Abstract:In ferruginous soils, nano-to microscale hematite (a-Fe 2 O 3 ) plays a central role in redox processes and contaminant cycling. Hematite is known to incorporate structural OH -and water, and the requisite charge balance is achieved by iron vacancies. Prior researchers have suggested that the defective hematite structures form unique phases called "protohematite" and "hydrohematite." Infrared and Raman spectroscopic studies have assigned a lower-symmetry space group to "hydrohematite" (R3c) relative to that of… Show more
“…On the other hand, the value of the quadratic elongation of the octahedral unit around Fe(3) is around 1.008 and remains almost unalterable with compression along the stability range of this phase. This parameter is quite similar to those observed in other polymorphs such as γ-Fe 2 O 3 (with λ equal to 1.0024 for the octahedron and 1.000 for the tetrahedron) 41 , the high pressure monoclinic-Fe 2 O 3 (with two out of the three octahedral units with a λ of 1.0025 and 1.0021, respectively) 42 , and ζ-Fe 2 O 3 (an octahedron with λ of 1.0109) 43 .Fig. 3Analysis of the volume and the quadratic elongation of the polyhedral units under pressure.…”
Iron oxides are among the major constituents of the deep Earth’s interior. Among them, the epsilon phase of Fe2O3 is one of the less studied polymorphs and there is a lack of information about its structural, electronic and magnetic transformations at extreme conditions. Here we report the precise determination of its equation of state and a deep analysis of the evolution of the polyhedral units under compression, thanks to the agreement between our experiments and ab-initio simulations. Our results indicate that this material, with remarkable magnetic properties, is stable at pressures up to 27 GPa. Above 27 GPa, a volume collapse has been observed and ascribed to a change of the local environment of the tetrahedrally coordinated iron towards an octahedral coordination, finding evidence for a different iron oxide polymorph.
“…On the other hand, the value of the quadratic elongation of the octahedral unit around Fe(3) is around 1.008 and remains almost unalterable with compression along the stability range of this phase. This parameter is quite similar to those observed in other polymorphs such as γ-Fe 2 O 3 (with λ equal to 1.0024 for the octahedron and 1.000 for the tetrahedron) 41 , the high pressure monoclinic-Fe 2 O 3 (with two out of the three octahedral units with a λ of 1.0025 and 1.0021, respectively) 42 , and ζ-Fe 2 O 3 (an octahedron with λ of 1.0109) 43 .Fig. 3Analysis of the volume and the quadratic elongation of the polyhedral units under pressure.…”
Iron oxides are among the major constituents of the deep Earth’s interior. Among them, the epsilon phase of Fe2O3 is one of the less studied polymorphs and there is a lack of information about its structural, electronic and magnetic transformations at extreme conditions. Here we report the precise determination of its equation of state and a deep analysis of the evolution of the polyhedral units under compression, thanks to the agreement between our experiments and ab-initio simulations. Our results indicate that this material, with remarkable magnetic properties, is stable at pressures up to 27 GPa. Above 27 GPa, a volume collapse has been observed and ascribed to a change of the local environment of the tetrahedrally coordinated iron towards an octahedral coordination, finding evidence for a different iron oxide polymorph.
“…8). This is consistent with a general observation that in hematite formed by goethite dehydroxylation, hydroxyls are trapped and the oxide can retain up to half the stoichiometric water content of the parent oxyhydroxide, giving a formula Fe 2-x-/3 (OH) x O 3-x [88,89]. Therefore, the ''hydrated'' product of goethite conversion in Less-intense peaks (I \ 10) have been omitted.…”
The processes occurring during roasting of bog iron ores were characterized using TG-DTG-DTA-QMS, XRD, FTIR and specific surface analysis. Removal of physically adsorbed water is followed by dehydroxylation of iron oxyhydroxides and oxidation of organic matter at 200-600°C. The main product of the transformations is disordered nanocrystalline (proto)hematite or hematite/-maghemite mixture, depending on organic matter content and heating conditions. The conversion of iron oxyhydroxides to hematite occurs at temperatures different than those reported for pure compounds. At higher temperatures, protohematite undergoes recrystallization to the stoichiometric hematite, and manganese oxides are partially reduced. At 1000°C, the roasting products consist of hematite and cristobalite together with Mn-Fe spinels if the initial ore contained Mn oxides. The admixtures of various secondary silicates were encountered as well. Low-to moderate-temperature roasting slightly affects the specific surface area and lowers volume of micropores. The hightemperature transformations lead to decrease in the specific surface area and to the destruction of porous texture of the bog iron ores. Although the general course of the processes during roasting was similar in all the samples, some of their details as well as mineralogy and properties of the products are highly dependent on the composition of the initial material.
“…2). Therefore, the proper (stoichiometric) hematite is developed at higher temperatures [24,40,63]. Magnetic susceptibilities are gradually lowered in the temperature range of 25-550°C, and then, i.e., after hematite crystallization, only slight further lowering is observed (Fig.…”
In this study, a series of synthetic ferrihydrite samples of Si/Fe molar ratios ranging from zero to 1.5 were heated up to 1000°C using simultaneous TG-DTA equipment. The XRD, FTIR, SEM-EDS and magnetic susceptibility measurements were carried out prior to and after heating. It has been found that silicate retards ferrihydrite transformation to hematite and affects crystallinity of the product. Low Si admixture in the precursor reduces hematite crystal size severely, but the increase in average crystal dimensions with increasing Si/Fe molar ratio was observed. High Si content results in the formation of hematite which exhibits a wide range of crystal habits. The conversion of pure ferrihydrite to hematite proceeds without any intermediate phase, whereas the increasing silicate content in the pristine oxyhydroxide strongly affects the transformation pathway. During annealing of high-Si ferrihydrites, the presence of two or three intermediate Fe 2 O 3 polymorphs (gamma, epsilon and beta) was demonstrated prior to the crystallization of final a-Fe 2 O 3 . The conditions favoring crystallization of intermediate phases result from progressive silica polymerization which forms separate matrix-type phase and impedes the aggregation of iron oxide nanoparticles.
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