perovskites. Samples obtained from furnace cooled and LN quenched undergo two compositionally driven phase transitions, which are supposed to be of second order. The x=0–0.3 members crystallize in orthorhombic parent SrSnO 3 structure (Space group Pbnm), whereas samples x=0.4–0.9 have a simple cubic perovskite cell and end-member SrFeO 2.74 composition crystallize orthorhombic structure (Space group Cmmm). The composition of the first phase transition ( x≈0.3) is slightly shifted to higher x with decreasing annealing temperature. Mössbauer data show that the Fe 4+/Fe tot ratio is depending on composition under constant synthesis conditions. The phase compositions have been discussed in terms of ternary solid solution of compounds SrSnO 3–SrFeO 2.74–SrFeO 2.5 superior to a simple binary solid solution (SrSnO 3–SrFeO 3).

Iron photochemistry in cloud droplets at the Puy de Dôme

Iron play a significant role in atmosphere. Its reactivity depends on oxidation state and speciation. In order to study photochemical cycle of iron in atmospheric waters, we collected cloud samples at the research station of puy de Dome. We found very low levels: iron concentrations are ranging from 0.2 μM to 3.4 μM. The light is not an important factor towards affecting the oxidation state, in contrast with many field studies, where clear dependence from light intensity was shown. Contrary to recent studies and to chemical models iron in the cloud droplets at Puy de Dome is mostly present as Fe(II): the ratio Fe(II)/Fe TOT is 0.77 on average. No clear relationship is found between Fe and carboxylic acids. Same samples are irradiated between 300 and 450 nm in a Teflon-made reactor: H 2 O 2 , TOC, Fe 2+ , Fe 3+ and pH are continuously monitored during the irradiation. We observed no tendency for the Fe(II)/Fe TOT ratio to increase or decrease and the DOC is not strongly varying during the irradiation. In consequence of the stability of the Fe(II)/Fe(III) ratio, we can assume that cloud water contains compounds that undergo the action of oxidants, and possibly, organic substances play an important role in this processus.

Selected nutrients and iron in interstitial waters of the estuary of southern Baltic (Gulf of Gdañsk and the Pomeranian Bay) in relation to redox potential

The following parameters were determined in the interstitial waters of the Gulf of Gdansk and the Pomeranian Bay: nutrients (ammonia and phosphates), sulphates, total iron and iron(II) concentrations. Redox potential (Eh), pH, humidity and organic matter content measured as a loss on ignition were determined in bottom sediments from these regions. Interstitial waters of both regions were found to have reducing character (Eh < 250 mV). Reduction processes were more advanced in the Gulf of Gdansk, rich in organic matter, as evidenced by lower values of the redox potential, lower Fe(III)/Fetot ratio, and incidents of sulphates exhaustion.

Experimental alteration of vivianite to lepidocrocite in a calcareous medium

AbstractVivianite [Fe3(PO4)2·8H2O] is easily oxidized in the presence of air. In this work, we studied the oxidation and incongruent dissolution of vivianite in a calcareous medium containing an anion-exchange resin (AER) that acted as a sink for phosphate. Freshly prepared vivianite suspensions with calcite sand and an AER membrane were oxidized and stirred by bubbling air or CO2-free O2. Experiments were finished when oxidation rate and P removal by the AER became slow, which was at 53 days (air system) or 28 days (CO2free O2 system). At these times, the respective values of the Fe(II)/total Fe ratio were 0.29 and 0.12, and the respective values of the atomic P/Fe ratio were 0.15 and 0.11. The final product of the oxidation was poorly crystalline lepidocrocite in the form of thin (1 ­ 5 nm), irregular lamellae that were soluble in acid oxalate. The unit-cell edge lengths of this lepidocrocite were a= 0.3117, b= 1.2572, and c= 0.3870, vs. a= 0.3071, b= 1.2520 and c= 0.3873 nm for the reference lepidocrocite. The lepidocrocite lamellae contained occluded P (P/Fe atomic ratio = 0.03 ­ 0.04). The results of this and a previous study made us hypothesize that this occluded P is structural and occupies tetrahedral sites adjacent to the empty octahedral sites in the sheets extending on ﹛010﹜.

Infrared Study of Reduced and Reduced-Reoxidized Ferruginous Smectite

AbstractOxidation-reduction processes within natural systems greatly influence the properties of sediments, soils and clays. The objective of this experimental study was to gather new evidence for the effects of changes in redox conditions (reduction and reoxidation) on structural properties of ferruginous smectite and to understand better the mechanisms involved. The &lt;2 µm fraction of a ferruginous smectite (sample SWa-1), which contains 17.3 wt.% of total structural Fe, was studied by infrared (IR) spectroscopy. The pure Na-saturated clay was reduced by Na dithionite for 10 to 240 min to obtain various Fe(II):(total Fe) ratios ranging from 0 to 1.0. Selected reduced samples were then reoxidized completely by bubbling O2 gas through the suspensions for up to 12 h. Infrared spectra of the initially unaltered, reduced and reduced-reoxidized samples were collected. Reduction generated changes in the three studied spectral regions (O-H stretching, M-O-H deformation, and Si-O stretching), indicating that major modifications occurred within the clay crystal beyond merely a change in Fe oxidation state. partial dehydroxylation and redistribution of Fe, and perhaps Al, cations occurred upon reduction of SWa-1, changing the structural properties of its tetrahedral and octahedral sheets. Water molecules, probably generated by dehydroxylation within the octahedral sheet upon reduction, were tightly bound to the clay surface and were possibly trapped within the clay structure. Except for dehydroxylation and the Fe oxidation state, all these modifications were largely irreversible. The tightly bound water was not completely removed upon reoxidation and the cationic rearrangements generated during reduction were not reversed: either they were preserved as in the reduced state or cations were redistributed into a different configuration from the unreduced clay.

La, Ca and Fe oxide perovskites: preparation, characterization and catalytic properties for methane combustion

La1−xCaxFeO3 (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K have been investigated as catalysts for methane combustion. The formation of the perovskite structure has been shown by X-ray diffraction (XRD) for all samples. The surface area (SA) values are in the range 3–6m2g−1 for the samples up to x=0.4, whereas SA is 0.7m2g−1 for the specimen with x=0.5. The atomic susceptibility increases with increasing in Ca content. The Fe4+/Fetotal ratio has been determined by both redox titration and TPR analysis. The amount of Fe4+ enhances with Ca substitution but the Fe4+/Ca2+ ratio is quite constant giving rise to an increasing formation of oxygen vacancies to preserve charge neutrality. All perovskites show a similar intrinsic activity in CH4 combustion with activation energy, Ea, of about 23kcalmol−1. A slightly lower value of Ea is shown by LaFeO3 which, in contrast with Ca substituted samples, exhibits a small cationic defectivity. Methane reaction order lower than one was found for all catalysts.

Thermodynamic analysis of Fe and Mg partitioning between plagioclase and silicate liquid

Thermodynamic analysis of Fe- and Mg-bearing plagioclase and silicate liquid was carried out based on reported element partitioning data between plagioclase and silicate liquid in reduced conditions, solution properties of ternary feldspar, standard state properties of plagioclase endmembers and solution properties of multicomponent silicate liquid. Derived mixing properties of Fe- and Mg-bearing plagioclase are in harmony with estimated results from synthetic experiments in the systems CaAl2Si2O8-CaFeSi3O8 and CaAl2Si2O8-CaMgSi3O8. Based on the determined solution properties of the plagioclase, a computer program to calculate the element partition relationships between Fe- and Mg-bearing plagioclase and multicomponent silicate liquid was developed. The FeO, MgO and MgO/(MgO + FeO) in plagioclase predicted from known liquid compositions and pressure are in agreement with measurements within 0.2 wt%, 0.1 wt% and 0.1 (mol ratio), respectively. The Fe3+ content in plagioclase crystallized at high oxygen fugacity can be estimated with this program. The Fe3+/total Fe ratio in plagioclase crystallized near the quartz-fayalite-magnetite buffer ranges from 0 to 0.5, which is consistent with previous study on natural plagioclase in submarine basalt. Derived solution properties of the Fe- and Mg-bearing plagioclase are also used to calculate equilibrium composition relationship between olivine and plagioclase. Change of XFo in olivine coexisting with plagioclase affects MgO and FeO contents in plagioclase greatly. The present model predicts XFo of coexisting olivine from the chemical composition of plagioclase to ±0.1 accuracy at given pressure and temperature.

Oxygen Evolution and Chemical State Changes of Iron during Firing of Red Clay Bodies at Low Oxygen Pressure.

A red clay body containing 7mass% Fe2O3 was fired at PO2=0.1kPa and its bloating and blackening mechanisms were investigated.Blackening started at around 1100°C and the Fe2+/total Fe ratio in the fired body was about 5%. The disappearance of open pores started at about 1200°C, and the sealed porosity began to increase above 1200°C. The Fe2+/total Fe ratio in the body fired at 1200°C was 16%, indicating that a considerable amount of oxygen had been released. Although the evolution of oxygen started at around 1100°C as judged from the results of chemical analysis of Fe, the evolution of oxygen was detected only at around 1080°C and at the soaking temperatures probably due to the high heating rate (5°C/min).The results of thermodynamic calculations suggested that oxygen evolution was caused or at least assisted by the reaction of iron oxides with Al2O3 to form FeAl2O4, a stable spinel. It is, however, assumed that FeAl2O4 reacted with silicate compounds during sintering at high temperatures.