Iron Silicate Phase Research Articles

Iron silicate perovskite and postperovskite in the deep lower mantle

Ferromagnesian silicates are the dominant constituents of the Earth's mantle, which comprise more than 80% of our planet by volume. To interpret the low shear-velocity anomalies in the lower mantle, we need to construct a reliable transformation diagram of ferromagnesian silicates over a wide pressure-temperature (P-T) range. While MgSiO3 in the perovskite structure has been extensively studied due to its dominance on Earth, phase transformations of iron silicates under the lower mantle conditions remain unresolved. In this study, we have obtained an iron silicate phase in the perovskite (Pv) structure using synthetic fayalite (Fe2SiO4) as the starting material under P-T conditions of the lower mantle. Chemical analyses revealed an unexpectedly high Fe/Si ratio of 1.72(3) for the Pv phase in coexistence with metallic iron particles, indicating incorporation of about 25 mol% Fe2O3 in the Pv phase with an approximate chemical formula (Fe2+0.75Fe3+0.25)(Fe3+0.25Si0.75)O3. We further obtained an iron silicate phase in the postperovskite (PPv) structure above 95 GPa. The calculated curves of compressional (VP) and shear velocity (VS) of iron silicate Pv and PPv as a function of pressure are nearly parallel to those of MgSiO3, respectively. To the best of our knowledge, the iron silicate Pv and PPv are the densest phases among all the reported silicates stable at P-T conditions of the lower mantle. The high ferric iron content in the silicate phase and the spin-crossover of ferric iron at the Si-site above ~55 GPa should be taken into account in order to interpret the seismic observations. Our results would provide crucial information for constraining the geophysical and geochemical models of the lower mantle.

Exploring the secondary mineral products generated by microbial iron respiration in Archean ocean simulations.

Marine chemical sedimentary deposits known as Banded Iron Formations (BIFs) archive Archean ocean chemistry and, potentially, signs of ancient microbial life. BIFs contain a diversity of iron- and silica-rich minerals in disequilibrium, and thus many interpretations of these phases suggest they formed secondarily during early diagenetic processes. One such hypothesis posits that the early diagenetic microbial respiration of primary iron(III) oxides in BIFs resulted in the formation of other iron phases, including the iron-rich silicates, carbonates, and magnetite common in BIF assemblages. Here, we simulated this proposed pathway in laboratory incubations combining a model dissimilatory iron-reducing (DIR) bacterium, Shewanella putrefaciens CN32, and the ferric oxyhydroxide mineral ferrihydrite under conditions mimicking the predicted Archean seawater geochemistry. We assessed the impact of dissolved silica, calcium, and magnesium on the bioreduced precipitates. After harvesting the solid products from these experiments, we analyzed the reduced mineral phases using Raman spectroscopy, electron microscopy, powder x-ray diffraction, and spectrophotometric techniques to identify mineral precipitates and track the bulk distributions of Fe(II) and Fe(III). These techniques detected a diverse range of calcium carbonate morphologies and polymorphism in incubations with calcium, as well as secondary ferric oxide phases like goethite in silica-free experiments. We also identified aggregates of curling, iron- and silica-rich amorphous precipitates in all incubations amended with silica. Although ferric oxides persist even in our electron acceptor-limited incubations, our observations indicate that microbial iron reduction of ferrihydrite is a viable pathway for the formation of early iron silicate phases. This finding allows us to draw parallels between our experimental proto-silicates and the recently characterized iron silicate nanoinclusions in BIF chert deposits, suggesting that early iron silicates could possibly be signatures of iron-reducing metabolisms on early Earth.

High-temperature performance of slag-based Fe-rich alkali-activated materials

Fe-rich alkali-activated materials have been studied for their suitability for high-temperature application up to 1000 °C. The evolution of compressive strength, mineral composition and microstructure was monitored using FT-IR, XRD, TGA and SEM-WDS for the pastes and mortars obtained by alkali activation of fayalite and wüstite non-ferrous slags. For both binders, the changes in compressive strength revealed moderate fluctuations below 400 °C due to water evaporation, a sharp drop in strength to 25 MPa at 600 °C due to collapse of the original gel structure, and reinforcement of the strength above 600 °C due to sintering densification and formation of gehlenite-åkermanite or diopside-albite ceramic. An extensive reaction of the quartz aggregate with a sodium-rich binder resulted in the formation of an iron silicate phase that contributed to a further increase in the strength up to 70 MPa by increasing both temperature and time of heating.

Metallization of Extruded Briquettes (BREX) in Midrex Process

The results of the full-scale testing of the Extruded Briquettes (BREX) as the charge components of the industrial Midrex reactor are discussed. The influence of the type of binder on the degree of metallization of BREX is analyzed. Magnesium sulfate-based binder helps to reach highest metallization degree of BREX. Mineralogical study shows the difference in the iron-silicate phase’s development as well as in the porosity change during metallization depending on the binder used.

Interactions between oxygen carriers used for chemical looping combustion and ash from brown coals

Iron-based oxides are considered as potential oxygen carriers in coal chemical looping combustion. However, their redox or oxidation behaviour can be affected by coal mineral matters. The interactions between oxygen carriers and coal ashes depend strongly on the type of ash, which varies significantly in Australian brown coals.We studied the interactions between two types of coal ash (silica-rich and iron and magnesium-rich) and two types of iron-based oxygen carriers (iron ore and industrial-grade ilmenite) via microstructural analysis of samples processed in reducing and oxidising atmospheres at 900 and 950°C, respectively.Both types of ash reacted locally with iron-based oxides, forming alumina silicates, spinels, magnesioferrites, and iron silicate phases as solid solutions or through liquid phase formation. These interactions can affect the oxidation and reduction kinetics of oxygen carriers. Our results show that iron-rich ash improves the oxidation and reduction kinetics of iron ore. It does not affect the reduction kinetics of ilmenite, but significantly increases ilmenite oxidation time. Silica-rich ash decreases oxidation rates of iron ore, but has less of an effect on the kinetics of ilmenite processing.

Formation of the active surface of Ag/SiO2 catalysts in the presence of FeO x additives

Supported FeO x -modified silver catalysts based on commercial silica gel and prepared via impregnation with a solution of Fe- and Ag-containing salts while varying the amount of modifier from 1 to 10 wt % are studied. It is found that the introduction of Fe-containing compounds leads to the distribution of silver in the form of clusters/ions on the surface of SiO2, improving the reducibility of the systems in the H2 TPR mode. After reduction at 800°C, the catalysts containing 10 wt % iron and 5 wt % silver comprise a Fe2SiO4 iron silicate phase.

Influence of calcination on magnetic honeycomb briquette cinders composite for the adsorptive removal of As(III) in fixed-bed column

• MHBC was successfully synthesized and calcined at 1000 °C under air and nitrogen. • Dynamic nitrogen-flow calcination increased the adsorbent hardness and particle size. • As(III) adsorption using un- and calcined MHBC was performed in fixed-bed column. • As(III) adsorption process in fixed-bed column was well described by Thomas model. • Selective calcination guaranteed phase transformation for enhanced arsenic removal. Elevated concentration of arsenic in water supplies is detected worldwide and therefore becomes a global issue. Calcination process is known to increase the adsorbent hardness for its effective application in column-based treatment. In this study, magnetic honeycomb briquette cinders (MHBC) and its calcined products: MHBC(A) and MHBC(N) were employed for the adsorptive removal of As(III) in fixed-bed column. Characterizations revealed that the calcination at 1000 °C under nitrogen has significantly increased the adsorbent particles size; favored phase transformation and improved saturation magnetization (>20 emu g − 1 ). Additionally, the new iron silicate phase (Fe 2 SiO 4 ) in aqueous medium generated highly reactive iron oxide species (=FeOOH), which effectively bind As(III) from the influent water by ligand exchanges. In contrast, calcination under dynamic air drastically reduced the saturation magnetization (<1 emu g − 1 ) and assisted to form segregated magnetite, quartz and hematite, as revealed in XRD patterns. The breakthrough curve of each column was compared with Thomas model and found that the model could be applied to estimate As(III) adsorption in fixed-bed column. Thomas model suggested parameters follow the order: MHBC(N) > MHBC > MHBC(A) and the maximum solid phase concentration ( q T ) was found to be about 56.07 mg g − 1 for MHBC(N). The column beds could be successfully regenerated using 200 bed volume of 10% NaOH solution. This study suggests that the selective calcination process need to be integrated with the adsorbent development process for the efficient removal of As(III) from contaminated water using column-based treatment.

Mineralogy and Geochemistry of A Low Grade Iron Ore Sample from Bellary-Hospet Sector, India and Their Implications on Beneficiation

The gradual depletion of high-grade iron ores has necessitated the exploitation of low/off grade iron ore reserves of India. The role of geochemical and mineralogical characterization is paramount to arrive at the process flow sheet development for such complex ores. Detailed studies were conducted on iron ores of Bellary-Hospet sector using microscope, XRD, TG, and EPMA techniques. The results indicate that hematite is the major iron oxide mineral with minor amounts of goethite, magnetite, martite and limonite with quartz and clay as major gangue. There is no evidence of the presence of any iron silicate and iron carbonate minerals. Trace amounts of pyrite were observed under the microscope and is the only iron sulphide phase. Microscopic studies also indicated that most of the quartz grains are present as inclusions within the hematite and magnetite grains. XRD studies revealed hematite as the major mineral with subordinate amounts of goethite, quartz and kaolinite confirming to the microscopic findings. Qualitative mapping and quantitative EPMA studies on these ores indicated the presence of gibbsite as the only alumina bearing phase and apatite as phosphorous bearing mineral. Traces of alumina, present as solid solution in the iron oxide minerals, has also contributed Al2O3 to the ores. Electron microscopic studies revealed that gibbsite grains are in the size range of 10 to 50 microns and are intimately and intricately associated with the iron oxide phases. Major elemental analyses of 47 representative iron ore samples of various types were analyzed to deduce the source of silica and alumina’s contributing phases in the ore and their interrelationships. The geochemistry data revealed negative correlation of Fe2O3 with silica and alumina thus indicating there is no iron silicate phase as well as alumina contribution from iron

Morphological changes accompanying gaseous reduction of SiO2 doped wüstite compacts

Pure wüstite was prepared from the gaseous reduction of reagent grade (99·5%) Fe2O3 at 1000°C, in 50CO–50CO2 gas mixture. Compacts of 0·5% and 2%SiO2 doped wüstite were made and annealed for 3 h at 1000°C in 50CO–50CO2 gas mixture. These compacts were isothermally reduced in H2, CO and 50H2–50CO mixture at a temperature range 900–1050°C. The oxygen weight loss during reduction process was recorded as a function of time. X-ray diffraction (XRD), SEM and porosity measurements were used to characterise the annealed and reduced samples. The influences of temperature, gas composition and silica contents on the reduction behaviour and the morphology were investigated. It was observed that the addition of silica decreased the porosity owing to the formation of iron silicate phases. The reduction rate increases with temperature and in presence of hydrogen in the gas mixture while it decreases with increasing silica content owing to decreasing porosity of compacts. The apparent activation energy values were calculated and correlated with the gas–solid reaction formulations to elucidate the corresponding mechanism at both early and final reduction stages. The reduced iron was formed in different morphology including whiskers and platelets structures which are temperature and gas composition dependent.

Design of Iron Oxide/Silica/Alginate HYbrid MAgnetic Carriers (HYMAC)

A large number of natural and synthetic polymers have already been evaluated for the design of nanomaterials incorporating magnetic nanoparticles for biomedical applications. The possibility to use hybrid (bio)-organic/inorganic nano-carriers have been much less studied. Here we describe the design of Hybrid MAgnetic Carriers (HYMAC) consisting of alginate/silica nanocomposites incorporating magnetite nanoparticles, based on a spray-drying approach. Transmission electron microscopy and X-ray energy dispersive spectrometry confirm the successful incorporation of magnetic colloids within homogeneous hybrid capsules. X-ray diffraction data suggest that surface iron ions are partially desorbed by the spray-drying process, leading to the formation of lepidocrocite and of an iron silicate phase. Magnetic measurements show that the resulting nanocomposites exhibit a superparamagnetic behaviour with a blocking temperature close to 225 K. Comparison with un-silicified capsules indicate that the mineral phase enhances the thermal stability of the polymer network and do not modify of the amount of incorporated iron oxide nanoparticles. Moreover, evaluation of nanocomposite up-take by fibroblasts indicates their possible internalization. A selective intracellular alginate degradation is observed, suggesting that these HYMAC nanomaterials may exhibit interesting properties for the design of drug delivery devices.

X-ray photoelectron spectroscopy studies of enameled steels

X-ray photoelectron spectroscopy is used for studying, both qualitatively and quantitatively, the chemistry of steel/enamel interfaces. Experimental evidence is presented that high adherence is achieved when the interface consists of a single, fully reacted iron silicate plase. Conversely, poor adherence is characterized by an iron silicate phase plus some unreacted FeO. The presence of this latter phase derives from the fact that oxide formation by the steel is unbalanced by oxide dissolution through glass attack, whereas there is no evidence for FeO diffusion into enamel. Our results lend experimental support to the chemical theories of adherence at steel/enamel interfaces.

Microchemistry and Mechanisms of Adherence in Steel/Enamel Interfaces

The microchemistry of steel/enamel interfaces was investigated, on a nearly atomic scale, by means of X‐ray photoelectron spectroscopy. Experimental evidence is given that good adherence is characterized by a fully reacted iron silicate phase, which produces a local chemical continuity between steel and enamel, whereas bad adherence is associated with a heterogeneous mixture of an iron silicate phase plus unreacted FeO.

Iron and manganese micro-precipitates within a cretaceous biosiliceous ooze from the Arctic Ocean: possible hydrothermal source

The oldest sediment most recently recovered from the Arctic Ocean is a biosiliceous ooze nearly devoid of nonbiogenic particles and exhibiting small-scale color changes. Color variations are due to changes in iron and manganese content. These elements are probably of local hydrothermal origin, and the Mn precipitation may be bacteria-mediated. An iron silicate phase seems to form at the expense of biogenic silica. The ooze deposited slowly until a sudden sediment input, probably a volcanigenic deposit now weathered to clay minerals, induced dissolution of siliceous microfossils. This clay layer contains calcium phosphate microspheres enriched in rare earth elements.

XPS studies of iron sodium borosilicate glasses

X-ray photoelectron spectroscopy (XPS) and x-ray induced Auger electron spectroscopy have been used to study borosilicate glasses of various iron oxide contents. The iron is present as both Fe(II) and Fe(III) ions, and its distribution between the two oxidation states appears to be constant throughout the series of samples. Ols XPS spectra and OKVV Auger spectra show that the iron ions separate between iron oxide phases and iron silicate phases. The extent of this separation was measured by quantitative XPS analysis .