Structure Of Iron Oxide Research Articles

High-Temperature Oxidation Behavior of Fe-1Cr-0.2Si Steel.

In the case of Fe–1Cr–0.2Si steel, tube furnace oxidation was carried out for 120 min and 30 min. These studies, along with the high-temperature oxidation behavior of Fe–1Cr–0.2Si steel, were examined from 700 to 1100 °C. It has been observed that with an increase in the oxidation time, the oxidation weight gain per unit area of Fe–1Cr–0.2Si steel changed from a linear to a parabolic relationship. The time was shortened when the oxidation phase was linear. When the oxidation temperature exceeded 900 °C, the value of WTransition decreased, and the oxidation rule changed. It could be considered that overall, the iron oxide structure of Fe–1Cr–0.2Si steel is divided into two layers. The formation of an outer oxide of iron is mainly caused by the outward diffusion of cation, while the inward diffusion of O ion forms the inner oxides of chromium and silicon. As the temperature increases, the thickness of the outer iron oxide gradually increases, and the thickness ratio of the inner mixed layers of chromium- and silicon-rich oxides decreases; however, the degree of enrichment of Cr and Si in the mixed layer increases. After high-temperature oxidation, Cr and Si did not form a composite oxide but were mechanically mixed in the form of FeCr2O4 and Fe2SiO4, and no significant delamination occurred.

Quantum Magnetic Imaging of Iron Biomineralization in Teeth of the Chiton Acanthopleura hirtosa

AbstractIron is critical for life. Nature capitalizes on the physical attributes of iron biominerals for functional, structural, and sensory applications. Iron biomineralization is well exemplified by the magnetite‐bearing radula of chitons, the hardest known biomineral of any animal. Although magnetism is an integral property of iron biominerals, limited information exists on the magnetic state, structure, and orientation of these nanoscale materials during mineralization. The advent of quantum‐based magnetic microscopy provides a new avenue to probe these biological systems directly, providing detailed magnetic information of the iron oxide structures. Here two complementary quantum magnetic microscopy methods are applied, based on nitrogen‐vacancy centers in diamond, to spatially map the mineral phases ferrihydrite and magnetite in the developing teeth of the chiton Acanthopleura hirtosa. The images reveal previously undiscovered long‐range magnetic order, established at the onset of magnetite mineralization. This is in contrast to electron microscopy studies that show no strong common crystallographic orientation. The implications of these results are important, not just for the insights gained in biomineralization of the target organism, but also for the study of a broad range of iron minerals in the physical and biological sciences.

Eco-Friendly, Green Packaging Materials from Akaganeite and Hematite Nanoparticle-Reinforced Chitosan Nanocomposite Films

In this study, chitosan nanocomposite thin films were successfully fabricated by incorporating hematite nanoparticles (HNPs) and akaganeite nanoparticles (ANPs) as reinforcing fillers using the solution casting method. HNPs and ANPs were synthesized via a urea-assisted synthesis route using naturally occurring ferruginous laterites. Scanning electron microscopic (SEM) images indicated the spherical to subhexagonal morphology of the HNPs and rice-like morphology of the ANPs. X-ray diffractograms indicate the crystalline structure of iron oxides as hematite and akaganeite. Tensile tests were carried out to evaluate the mechanical properties of the nanocomposite films where maximum tensile stress of the chitosan/HNP composites was improved as high as 35.7% while chitosan/ANP composites indicated 43.5%. Thermal decomposition curves obtained by thermogravimetric analysis (TGA) indicate that the thermal stability of the nanocomposites has improved remarkably compared to neat chitosan films. Furthermore, these nanocomposites exhibited excellent UV barrier properties as identified by UV-visible spectrometry. Fourier-transform infrared (FTIR) spectroscopic results are evident in the presence of Fe-O bond in the wavenumber around 480-500 cm-1, and the result also indicated that the nanofillers interact with the chitosan matrix via hydrogen bonding, which enhanced the physical properties of the nanocomposites. Incorporation of iron oxide nanoparticle varieties into chitosan has led to improvements of certain physical and chemical properties, which make chitosan a promising material for packaging applications.

Enhanced Structural, Optical, and Multiferroic Properties of Rod-Like Bismuth Iron Oxide Nanoceramics by Dopant Lanthanum

Herein, the influence of lanthanum concentration on the structural, thermal, optical, ferroelectric, and magnetic properties of Bi1-xLaxFeO3 (BLFO, x = 0, 0.05, 0.1, 0.15, 0.2, and 0.25) nanorods prepared by hydrothermal method was investigated. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy were performed to study the structural properties of BLFO nanoparticles for different lanthanum concentration. The Rietveld refinement and analysis of crystal structure revealed a single-phase perovskite structure with space group R3c. Transmission electron microscope (TEM) image of bismuth iron oxide nanorods exhibits average diameter of ~ 40 nm and length of ~ 2 μm. The ferroelectric Curie temperature of 833 °C (829 °C) was determined for BiFeO3 (Bi0.8La0.2FeO3) by differential thermal analysis (DTA). Diffuse reflectance UV-Vis spectra indicated that the optical band gap is decreased by increasing the lanthanum concentration. The ferroelectric properties of BiFeO3 nanoparticles also improved by adding lanthanum into the structure of bismuth iron oxide. Moreover, J-E curves showed that lanthanum doping can significantly decrease the leakage current of bismuth ferrite nanorods. Finally, vibrating sample magnetometer (VSM) measurements indicated that the weak ferromagnetic behavior is observed for as-prepared BLFO nanorods with x ≤ 0.15. However, for the 20 mol% lanthanum doping, a magnetic transition from weak ferromagnetic to antiferromagnetic occurred.

Preference of Co over Al for substitution of Fe in goethite (α-FeOOH) structure: Mechanism revealed from EXAFS, XPS, DFT and linear free energy correlation model

Goethite is the most stable iron hydroxide mineral in geological environments, and it is usually crystallized with more than one type of substituent. However, the synergetic or antagonistic effects of coexisting cations on substitution for lattice Fe in gothite are unclear. In this study, a series of Co-, Al-, and Co + Al co-substituted goethite samples were synthesized at room temperature and then investigated by powder XRD, X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure spectroscopy (XAFS), density function theory (DFT) calculations, and acid dissolution experiments. All the obtained samples were pure goethite, except the one with a high total content of Co + Al that contained a portion of 2-line ferrihydrite. Lattice parameter a of Co + Al co-substituted goethite was linearly decreased with the final Co content, lattice parameter b and calculated crystal density had the best negatively linear relationships with Al content while lattice parameter c, cell volume, average edge-sharing Fe − Me (Me = Fe, Al, and Co) distances along c⁎ and b⁎ axis (E′ and E) and average corner-sharing Fe − Me (DC) distances had the best linear correlations with the total contents of Co + Al. Aluminum and Co were found to be evenly distributed in goethite structure, and both reduced the mineral particle size. Co coexisted in a mixed valance of +2 and +3, and the proportion of Co3+ increased with the increase of Co content in Co-substituted goethite owing to relatively high electric potential energy and decreased with the increase of coexisting Al3+ content in Co + Al co-substituted goethites because of relatively low alkalinity in local environments around Co2+. Incorporation of Co greatly promoted the mineral dissolution in 2 M HCl solution at 298 K. Fe K-edge EXAFS analysis indicated almost constant local order around Fe in these samples. Coexisting Al had almost no effect on Co incorporation into goethite structure but Co suppressed Al substitution for lattice Fe. This preference of Co over Al to substitute for Fe was explained by the predicted partition coefficients (Kd) of common trivalent substituents in goethite by using the linear free energy relationship. It highlights the necessity to take into consideration not only the substituent physical properties (size and charge) but also their chemical properties (chemical bonding energy and chemical potential). These results enhanced our understanding of the incorporation of exotic cations into the iron oxide structures and the mutual effects of coexisting cations on substitution for lattice Fe.

Structure-Activity Relationship of Iron Oxides for NO Reduction in the Presence of C3 H6 , CO, and O2.

Spinel-type NiFe2 O4 exhibited the highest NO reduction activity among base-metal oxides under simulated exhaust of a gasoline-powered vehicle. The structure-activity relationship of iron oxides has been investigated through both experimental and computational studies. Spinel iron oxide (γ-Fe2 O3 ) exhibited a much higher NO reduction activity than that of iron oxide with other structures (α-Fe2 O3 and LaFeO3 ). Operando IR measurements clarified that the spinel structure facilitated the reaction between NOx and adsorbed oxidized hydrocarbon or cyanide species. The high reactivity of the spinel structure was ascribed to the high adsorption energy of NO, as elucidated by DFT calculations. Furthermore, molecular orbital calculations demonstrated that the local coordination structure of the spinel iron oxide induced the involvement of not only σ but also π orbitals during NO adsorption on Fe atoms. This work clarified the origin of the structure-dependent activity of metal oxides, with a focus on their local coordination structures.

Effects of initial crystal structure of Fe2O3 and Mn promoter on effective active phase for syngas to light olefins

Although it is generally acknowledged that iron carbides are the active phase in Fischer-Tropsch synthesis (FTS), adjusting the formation of active phase and catalytic performance via the initial crystal structure of iron oxides has never been studied. Herein, we take one-dimensional pure phase α-Fe2O3 and γ-Fe2O3 nonporous nanorods to explore the effects of initial crystal phase of Fe2O3 on both formation of active phase and corresponding catalytic performance, in which the influence of diffusion limitation on selectivity can be ignored. In situ characterizations uncovered that the formation of iron carbides with high selectivity of light olefins strongly depends on the initial crystal phase of iron oxide and its induced metal-promoter interaction. Responding to the morphology effect and crystallographic phase effect, Mn located on surface of γ-Fe2O3 nanorods exhibit a low oxidation state due to a strong Fe-Mn interaction. This interaction was beneficial to the formation of C-poor iron carbide species at the metal-promoter interface during carburization. Hence, outstanding selectivity for light olefins (61.2%) was achieved on 0.5 Mn/γ-Fe2O3 nanorods catalyst at a CO conversion of 55.1% under industrially relevant conditions (320 °C, 1 MPa, H2/CO ratio of 1, and W/F = 5 gcat h mol−1). This finding enriches the fundamental understanding of active phase evolution and promoter effect in STO process and may guide the design of a catalyst with high selectivity.

One step synthesis of N-doped carbon/amorphous iron oxide composite for enhanced photodegradation activity

In this work, the N-doped carbon/amorphous iron oxide composite was prepared in one-step. The XRD, Raman, XPS, HRTEM, SEM and EDS mapping were used to characterize the composites and the results indicate that the Fe, C, O and N elements disperse uniformly in N-doped carbon/amorphous iron oxide composite. The photocatalytic performance of as-prepared composite was studied by the photodegradation of MB under solar light, indicated that the composites with nitrogen doped and amorphous structure significantly improved the photocatalytic activity. The amorphous structure of iron oxide played a key role in the improvement of photocatalytic activity. Based on photodegradaiton and photoluminescence spectra results, they were implied that the enhanced performance should be attributed to higher separation efficiency and a longer life time of photoproduced electron and hole pairs by the existence of amorphous structure. The composite prepared by this work has significant application value in photodegradation field.

Surface Anchoring Approach for Growth of CeO2 Nanocrystals on Prussian Blue Capsules Enable Superior Lithium Storage.

Prussian blue (PB) and its analogues (PBAs) have been acknowledged as promising materials for the catalysis, energy storage, and bioapplications because of different constructions and tunable composition. The approach for surface modification with metal oxides for boosting the performance, however, is rarely reported. Herein, a facile surface anchoring strategy has been proposed to realize CeO2 nanocrystals uniformly depositing on the surface of PB. Besides, the size, thickness, and depositing density of CeO2 nanocrystals can be regulated by adjusting the amount of the precursor and the proportion of ethanol and deionized water. Furthermore, after a step of confined pyrolysis treatment under an air atmosphere, CeO2 nanocrystals with an encapsulated iron oxide structure have been obtained. This shows a remarkable cycling and rate performance when evaluated as an anode of the lithium-ion battery. The surface anchoring approach of the CeO2 nanocrystals may not only promote the various applications of PB-based materials but also provide an opportunity for developing the architecture of other CeO2-based core-shell nanostructures.

Local structure of iron oxide sensitizing Nb2O5 photocatalysts

We here couple Nb2O5 with Fe2O3 to form visible-light active heterojunctions. The formed heterojunctions are responsive to visible light, and more importantly, photoactive for the mineralization of refractory organic compounds upon exposure to visible light. The sensitizer, Fe2O3, is found to govern the photocatalytic activity of the heterojunction. Therefore, the local structure of the sensitizer is investigated in-depth with X-ray absorption spectroscopy, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). Results show that the local structure of Fe2O3 is generally insensitive to the Fe2O3 loading amount, as confirmed by EXAFS. The lengths of the Fe–O coordination and Fe–Fe coordination in the heterojunctions are 1.93 and 2.92 Å, respectively, which are shorter than those in bulk Fe2O3. At a Fe2O3 loading amount that is higher than the optimum value, trace amounts of isolated Fe2O3 and bulk Fe2O3 are likely formed at the heterojunction. This is unfavorable for electron-hole separation, and hence, detrimental to the photocatalytic activity.

Structural Evolution of α-Fe2O3(0001) Surfaces Under Reduction Conditions Monitored by Infrared Spectroscopy.

The precise determination of the surface structure of iron oxides (hematite and magnetite) is a vital prerequisite to understand their unique chemical and physical properties under different conditions. Here, the atomic structure evolution of the hematite (0001) surface under reducing conditions was tracked by polarization-resolved infrared reflection absorption spectroscopy (IRRAS) using carbon monoxide (CO) as a probe molecule. The frequency and intensity of the CO stretch vibration is extremely sensitive to the valence state and electronic environments of surface iron cations. Our comprehensive IRRAS results provide direct evidence that the monocrystalline, stoichiometric α-Fe2O3(0001) surface is single Fe-terminated. The initial reduction induced by annealing at elevated temperatures produces surface oxygen vacancies, where the excess electrons are localized at adjacent subsurface iron ions (5-fold coordinated). A massive surface restructuring occurs upon further reduction by exposing to atomic hydrogen followed by Ar+-sputtering and annealing under oxygen poor conditions. The restructured surface is identified as a Fe3O4(111)/Fe1−xO(111)-biphase exposing both, Fe3+ and Fe2+ surface species. Here the well-defined surface domains of Fe3O4(111) exhibit a Feoct2-termination, while the reduced Fe1−xO(111) is Fe2+(oct)-terminated. These findings are supported by reference IRRAS data acquired for CO adsorption on magnetite (111) and (001) monocrystalline surfaces.

Complete degradation of ciprofloxacin over g-C3N4-iron oxide composite via heterogeneous dark Fenton reaction

Graphitic carbon nitride (g-C3N4) supported iron oxide (CN@IO) composite was first fabricated via synthesizing g-C3N4 in-situ onto iron oxide. The fabricated CN@IO composite was characterized by several techniques including XRD, XPS, TEM and nitrogen adsorption-desorption analysis. This composite was then used as a catalyst for the dark Fenton oxidative degradation of ciprofloxacin (CIP). Results demonstrated that the incorporation of g-C3N4 profoundly changed the structure and chemical properties of iron oxide, endowing CN@IO composites with high-efficient catalytic activity in dark Fenton system. In the synthesis process of CN@IO composites, iron oxide nanoparticles were successfully intercalated into the layers of g-C3N4, enlarging the surface area and thus providing more active sites for the reactions. Meanwhile, the existence of g-C3N4 can accelerate the Fe3+/Fe2+ redox cycle during the Fenton reaction, which further facilitated CIP degradation. In addition, the effects of reaction parameters, including pH, catalyst dosage, initial concentration of CIP and H2O2, on CIP degradation were investigated. Without any assistance of light irradiation, complete degradation and 48.5% mineralization of CIP were achieved under the best conditions of pH 3.0, 1 g/L CN@IO-2, 20 mg/L CIP and 0.0056 M H2O2. The trapping of iron oxide between g-C3N4 layers helped to stabilize iron oxide so the metal leaching problem that usually occurred in acidic media (pH = 3) can be effectively overcome. This work provides a new thought to develop environmental-friendly and high-efficient catalysts for the degradation of refractory pollutants in dark Fenton system, which is much easier to scale up for industrial application comparing with the photo-Fenton reaction.

Structure of single sheet iron oxides produced from surfactant interlayered green rusts

Ultrathin iron oxide particles are of high interest due to their large surface areas and unusual physical and chemical properties. Previous works have shown that single sheet iron oxides (SSI) can be formed via delamination of oxidized layered iron (hydr)oxides (green rusts, GR) interlayered by dodecanoate. However, there is considerable uncertainty on the true structure of both starting material and final products, and the reaction pathway. In this work, we describe a robust method for SSI synthesis and provide detailed structural characterization of the initial, intermediate and final phases to decipher the reaction mechanism. The SSI product has the formula FeO0.82(OH)1.38·0.7H2O, and consists of platelets with a height of 1 nm and lateral dimensions of 20 to 100 nm as observed by Atomic Force Microscopy and Transmission Electron Microscopy. Mössbauer spectroscopy from 300 to 9 K shows that SSI is distinct from goethite, ferrihydrite and feroxyhite. Pair distribution function (PDF) analysis of high energy X-ray scattering data reveals that SSI has two distinct nearest neighbor FeFe distances in contrast to the single distance in the parent FeII-FeIII (hydr)oxide composed of entirely edge-sharing octahedra. Modeling of the SSI PDF data indicates that oxidation of FeII to FeIII of dodecanoate-intercalated green rust results in displacements of Fe atoms perpendicular from the parent iron (hydr)oxide layer, forming a material that consists of iron polyhedra linked by both corner- and edge-sharing. This model which is different from the previously published model, matches the measured SSI thickness and electron diffraction pattern. This elucidated reaction pathway confirms that the dodecanoate interlayers in GR hinders Fe polymerization across interlayers and thus restrict chemical transformations to largely two-dimensional space. The increase of single- and double- coordinated O/OH groups in the SSI compared with the parent GRs is expected to give a high reactivity of SSI as surface complexation sorbents.

Defects Behavior in Disordered Iron Oxide Synthesized from Grain-Oriented Iron Foils

ABSTRACTDisordered iron oxide thin-films synthesized from grain-oriented iron foils were grown on both glass and Si (100) n-type substrates by vacuum evaporation followed by thermal oxidation at low temperatures. Defects such as vacancies formation has been studied using Atomic Force Microscopy (AFM) and Raman Spectroscopy. The kinetic of oxidation as a function of surface parameters was investigated by AFM studies. The vibrational modes (bands) connected with the vacancies formation and magnetic ordering into the iron oxide structure were validated by Raman spectroscopy. Space-charge effects can be influenced by discontinuous growth of iron oxide and correlated with their structure parameters. Finally, the disordered iron oxide will be useful for the next generation of adaptive oxide devices.

Impact of calcium doping on structure, catalytic and conductive properties of lanthanum strontium iron oxide

A set of dense structure materials based on La0.5Sr0.5-xCaxFeO3 with different amount of calcium (where x varies from 0 to 0.1) were prepared. It was found that with calcium addition the hexagonal perovskite La0.5Sr0.5FeO3 structure is preserved and Ca2+ ions accommodate themselves easily in the lattice. The sintered ceramics have homogeneous microstructure and uniform grain size distribution with the average size which increases with Ca addition. Electrical properties of the sintered materials determined by the DC method revealed an increase in sample conductivity with Ca2+ doping. An increase in structure stability with the increase of calcium addition was also confirmed by the temperature-programmed studies.

On the structure and chemistry of iron oxide cores in human heart and human spleen ferritins using graphene liquid cell electron microscopy.

Ferritin is a protein that regulates the iron ions in humans by storing them in the form of iron oxides. Despite extensive efforts to understand the ferritin iron oxide structures, it is still not clear how ferritin proteins with a distinct light (L) and heavy (H) chain subunit ratio impact the biomineralization process. In situ graphene liquid cell-transmission electron microscopy (GLC-TEM) provides an indispensable platform to study the atomic structure of ferritin mineral cores in their native liquid environment. In this study, we report differences in the iron oxide formation in human spleen ferritins (HSFs) and human heart ferritins (HHFs) using in situ GLC-TEM. Scanning transmission electron microscopy (STEM) along with selected area electron diffraction (SAED) of the mineral core and electron energy loss spectroscopy (EELS) analyses enabled the visualization of morphologies, crystal structures and the chemistry of iron oxide cores in HSFs and HHFs. Our study revealed the presence of metastable ferrihydrite (5Fe2O3·9H2O) as a dominant phase in hydrated HSFs and HHFs, while a stable hematite (α-Fe2O3) phase predominated in non-hydrated HSFs and HHFs. In addition, a higher Fe3+/Fe2+ ratio was found in HHFs in comparison with HSFs. This study provides new understanding on iron-oxide phases that exist in hydrated ferritin proteins from different human organs. Such new insights are needed to map ferritin biomineralization pathways and possible correlations with various iron-related disorders in humans.

Au-Fe3O4 heterostructures for catalytic, analytical, and biomedical applications

Heterostructures are a series of nanomaterials combining different components into a single nanostructure. Au-Fe3O4 heterostructures have received considerable attentions because of their superior properties coming from both individual and combinational features of gold and iron oxide nanoparticles. Their intrinsically peculiar magnetic, optical properties, and structure designability greatly enhance and broaden their potential applications in catalysis, assay, multimodal imaging, and synergistic treatment for tumor. In this review, we systematically introduce the preparation methods of Au-Fe3O4 heterostructures and their potential applications in the biomedical field, focusing on the unique synergistic effect caused by the combination of gold and iron oxide structures. This review will provide insights into the structure control in adjusting the function of heterogeneous or hybrid material, such as Au-Fe3O4 heterostructures, to implement their biomedical applications.

Synthesis of Iron Oxide Nanoparticles Optimized by Design of Experiments

Magnetic iron oxide nanoparticles have been widely studied for technological and biomedical applications due to its small size and the possibility of functionalizing the surface with intended molecules. Magnetite is the most used phase because of superparamagnetic characteristic, and it is synthesized by co-precipitation, starting from ferrous and ferric ions in alkaline media. However, depending on parameters of chemical reactions, such as molar ratio between iron and hydroxyl groups, stirring rate, and temperature, different iron oxide structures are formed through distinct pathways. Since several variables can affect the size, chemical composition, and crystalline structure of iron oxide nanoparticles during the synthesis, it is very important to apply experimental routines with a well-defined protocol and planning practical steps can significantly improve the quality of products. Plackett-Burman methodology from design of experiments is a filtration analysis, used in the initial stages of a process to investigate the main effects of factors over a characteristic of the final material. In this work, Plackett-Burman technique was employed in order to optimize bare iron oxide nanoparticle production by co-precipitation method, relating the different crystalline phases produced to the experimental routine. Dynamic light scattering, Fourier transform infrared spectroscopy, and X-ray diffractometry were used to characterize the samples.

A simple way to synthesize tartaric acid, ascorbic acid and their mixture coated superparamagnetic iron oxide nanoparticles with high saturation magnetisation and high stability against oxidation: Characterizations and their biocompatibility studies

Abstract A simple way to synthesise biocompatible superparamagnetic iron oxide nanoparticles coated with ascorbic acid, tartaric acid and a mixture of ascorbic acid and tartaric acid is proposed. The iron oxide nanoparticles were synthesized and coated with the surfactants in one step by co-precipitation method at room temperature in air atmosphere. The samples have the characteristic (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0) peaks of cubic spinel structure of iron oxide. Fourier Transform Infrared Spectroscopy analysis revealed that the nanoparticles were coated with the surfactants. The mean physical sizes of core obtained from the transmission electron microscopy images were 7 ± 2 nm. Magnetic measurements by vibrating sample magnetometer indicate that all samples are superparamagnetic at room temperature. And, the saturation magnetization, Ms of coated nanoparticles was around 62 emu/g. The effectiveness of the coating on the surface of nanoparticles against oxidation is also an important parameter. The coating agents successfully protected the core against oxidation since the Ms stayed almost stable (∼62) emu/g by 12 weeks and then decreased to ∼57 emu/g by 48 weeks. It can be concluded that coated superparamagnetic nanoparticles have high saturation magnetization and high stability against oxidation. Furthermore, the MTT assays were also carried out for cytotoxicity of various concentration of coated nanoparticles by using Hep3B, Saos-2 and HUVEC cell lines. From the MTT assay results, it is observed that the coated nanoparticles are non-toxic for all types of cells investigated and therefore can be biocompatible for potential biomedical applications.

Ferromagnetic Resonance Spectroscopy and Rock Magnetic Characterization of Fossil Coral Skeletons in Ishigaki Islands, Japan

Skeletons of hermatypic corals (e.g., Porites) might have enormous potential as a high-resolution paleomagnetic recorder owing to their rapid and continuous growth over hundreds of years at a rate of up to 2 cm/year, although typical corals show an extremely weak intensity of remanence and low stability. We found that coral tsunami boulders with negligible amounts of calcite on Ishigaki Island show a measurable intensity of remanence; thus, we attempted to characterize the magnetic assemblages in this coral skeleton to determine whether it is of biogenic or detrital magnetite using first-order reversal curve (FORC) measurements, ferromagnetic resonance (FMR) spectroscopy, and petrological observations through field-emission type scanning electron microscope (FE-SEM) with an acid treatment. The FMR derivative spectra of coral skeleton samples represent multiple derivative maxima and extended low-field absorption, indicating the presence of intact biogenic magnetite chains. FORC diagrams represent a “central ridge” signature with a vertical spread. These FMR and FORC features indicate the magnetization of these coral skeletons that are mainly created using intact biogenic magnetites and mixtures of grains from collapsed biogenic magnetites, pseudo-single domain grains, and multi-domain grains such as detrital magnetite. FE-SEM observations confirm the presence of a chain-like structure of iron oxides corresponding to the features of biogenic magnetite. Therefore, the magnetic mineral assemblage in coralline boulders from Ishigaki Island consists of dominant biogenic-origin single-domain magnetite and a trace amount of detrital component, indicating that fossil coral skeletons in Ishigaki Island have potential for utilization in paleomagnetic studies.