Goethite Crystals Research Articles

Dehydration of goethite during vacuum step-heating and implications for he retentivity characterization

Uncertainty exists over what environmental conditions and mineralogical/chemical properties are required to ensure retention of helium such that (UTh)/He dates record the time of goethite crystallization. We undertook vacuum step heating experiments to determine He diffusion parameters for extrapolation to Earth surface conditions on 10 goethite specimens in which we had created a uniform distribution of 3He. Arrhenius plots of apparent diffusion coefficients on all samples follow the same pattern. At temperatures <200 °C the data define arrays consistent with progressive degassing of increasingly large crystallites. However, above 200 °C the computed diffusivities increase dramatically until about 80% of the helium is extracted, after which they suddenly decline. The sudden increase in diffusivity at 200 °C coincides with the onset of dehydration of the goethite structure, a process which continues throughout the remainder of the step heat. There is a strong correlation between evolved water and 3He amounts. These observations likely reflect previously reported processes of formation, growth, and coalescence of pores as the phase transition to hematite proceeds. While significantly higher dehydration temperatures of ∼270 °C are observed using techniques such as thermogravimetric analysis in air, the long step durations, and vacuum conditions of our experiments destabilize goethite. Orders of magnitude differences in the computed diffusivities among our samples may reflect crystallite-size-distribution control on dehydration kinetics, and the qualitative size distributions we infer from the step heats are consistent with SEM observations of crystallite lengths. Vacuum step-heating experiments in which goethite is decomposing cannot be used to determine He diffusion behavior in nature, but the inferred crystallite size distribution provides some useful indirect evidence. A strong relationship exists between metrics of the crystallite size distribution from step heating results and the degree of He retention in nature inferred from the 4He/3He method. This observation provides evidence supporting the validity of the 4He/3He technique for estimating retention, and further suggests that the dominant control on He retention in nature is the crystallite size distribution. Experiments under hydrothermal conditions in which goethite remains stable may be an alternative approach to address the question of He diffusion behavior in nature.

Nonclassical crystallization of goethite nanorods in limpet teeth by self-assembly of silica-rich nanoparticles reveals structure–mechanical property relations

The intricate organization of goethite nanorods within a silica-rich matrix makes limpet teeth the strongest known natural material. However, the mineralization pathway of goethite in organisms under ambient conditions remains elusive. Here, by investigating the multi-level structure of limpet teeth at different growth stages, it is revealed that the growth of goethite crystals proceeds by the attachment of amorphous nanoparticles, a nonclassical crystallization pathway widely observed during the formation of calcium-based biominerals. Importantly, these nanoparticles contain a high amount of silica, which is gradually expelled during the growth of goethite. Moreover, in mature teeth of limpet, the content of silica correlates with the size of goethite crystals, where smaller goethite crystals are densely packed in the leading part with higher content of silica. Correspondingly, the leading part exhibits higher hardness and elastic modulus. Thus, this study not only reveals the nonclassical crystallization pathway of goethite nanorods in limpet teeth, but also highlights the critical roles of silica in controlling the hierarchical structure and the mechanical properties of limpet teeth, thus providing inspirations for fabricating biomimetic materials with excellent properties.

Crystallization between (100) Goethite and (001) Orientation of Hematite – A Review

Abstract The purposes of this study were: (1) to review the preparation and characterization of the intergrowth between goethite and hematite crystals; and (2) to propose a schematic diagram of the epitaxial relationships among three sets of (100) goethite twin crystals associated with the (001) orientation of the hexagonal prism of hematite. The Fe(ClO4)3 solution was prepared and aged at 70°C, which precipitated goethite initially and produced hematite later with prolonged aging. Goethite and hematite aged for 20 days were observed as star-shaped and hexagonal prisms, respectively. The results suggest that hematite could form later using goethite as a template surface. A selected area electron diffraction (SAED) pattern showed the epitaxial relationship among three sets of (100) goethite intergrowth crystals and hexagonal prisms with the (001) orientation of hematite. Goethite can be produced as lath-, X-, K-, or star-shaped crystals on the (100) orientation, depending on the Fe(ClO4)3 concentrations and the addition of HClO4 to Fe solution samples which were aged for a prolonged period at room temperature. The initial solubility products [(Fe3+)(OH–)3] of the sample solution, rather than the nature of the nuclei, are the key factors governing the formation of goethite or hematite. The addition of acids and high concentrations of iron solutions extend the secondary hydrolysis and induction period (IP) and favor the formation of hematite. The index of the SAED pattern of the star-shaped goethite intergrowth twin crystal has a (100) plane parallel to this basal plane and rotates at a 60° angle between two or three sets of lath-shaped goethite crystals, which share the (011) plane and form goethite twins with ‘interpenetrated’ crystal growth. Stereoscopic viewing using Oak Ridge Thermal Ellipsoid Plot (ORTEP) and CrystalMaker software was deployed to explore the relationship and configuration of oxygen atoms between pseudo-hexagonal (100) goethite associated with hexagonal (001) hematite lattice planes. A schematic diagram of the epitaxial relationship between star-shaped (100) goethite, which is acting as a template facilitating later precipitation of (001) hexagonal prisms of hematite on it, is presented.

A novel protein CtCBP-1 functions as a crucial macromolecule during mineralization of limpet teeth

Limpets are a class of marine mollusks that use mineralized teeth, one of the hardest and strongest biological materials, to feed on algae on rocks. By combining proteomics and RNA-seq analysis of limpet radula, we identified a novel chitin-binding protein (CtCBP-1) that may play a regulatory role in radula mineralization of Cellana toreuma. In this study, the full-length cDNA of CtCBP-1 gene was cloned for the first time, and the protein was successfully expressed in vitro. In vitro experiments demonstrated that CtCBP-1 binds well to both goethite and chitin, which are key components of the cusp. We studied the function of CtCBP-1 on goethite crystallization in vitro, revealing that it changed the morphology of goethite crystals. We also used fluorescence higher resolution imaging to map the binding of CtCBP-1 in radula and found that the distribution of CtCBP-1 on radula was specific, which consistent with the SEM results finding tightly aligned goethite. In this study, a novel protein CtCBP-1, which regulates the distinctive biomineralization process of limpet teeth, is identified for the first time. This protein’s identification may inform biomimetic techniques for creating hard materials that can withstand ambient temperature.

Influencing factors and environmental effects of interactions between goethite and organic matter: A critical review

This paper reviews progresses in the interactions between goethite and organic matter (OM) and their environmental effects in recent decades. The interactions mainly include the effect of organic matter on the surface properties and the crystallization of goethite, molecular changes of OM caused by goethite, and their interaction mechanisms, which can be depicted by the commonly used Langmuir model, the charge distribution multi-site complexation model (CD-MUSIC model), ligand charge distribution model (LCD model), and natural organic matter charge distribution model (NOM-CD model). The influencing factors of the interactions are summarized with emphasis on the external, including pH, ionic strength, carbon dioxide (CO2), and the internal, including the structure of OM and iron species. The goethite—OM complexes caused by the interactions will affect migration and transformation of conventional heavy metals and emerging antibiotics. The complexes, as the carrier of carbon and iron, are also the critical parts of the carbon and iron cycles, which are associated with climate change. This review provides a basis for future mechanism studies of formation, transformation, and effects of goethite—OM complexes (particulate OM or carbon-contained minerals) in different environmental systems at a molecular level.

Disparate relationships between pH and goethite-water 18O/16O and D/H fractionation factors

For goethites synthesized at low (∼1–2) and high (∼13–14) extremes of pH, the values of 1000ln18αgt-w at ca. 25 °C are, respectively, +6.2(±0.6) and + 0.9(±0.6). Note that 18αgt-w = (18O/16O)gt/(18O/16O)w and gt = goethite (α-FeOOH); w = liquid water. A steady-state, flux-balance model indicates that this pH-related difference in values of 1000ln18αgt-w arises because of an effectively irreversible oxygen flux associated with aqueous Fe(III) hydroxide adsorption on, and incorporation in, goethite during growth at crystal surfaces. The model predicts that the transition from a low-pH to a high-pH value of 1000ln18αgt-w is relatively abrupt. At 22 to 25 °C, extant data indicate that the transition occurs in the interval of pH from about 6 to 8. Consequently, uncertainty about the pH of ancient waters could be a factor in interpretations of goethite δ18O values. However, an experimentally determined equation for the temperature dependence of 1000ln18α(LpH) has been applied with apparent success to natural goethites that formed in the presence of meteoric waters (18α(LpH) = low-pH 18αgt-w). This suggests that those ancient waters may have been acidic (pH < 6) in the immediate environments of goethite crystallization.Published values of D/H fractionation between synthetic goethite and water (Dαgt-w) show no systematic effect of either pH or temperature (T) over a range of pH from 1 to 14 and a range of T from 22 °C to 62 °C. Within these ranges, the average value of Dαgt-w is 0.905(±0.005) – a value which seems to characterize the D/H fractionation observed in natural goethites. If hydrogen in transitional, chemisorbed Fe(III) hydroxides – represented here as Fe(OH)3A – rapidly exchanges D/H with ambient water during crystal growth, there is a model-based expectation that Dαgt-w will not be affected by pH. This postulated D/H exchange would cease when Fe(OH)3A irreversibly transitions to FeOOH during crystallization – i.e., when Fe, O, and H are incorporated in the structure of the growing crystal.Thus, the flux balance model suggests that the observed dichotomy in the effects of pH on 18αgt-w and Dαgt-w is a result of distinctly different exchangeabilities of 18O/16O and D/H in transitional Fe(III) hydroxides that are chemisorbed on growing surfaces of goethite crystals. Observed values of 18αgt-w and Dαgt-w may represent steady-state – but not necessarily thermodynamic equilibrium – distributions of isotopes between H2O and the oxygen or hydrogen in the interiors of crystallizing goethites. Nevertheless, these steady-state values of Dαgt-w and 18αgt-w are generally reproducible and systematically relate δ18O and δD values of goethite to environmental variables.

Revealing the radiation damage and Al-content impacts on He diffusion in goethite

Over the past few decades, the (U-Th)/He geochronological method on goethite has been more and more applied to date laterite formation and evolution or ore-deposit formation. However, questions remain on possible He loss by diffusion due to the polycrystalline structure of goethite and associated underestimation of the goethite crystallization date given by the (U-Th)/He age. Prior studies estimated helium loss in goethite to range from 2 to 30%, but no relation or models have been produced to explain such values. To clarify the situation, we firstly performed a complete review of experimental He diffusion data in natural goethite, that reveals the link between activation energy and He loss with the damage dose. To understand He diffusion behavior in goethite and model the He loss, natural defect and alpha damage as well as the chemical composition and growth structure effect on He diffusion have been investigated thanks to a multi-scale study. We used numerical simulations combining the Density Functional Theory (DFT) at the atomic scale and Kinetic Monte Carlo (KMC) simulations at the macroscopic scale. We found that He diffusion is purely anisotropic along the preferential elongated axis (i.e., b-axis) and He leaks out easily in defect-free goethite and Al-goethite. The consequence of this anisotropy is that crystallographic defects and alpha damage strongly lower the He diffusivity in goethite and Al-goethite by obstructing the diffusion channel or trapping He along the b-axis. Defect and damage impact on He diffusion is even larger for Al-goethite. The obtained He diffusion parameters for goethite containing defect and damage are similar to the activation energy and He diffusional loss obtained in natural goethite from the literature. We demonstrate the systematic dependence of the diffusion coefficient with damage dose and the impact of Al on He retention. He atoms are retained only at the favor of obstructions blocking the diffusion and vacancies trapping them in the goethite structure. The consequence of the diffusive behavior is that a part of He diffuses out of the crystal until sufficient damage accumulated along the b-axis. The diffusion domain size is the channel length along the b-axis rather than the whole crystallite size. To correct the He loss this study proposes estimation of the He retention and needed corrections for different types of goethite.

Dissolution behavior and porous kinetics of limonitic laterite during nitric acid atmospheric leaching

In our previous study, the mineral evolution and porous kinetics during nitric acid pressure leaching of limonitic laterite has been investigated. In order to optimize the nitric acid leaching technology, this paper investigated the dissolution behavior and porous kinetics of limonitic laterite during nitric acid atmospheric leaching. The experimental results showed that Ni and Fe had consistent leaching behavior, whereas Co was only consistent with Mn in the early stages of leaching. The DFT (Density functional theory) simulations of goethite dissolution revealed that H+ attacked the bridging O first with adsorption energy of −1.37 eV, while NO3– attacked the Fe with adsorption energy of −3.07 eV, hence promoting the dissolution of Fe from the goethite crystals. Based on meticulous modeling analysis, the leaching of Ni could be described by grain model-cylindrical particle. In the early stage of leaching, the Ni extraction is regulated by chemical reactions with an activation energy of 88.4 kJ/mol. In the latter stage, the leaching is regulated by pore diffusion with an effective diffusivity of 1.37 × 10-11 m2/min at 90 °C. This study clarified the dissolution behavior of limonitic laterite, and described nickel leaching using a porous kinetic model.

Genesis of alunite-bearing kaolin deposit in Mudamköy member of the Miocene Göbel Formation, Mustafakemalpaşa (Bursa), Turkey

Tectonic-controlled hydrothermal alteration of Lower Miocene volcanic rocks in the Mustafakemalpaşa (Bursa) region resulted in the development of kaolin deposit, which have an economical potential. This study will be important for the mineralogical, geochemical, and genetic contributions to the ongoing mining operation of this kaolin deposit and its possible application fields. The upward mineralogical and geochemical zonation of the kaolin deposit is as follows: kaolin, alunite-bearing kaolin, silicified kaolin, and silcrete; associated with Fe-oxide bearing veins and silicification. The characteristics of the kaolinized and alunitized units and their parent rocks were examined using polarized-light microscopy, X-ray powder diffractometry (XRD), scanning electron microscopy (SEM–EDX), chemical (ICP–AES and –MS), and isotopic (H–O, S–O) methods. Kaolinite and alunite are abundant and generally associated with quartz, feldspar, volcanic glass, and mica, locally smectite, accessory halloysite, and goethite. The kaolinite content increases up to 75% in the kaolin zone. Kaolinite shows both ideal hexagonal book-like and non-idiomorphic (initial stage) crystal forms associated with rhombohedral blocky alunite are formed authigenically on altered volcanic materials and locally covered by accessory star-like goethite and tube-like halloysite crystals. Under the open hydrological system and acidic conditions, the Al ± Fe/Si ratio and Al + K + S values increased compared to the parent rocks, favoring the formation of kaolinite and alunite, respectively. The enrichment of Sr, Ba, and depletion of Fe, Rb, Ce, Y, and La, and the negative Eu anomaly shows that the fractionation of feldspar and mica originated from Miocene volcano-sedimentary components during the magmatic and hydrothermal alteration process(es) were the main sources for the formation of kaolinite and alunite. The δD and δ18O composition and formation temperatures of kaolinite are 106–167 °C, suggesting an epithermal alteration process(es). The negative δ34S and positive δ18O values for alunite exhibit local contribution of meteoric water mixed with magmatic fluid during hydrothermal activity.

Formation of iron oxyhydroxides as a result of glauconite weathering in soils of temperate climate

The aim of this study was to investigate glauconite weathering in soils of temperate climate, with special attention paid to formation of iron oxyhydroxides at the expense of the glauconite. Five soil profiles developed on parent material in which glauconite was the main (if not the only) phyllosilicate and source of Fe were selected for the study. Basic physical and chemical properties of bulk (<2 mm) soil samples (color, texture, pH, and carbonate and soil organic carbon content) were determined. Clay fractions (<2 µm and <0.2 µm) and green pellets were separated from the studied soils and analyzed using X-ray diffractometry, Fourier-transform infrared spectroscopy, Mössbauer spectroscopy, and inductively coupled plasma - optical emission spectrometry. Thin sections prepared from selected undisturbed soil materials were observed under optical microscope. Three of the soils were acidic with pH ranging from 4.3 to 5.5 and the remaining two were alkaline with pH from 6.8 to 8.6. Parent glauconites from each profile showed quite similar chemical composition, except that in one case Fe(II) was much more abundant than in the others. In both acidic and alkaline soils, glauconite weathering products included Al-smectite-rich glauconite-smectite mixed-layered minerals (G-S), kaolinite, and Fe oxyhydroxides. G-S were likely formed by transformation of glauconite, whereas kaolinite and iron oxyhydroxides (i.e. goethite, lepidocrocite, and perhaps ferrihydrite) crystallized in the studied soils from substances dissolved from parent glauconites. Leaching of Fe and Mg from the glauconite structure and loss of K were indicated. In acidic soils the weathering products concentrated in the clay fractions, whereas in alkaline soils they occurred both in clays and in the glaucony pellets. Mössbauer spectroscopy indicated that fine crystalline and especially nanocrystalline iron oxyhydroxides are substantial products of glauconite weathering in all the studied clays. Formation of the Fe oxyhydroxides was controlled by hydrological conditions (and related redox activity) and by soil pH. Alkaline conditions seemed to favor goethite crystallization, whereas redox activity led to formation of lepidocrocite and likely ferrihydrite. Soil color was mostly controlled by the presence of glauconite. Only the relatively significant lepidocrocite content in a few of the studied soil horizons was marked by brown or rusty colors indicating high redox activity.

Determination of Band Structure of Naturally Occurring Goethite with Al Substitution: A Case Study of Zhushan Iron Zone.

The photocatalytic property of Fe oxide minerals has long been considered to play an important role in shaping modern terrestrial environments. However, due to the complexity of natural settings, a precise determination of the band structure of natural goethite has not been achieved. In this work, the mineralogical characteristics of natural goethite samples obtained from Zhushan, China, were systematically studied through X-ray diffraction, transmission electron microscopy, X-ray energy dispersive spectroscopy, and X-ray fluorescence spectroscopy. Afterward, the band structure for both natural and synthetic goethite samples was determined by synchrotron-based X-ray absorption and emission spectra and photoelectron spectroscopy. The band gap of natural goethite (2.25 eV) was narrower than that of its synthetic counterpart (2.55 eV), and the valence band position of natural goethite was slightly lifted (−5.06 eV) compared to that of synthetic goethite (−5.38 eV). Al doping in natural goethite crystal, as revealed by the mineralogical tests, was the main reason that contributed to this difference. The theoretical calculation showed the narrowed band gap was caused by the contribution of Al-2p orbits at the top of the valence band. Therefore, free electrons can be created under light irradiation with a shorter wavelength. The experiments showed that natural goethite can photo-catalytically degrade methyl orange, and the degradation efficiency was better (47.5%) than that of the synthetic goethite group (31.5%). This study, for the first time, revealed the band structure and confirmed the photocatalytic properties of natural goethite, which should play an important role in surface substance evolution and elemental cycling.

Geochemical insights into formation of enigmatic ironstones from Rio Grande rise, South Atlantic Ocean

Rio Grande Rise (RGR) is an intraplate oceanic elevation in the South Atlantic Ocean that formed at a hotspot on the Mid-Atlantic Ridge during the Cretaceous. In spreading center and hotspot environments, ironstones form mainly by biomineralization of reduced Fe from hydrothermal fluids or oxidation of sulfide deposits. However, RGR has been considered aseismic and volcanically inactive for the past 46 Ma. Here, we investigate the origin of ironstones collected from the summit of RGR using multiple techniques: petrographic observations, X-ray diffraction, U-Th/He geochronology, and chemical composition. The ironstones from RGR consist of finely laminated goethite containing igneous rock fragments, carbonate fluorapatite, and calcite. Our results suggest that Fe oxyhydroxides were precipitated by Fe-oxidizing bacteria forming bacterial mats. The bacterial Fe mats underwent compaction, dewatering, goethite crystallization, and cementation that created the ironstone deposits. U-Th/He geochronology reveals protracted goethite minimum ages extending from the late Miocene to the Quaternary, probably due to multiple generations of mats, slow mineralization rates, and Fe-oxide dissolution-reprecipitation cycles. Flame-like goethite structures underneath FeMn crusts and a chimney-shaped goethite sample with a central channel indicate that the dewatering fluid flowed upward through the deposits, or a thermal fluid source may have been introduced from below the ironstone deposits. High Fe/Mn ratios, low trace metals contents (Ni + Co + Cu), and very low Fe/REY ratios suggest ironstone precipitation from a hydrothermal fluid; however, REYSN plots and bivariate CeSN /CeSN⁎ versus YSN/HoSN and CeSN /CeSN⁎ versus Nd plots are inconclusive, and a proximal source of magma was unlikely during the period of mat formation. Given this evidence, we hypothesize that a geothermal circulation system may have facilitated ironstone mineralization at RGR.

Inclusions in calcite phantom crystals suggest role of clay minerals in dolomite formation

AbstractMicro- and nano-inclusions embedded in calcite phantom crystals from Gemerská Ves, Slovak Republic, have been characterized by a combination of Raman spectroscopy, scanning and transmission electron microscopy, X-ray powder diffraction, and C and O isotope analysis. Whereas the outer, colorless part of the phantom crystal is relatively homogeneous and cavity and inclusion-free, the inner terracotta-colored part contains abundant cavities, dolomite, hematite, goethite, titanite, phyllosilicates (mainly kaolinite and illite), and apatite inclusions and nanostructures that have formed on the walls of cavities. The nanostructures comprise hematite and goethite particles sandwiched between either two phyllosilicate crystals or a phyllosilicate and a carbonate (calcite or dolomite) crystal. Our observations suggest that all inclusions in the terracotta calcite originate from the terra rossa (a common soil type in karstic areas) and limestone outcropping adjacent to the calcite crystals. While the micrometer-sized phyllosilicate and hematite particles were likely transported from the terra rossa and attached to the surface of growing calcite, the presence of phyllosilicates that are only a few atomic layers thick and of euhedral hematite, goethite, and dolomite crystals suggests that these particles precipitated along with the phantom calcite in situ, from an aqueous solution carrying terra rossa-derived and limestone-derived solutes. The compositional differences between the terra rossa (e.g., smectite as the only major Mg-rich phase) and terracotta calcite inclusions (e.g., dolomite as the only major Mg-rich phase and the presence of only Mg-free clays) hint that a smectite-illite conversion provides the Mg necessary for the precipitation of dolomite and possibly the Fe associated with the iron oxyhydroxide nanostructures. Phyllosilicate nucleation on calcite and dolomite nucleation on phyllosilicates, as inferred from nanoscale mineralogical associations, suggest that carbonates and phyllosilicates may mutually enhance nucleation and growth. This enhancement may result in the formation of large-scale clay-carbonate successions in aqueous settings, including the enigmatic, pink-colored cap dolostones succeeding late Neoproterozoic “Snowball Earth” deposits. The distribution of inclusions in the terracotta calcite and the preferred nucleation of hematite and goethite on phyllosilicate, rather than on carbonate surfaces, indicates that phyllosilicates have a potential to not only disrupt crystal growth and trigger the formation of cavities in the structure of the calcite host, but also to provide surfaces for the precipitation of different phases in the cavities and to uniformly distribute otherwise incompatible materials in a calcite host crystal. This calls for further exploration of the potential application of phyllosilicates in composite structure development.

Reaction kinetics of shearing-enhanced goethite process for iron removal from zinc solution

A shearing-enhanced goethite process for iron removal is proposed to overcome the difficulties in process reaction control and non-uniform mass transfer. The reaction kinetics of the oxidation of Fe2+ to α-FeOOH and crystal growth of α-FeOOH were studied in a ZnSO4-FeSO4 system. Oxidation and crystal growth experiments were performed under the following conditions: shearing speed = 1000–4000 rpm, flow rate of oxygen = 0.3–1.5 L min−1, and reaction temperature = 70–90 °C. The results show that the oxidation rate increases with increasing shearing speed and reaction temperature, owing to the increase in oxygen mass transfer. The degree of crystallization of goethite improves and the needle-like structure is further refined as the shearing speed increases and the temperature decreases. The activation energy of nucleation is about 54 kJ mol−1. The rate equations are derived for the oxidation and oxygen mass transfer processes. The oxidation kinetics parameters are calculated by data fitting. The rate equations of oxygen mass transfer and Fe2+ oxidation are mO2=54.01kLO∙PO2HO2−CO2∙MO2∙vg∙n0.4 and r=47.08CFe2+2∙CO2CFe2++0.001865, respectively.

Nanometer scale insight on the analysis of limpets mineralized teeth: Special focus on the silica-containing regions

We report the electron microscopy-based analysis of the major lateral tooth of the limpet Colisella subrugosa during early and intermediate stages of development. We aimed to analyze the structural relationship among the needle-like crystals of the iron oxide goethite, the amorphous silica phase that forms the tooth base and occupy inter-crystalline spaces in the cusp, and the chitin fibers of the matrix. Goethite crystals followed the three dimensional organization pattern of the chitin fibers in the cusp. In the tooth base, spherical individual silica granules were found in regions where the chitin fibers cross. The spherical granules near the interface between the tooth base and the cusp (junction zone) formed an almost continuous medium that could easily be ultrathin-sectioned for further analysis. By contrast, the nearby silica-rich region localized on the other side of the junction zone contained needle-like goethite crystals immersed in the matrix and presented a conchoidal fracture. The chitin fibers from the silica granules of the tooth base were dotted or undulating in projection with a periodicity of about 6 nm when observed by high magnification transmission electron microscopy. Very thin goethite crystals were present in the base of the cusp near the junction zone surrounded by silica. On several occasions, crystals presented internal thin straight white lines parallel to the major axis, indicating a possible growth around fibers. We propose that silica and iron oxide phases mineralization may occur simultaneously at least for some period and that silica moderates the dimensions of the iron oxide crystals.

Raman spectroscopy of alpha-FeOOH (goethite) near antiferromagnetic to paramagnetic phase transition

Synthetic powder, ore samples, and mineral single crystals of goethite (α-FeOOH) were investigated with polarized Raman spectroscopy at temperatures from 293 K to 473 K. The symmetry of the vibrational modes, observed in different scattering configurations, was determined unequivocally. The assignment of the Raman-active modes to definite atomic vibrations is supported by two types of lattice-dynamical calculations: empirical shell model and ab initio density functional theory. The temperature dependencies of the line shape parameters of some Raman-active vibrations at temperatures near to the antiferromagnetic–paramagnetic phase transition infers for a significant spin–lattice coupling in this compound. The most informative in this aspect is the B3g phonon at 387cm−1, which overlays a broad scattering background and displays a pronounced asymmetric Fano-line shape. The asymmetry increases in the paramagnetic state above the Néel temperature (TN=393K), indicating a strong interaction of this mode with the underlying excitation continuum. The origin of the excitation background is discussed in light of our experimental results and the existing data for the magnetic structure and transport properties of α-FeOOH. By using the molecular-orbital dimer approach, we calculate the spin–phonon coupling constants for the B3g mode as J′=−0.2eV/Å and J″=+0.18eV/Å2, respectively. Thus, we rationalize that, most probably, the scattering background stems from magnetic excitations, and the asymmetric shape of the B3g band is a result of a linear spin–phonon coupling of this mode with the Fe–O1–Fe spin dimers. Another mechanism, a phonon interaction with thermally activated charge carriers above TN, is also considered.

Mineralogical-Geochemical Study of Corroded Iron-Based Metals from a Salt Mine Environment

Metal fragments (pipe, chain, valves), at advanced stages of corrosion, were collected underground in the Wieliczka salt mine. Macroscopically distinct zones of corroded material, as well as black blisters on the surface of different metal fragments, were studied using scanning electron microscopy with energy dispersive spectrometry (SEM-EDS), x-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and Mössbauer spectroscopy (MS). SEM-EDS studies showed various morphological forms with different chemical compositions. The original outer zone of the iron artifacts is mainly composed of aggregates of needle-shaped goethite crystals with idiomorphic crystals of halite. A subsequent zone, toward the inner margin, is composed mainly of fine granular aggregates of magnetite. Goethite (α-FeOOH) and akaganeite (β-FeOOH) form spherical, fibrous, and structureless aggregates in the next internal zone. Forms of aggregates seem to depend on the chloride content, 1 wt% to 3.5 wt% Cl in the structureless aggregates and 5 wt% to 9 wt% Cl in the regular ones. In addition, in the internal zone crystals of lepidocrocite form rosettes. Blisters are built of the acicular akaganeite crystals, which form fibrous aggregates in the shell and spherical ones in the interior. The relative concentrations of iron bearing minerals in the studied zones, i.e., akaganeite, hematite, goethite, magnetite, and lepidocrocite are established. Where they dominate, the zones are: black (magnetite), orange (goethite, lepidocrocite, akaganeite), and light brown (goethite).

Recovery and interpretation of the 18O/16O of Miocene oolitic goethites in multi-generational mixtures of Fe (III) oxides from a channel iron deposit of Western Australia

Published (U-Th)/He ages indicate that an oolitic Miocene channel iron deposit (CID) on Mesa J in the Hamersley province of Western Australia (∼22 °S latitude) contains three predominant generations of Fe (III) oxides – two generations of older hematite and one of younger goethite. Selective dissolution and sequential atom balance calculations were combined with chemical and oxygen isotope analyses to determine the δ18O values of hematite, goethite, and quartz in eight samples from a core drilled in that CID. For the quartz (Q), δ18OQ = +21.7(±0.9)‰. For the younger hematite (Hm1), δ18OHm1 ≥ +3.6(±0.6)‰, whereas for the older hematite (Hm2), δ18OHm2 ≤ +2.1(±0.8)‰. The oolitic CID goethite (G) crystallized in the late Miocene [7(±1) Ma] and is a solid solution represented as Fe(1−Y)AlY(CO3)XO(1−X)OH with values of δ18OG that range from −0.4‰ to +0.7‰. A mixing model for the solid solution expresses the value of δ18OG in terms of Y, X, relevant fractionation factors, and the δ18O of the endmember FeOOH (δ18Ogt). Mixing model-derived values of δ18Ogt range from −2.4‰ to −0.7‰.Paired values of δDgt and δ18Ogt in seven of eight late Miocene oolitic goethites from Mesa J indicate crystallization from meteoric waters at an average temperature of 20(±5) °C. In the late Miocene, Mesa J was in the subtropics at a paleolatitude of ∼29 °S, which invites comparison with the climate of the near-coastal community of Mingenew in Western Australia (presently at 29 °S latitude). The modern MAT at Mingenew is 20 °C – i.e., indistinguishable from the late Miocene Mesa J average of 20 °C. However, this similarity is not matched by the mean annual precipitation (MAP). The modern MAP at Mingenew is about 400 mm, but the lateritic weathering that produces abundant pedogenic Fe (III) oxides appears to require a seasonally contrasting climate with rainfall totals of at least 1300 mm/yr. In the late Miocene, southward-shifted summer season tropical cyclones might have delivered such large amounts of rain (with relatively low δD and δ18O values). If so, the δDgt and δ18Ogt of the oolitic goethites of Mesa J support the idea of a rainfall-driven, possibly microbially mediated, summer season bias in the timing of crystallization. Thermal buffering in the regolith can reconcile summer-dominated goethite crystallization with a temperature that would be analytically indistinguishable from the MAT. Comparison of the δ18Ogt values of the Mesa J goethites with published δ18O values of goethites of similar age from two other sites in the Hamersley province suggest a possible increase in δ18O of late Miocene rainfall with increasing distance from the coast.

Transformation of siderite to goethite by humic acid in the natural environment

Humic acid (HA) is particularly important in iron-bearing mineral transformations and erosion at the water-mineral boundary zone of the Earth. In this study, three stages of the possible pathway by which HA causes mineral transformation from siderite to goethite are identified. Firstly, a Fe(II)-HA complex is formed by chelation, which accelerates the dissolution and oxidation of Fe(II) from the surface of siderite. As the Fe(II)-HA complex retains Fe atoms in close proximity of each other, ferrihydrite is formed by the agglomeration and crystallization. Finally, the ferrihydrite structurally rearranges upon attachment to the surface of goethite crystals and merges with its structure. The influence of low concentrations of HA (0–2 mg/L) on phosphate adsorption is found to be beneficial by the inducing of new mineral phases. We believe that these results provide a greater understanding of the impact of HA in the biogeochemical cycle of phosphate, mineral transformation.

Enhancement of phosphate adsorption during mineral transformation of natural siderite induced by humic acid: Mechanism and application

Abstract The interfacial interactions between siderite, humic acid (HA), and phosphate, is critical to understand the transport of HA and phosphate in the environment. In this study, the influence of HA on the adsorption of phosphate by siderite not only includes competitive adsorption, but also includes indirectly facilitated adsorption. It was attributed to that humic acid as a representative natural organic matter possesses abundant carboxylic groups, which facilitate chelation between minerals and HA, and then the phosphate adsorption performance is affected. The experimental results concluded that three stages of the transformation from siderite to goethite under the action of HA can be divided. Firstly, chelation can be formed between HA and the ferrous ions from siderite, and it is favorable for the oxidation of Fe(II) by the O2 in water to form amorphous ferrihydrite; and then the ferrihydrite nano-particles were attached to the surface of goethite crystals; finally, the goethite crystal structure was formed by the structural rearrangement of ferrihydrite nano-particles. Meanwhile, the surface complex species of phosphate was formed on precipitate. It is revealed that the diprotonated bidentate binuclear complex is the main phosphate species when the HA concentration was 0.5 mg/L; as the HA concentration rose to 10 mg/L, the main phosphate species change to diprotonated monodentate mononuclear complex, which is attributed to the competitive adsorption of humic acid. We believe that these results are beneficial to guide the impact of HA for the biogeochemical cycling of phosphate, iron-bearing mineral transformation, as well as environmental remediation.