Simultaneous incorporation of Mn and Al in the goethite structure

Two series of (Al,Mn)-substituted goethites were synthesized from ferrihydrite made in alkaline media, with different Al/Mn mole ratios ([Al + Mn]/Fe molar ratio up to 0.12). Powder X-ray diffraction and extended X-ray absorption fine structure (EXAFS) techniques were used to assess the structural characteristics of the simultaneous substitution in goethite. XRD patterns revealed that all the obtained solids remain in a goethite-like structure. Rietveld refinement of X-ray diffraction data indicates that the increasing Mn substitution and consequent decrease of Al substitution causes an increase in the unit cell volume. This change is accompanied by the increment of the various Me-Me distances. XANES spectra at the Al and Mn K-edge confirm the octahedral coordination of Al and the trivalent oxidation state of the Mn ion in all the synthesized samples. EXAFS spectra at the Fe K-edge indicate that the local order around the Fe atom remains practically constant upon (Mn,Al) substitution. Measurements in the Mn K-edge show that distances Mn-Me suffer different changes with the increase in Mn substitution: a marked decrease in E and a slight decrease in E′, while DC remains constant. E and E′ values correspond to the distance between one Mn and one neighboring Me (Fe, Mn, Al) atom, both situated in two polyhedra linked by an edge. These polyhedra belong to the same double row of the goethite structure. DC value corresponds to the distance between one Mn and one Me (Fe, Mn, Al) atom, situated in two octahedral linked by one corner and belonging to two adjacent double chains. All the intermetallic distances are minor than the corresponding singly substituted goethites, this fact is attributed to the structure contraction due to the presence of Al(III) which restrains the axial distortion of Mn. Dissolution-time curves, resulting from exposure to 6 M HCl at 318 K, show that the dissolution rate slows with increasing Al substitution and consequent decrease of Mn substitution, and the shape of the curve becomes increasingly sigmoidal for mixed goethite with large Al content and Al-goethite. Dissolution kinetics of most samples are well described by the Kabai equation. Al dissolves almost congruently with respect to Fe, implying that it is homogeneously distributed in the structure. However, the convex χ_Mn:χ_Fe curve indicates that Mn tends to be concentrated in the outer layers of the goethite particles.     

Evidence for manganese substitution in synthetic goethite

Goethite (α-FeOOH) synthesized in the presence of Mn incorporates up to ca. 15 mole% Mn into its structure. This conclusion is based on a shift towards the isostructural groutite (α-MnOOH) of the unit cell dimensions a, b and c, and of the OH-bending modes. These results and a chemical test indicate trivalency of the incorporated Mn. The dehydroxylation (DTA) and the crystallinity (XRD line broadening) are also modified by Mn incorporation. All modifications of the goethites are analogous to the Al-for-Fe substitution.     

Coexisting goethite and gibbsite from a high-paleolatitude (55°N) Late Paleocene laterite: Concentration and C/C ratios of occluded CO2 and associated organic matter

A Late Paleocene (∼60 Ma BP) lateritic soil from Northern Ireland (the Antrim paleosol, herein referred to as Nire) contains coexisting goethite, gibbsite, phyllosilicate, and hematite. The Fe(III) oxides exhibit pisolitic and Liesegang-type morphologies that are mutually exclusive in hand specimens. X-ray diffraction (XRD) measurements of Al substituted for Fe in goethite indicate two populations: (1) low-Al, Liesegang-type goethites (∼0 mol% Al) and (2) high-Al, pisolitic goethites (∼9 to ∼24 mol% Al). Selective dissolution and incremental vacuum dehydration-decarbonation were used to determine the concentration and δ¹³C values of CO₂ occluded in the respective structures of the goethites and gibbsites in this complex mixture of Nire lateritic minerals. The Fe(CO₃)OH component in the high-Al goethites appears to retain a proxy carbon isotopic record of vadose zone CO₂ in the ancient soil. The δ¹³C values of CO₂ occluded in coexisting goethites and gibbsites indicate that these minerals did not form in equilibrium with the same environmental CO₂.The measured mole fractions (X) of Fe(CO₃)OH in the high-Al goethites range from 0.0059 (±0.0005) to 0.0077 (±0.0006) and correspond to soil CO₂ concentrations of ∼28,000 to ∼37,000 ppmV. The average values of X and δ¹³C for the four high-Al goethites are 0.0067 ± 0.0007 and −20.1 ± 0.5‰, respectively. The δ¹³C value of the organic matter undergoing oxidation in this midlatitude (∼55°N) Late Paleocene soil appears to have been ∼ −28.2‰. Taken together, these data indicate an atmospheric CO₂ concentration of ∼2400 ppmV (± ∼1200 ppmV) at ∼60 Ma BP. The inferred high concentration of atmospheric CO₂ would have been coincident with the warm global climate of the Late Paleocene and is consistent with the idea that CO₂ plays an important role in climate variation.     

Experimental tests of the effects of Al substitution on the goethite-water D/H fractionation factor

Twelve goethite samples with different degrees of substitution of Al for Fe were synthesized at 22-48 °C and pH values of 1.5-14 under closed system conditions and used to study the effects of Al substitution on the hydrogen isotopic fractionation between goethite and its ambient water. The syntheses followed two pathways: (1) Fe³⁺ hydrolysis in high pH aqueous solutions; (2) oxidation of Fe²⁺ to Fe³⁺ in mid to low pH solutions. XRD and SEM analyses indicated that, irrespective of temperature and pH, goethite was the predominant product of the syntheses in all of the experiments (with degrees of Al substitution as high as ∼13 mol %). “High temperature nonstoichiometric” (HTN) water is present in all of the samples and rapidly exchanges D/H with ambient vapor at room temperature. Uncertainties in the value of the apparent D/H fractionation factor (α_e-v) between HTN water and ambient exchange water at 22 °C lead to significant uncertainties in determinations of the δD values of structural hydrogen (δD_s) in goethites which contain high proportions of HTN water. As determined for the samples of this study, α_e-v has a nominal value of 0.942 (±0.02). δD_s values determined using an α_e-v value of 0.942 indicate that Al substitution increases the δD value of structural hydrogen in goethite by about 1.4 (±0.4)‰ for each increase in Al of 1 mol %. This dependence on Al is of the same sign as, but somewhat larger in magnitude than, the effect of Al predicted by a published model (∼0.7‰ per mol % Al). The overall uncertainties in the current results suggest that an increase of ∼1‰ per mol % Al, as adopted by previous studies, may be a reasonable estimate with which to adjust δ D_s values of natural goethites to those of the pure FeOOH endmember and could be valid for degrees of Al substitution of up to at least 15 mol %. These synthesis experiments also yield a hydrogen isotopic fractionation factor (^Dα_G-W) between pure goethite (α-FeOOH) and liquid water of 0.900 (±0.006), which is analytically indistinguishable from the published value of 0.905 (±0.004). Thus, use of an ^Dα_G-W value of 0.905 in applications to the FeOOH component of natural goethites is supported by the current study.     

Temperature dependence of goethite dissolution promoted by trihydroxamate siderophores

This article reports an investigation of the temperature dependence of goethite dissolution kinetics in the presence of desferrioxamine B (DFO-B), a trihydroxamate siderophore, and its acetyl derivative, desferrioxamine D1 (DFO-D1). At 25 and 40°C, DFO-D1 dissolved goethite at twice the rate of DFO-B, whereas at 55°C, the behavior of the two ligands was almost the same. Increasing the temperature from 25 to 55°C caused little or no significant change in DFO-B or DFO-D1 adsorption by goethite. A pseudo-first-order rate coefficient for dissolution, calculated as the ratio of the mass-normalized dissolution rate coefficient to the surface excess of siderophore, was approximately the same at 25 and 40°C for both siderophores. At 55°C, however, this rate coefficient for DFO-D1 was about half that for DFO-B. Analysis of the temperature dependence of the mass-normalized dissolution rate coefficient via the Arrhenius equation led to an apparent activation energy that was larger for DFO-B than for DFO-D1, but much smaller than that reported for the proton-promoted dissolution of goethite. A compensation law was found to relate the pre-exponential factor to the apparent activation energy in the Arrhenius equation, in agreement with what has been noted for the proton-promoted dissolution of oxide minerals and for the complexation of Fe³⁺ by DFO-B and simple hydroxamate ligands in aqueous solution. Analysis of these results suggested that the siderophores adsorb on goethite with a only single hydroxamate group in bidentate ligation with an Fe(III) center.     

Siderophore-promoted dissolution of cobalt from hydroxide minerals

Recent research has revealed that siderophores, a class of biogenic ligands with high affinities for Fe(III), can also strongly complex Co(III), an element essential to the normal metabolic function of microbes and animals. This study was conducted to quantify the rates and identify the products and mechanisms of the siderophore-promoted dissolution of Co from synthetic Co-bearing minerals. The dissolution reactions of heterogenite (CoOOH) and four Co-substituted goethites (Co-FeOOH) containing different Co concentrations were investigated in the presence of a trihydroxamate siderophore, desferrioxamine B (DFOB), using batch and flow-through experiments. Results showed that DFOB-promoted dissolution of Co from Co-bearing minerals may occur via pH-dependent ligand-promoted or reductive dissolution mechanisms. For heterogenite, ligand-promoted dissolution was the dominant pathway at neutral to alkaline pH, while production of dissolved Co(II) for pH <6. It was not possible from our data to decouple the separate contributions of homogenous and heterogeneous reduction reactions to the aqueous Co(II) pool. Cobalt substitution in Co-substituted goethite, possibly caused by distortion of goethite structure and increased lattice strain, resulted in enhanced total dissolution rates of both Co and Fe. The DFOB-promoted dissolution rates of Co-bearing minerals, coupled with the high affinity of Co(III) for DFOB, suggest that siderophores may be effective for increasing Co solubility, and thus possibly Co bioavailability. The results also suggest that siderophores may contribute to the mobilization of radioactive ⁶⁰Co from Co-bearing mineral phases through mineral weathering and dissolution processes.     

Aluminous hematite: A mössbauer study

Aluminous hematites prepared in three different ways have been examined at 300K and 4.2K using the Mössbauer technique. The results indicate significant differences between the behaviour of aluminous hematites that have been subjected to high temperatures (>600° C) and those which have not. The magnitude of the room temperature quadrupole splitting of the former increases with aluminium content, approaching at ～16 mole percent substitution the value (?0.22 mm/s) exhibited by all of the low temperature specimens. This variation may be explained qualitatively in terms of a preferential c-axis contraction of the lattice upon incorporation of aluminium, which does not occur unless a c-axis defect structure is removed by subjection of the hematite to high temperatures. The solid solubility limits of high and low temperature hematites (～15 mol % and ≥19 mol % respectively) also differ, as do the room temperature decreases in hyperfine splitting (?0.82 kOe/mol % Al and ?0.86 kOe/mol % Al). At 4.2 K only low temperature hematite exhibits a decrease in hyperfine splitting with increasing Al content (?0.40 kOe/mol % Al). The absolute values of the recoil free fractions of hydrothermally prepared aluminous hematites have been determined at 4.2 K (0.70±0.02 — pure hematite, 0.82±0.02, 14±2 mol % Al substitution) and exhibit a similar increase with Al content to that previously observed for aluminous goethites. The room temperature recoil free fraction of pure hematite has been measured to be 0.64±0.02. The effects of particle size on both hyperfine splitting and recoil free fraction have been investigated.     

Aluminum coprecipitates with Fe (hydr)oxides: Does isomorphous substitution of Al for Fe in goethite occur?

Iron (hydr)oxides are common in natural environments and typically contain large amounts of impurities, presumably the result of coprecipitation processes. Coprecipitation of Al with Fe (hydr)oxides occurs, for example, during alternating reduction-oxidation cycles that promote dissolution of Fe from Fe-containing phases and its re-precipitation as Fe-Al (hydr)oxides. We used chemical and spectroscopic analyses to study the formation and transformation of Al coprecipitates with Fe (hydr)oxides. In addition, periodic density functional theory (DFT) computations were performed to assess the structural and energetic effects of isolated or clustered Al atoms at 8 and 25 mol% Al substitution in the goethite structure. Coprecipitates were synthesized by raising the pH of dilute homogeneous solutions containing a range of Fe and Al concentrations (100% Fe to 100% Al) to 5. The formation of ferrihydrite in initial suspensions with ?20 mol% Al, and of ferrihydrite and gibbsite in initial suspensions with ?25 mol% Al was confirmed by infrared spectroscopic and synchrotron-based X-ray diffraction analyses. While base titrations showed a buffer region that corresponded to the hydrolysis of Fe in initial solutions with ?25 mol% Al, all of the Al present in these solutions was retained by the solid phases at pH 5, thus indicating Al coprecipitation with the primary Fe hydroxide precipitate. In contrast, two buffer regions were observed in solutions with ?30 mol% Al (at pH ∼2.25 for Fe³⁺ and at pH ∼4 for Al³⁺), suggesting the formation of Fe and Al (hydr)oxides as two separate phases. The Al content of initial coprecipitates influenced the extent of ferrihydrite transformation and of its transformation products as indicated by the presence of goethite, hematite and/or ferrihydrite in aged suspensions. DFT experiments showed that: (i) optimized unit cell parameters for Al-substituted goethites (8 and 25 mol% Al) in clustered arrangement (i.e., the formation of diaspore-like clusters) were in good agreement with available experimental data whereas optimized unit cell parameters for isolated Al atoms were not, and (ii) Al-substituted goethites with Al in diaspore-like clusters resulted in more energetically favored structures. Combined experimental and DFT results are consistent with the coprecipitation of Al with Fe (hydr)oxides and with the formation of diaspore-like clusters, whereas DFT results suggest isomorphous Al for Fe substitution within goethite is unlikely at ?8 mol% Al substitution.     

A combined FTIR and TPD study on the bulk and surface dehydroxylation and decarbonation of synthetic goethite

The thermal dehydroxylation of a goethite-carbonate solid solution was studied with combined Fourier-transform infrared (FTIR)-Temperature programmed desorption (TPD) experiments. The TPD data revealed dehydroxylation peaks involving the intrinsic dehydroxylation of goethite at 560 K and a low temperature peak at 485 K which was shown to be associated to the release of non-stoichiometric water from the goethite bulk and surface. The FTIR and the TPD data of goethite in the absence of adsorbed carbonate species revealed the presence of adventitious carbonate mostly sequestered in the goethite bulk. The release of carbonate was however not only related to the dehydration of goethite but also from the crystallization of hematite at temperatures exceeding 600 K. The relative abundance of surface hydroxyls was shown to change systematically upon goethite dehydroxylation with a preferential stripping of singly-coordinated OH sites followed by a dramatic change in the dominance of the different surface hydroxyls upon the formation of hematite.     

Seasonal variations of ochreous precipitates in mine effluents in Finland

Ochreous precipitate and water samples were collected from the surroundings of seven closed sulphide mines in Finland. In the Hammaslahti Zn–Cu–Au mine, Otravaara pyrite mine and Paroistenjärvi Cu–W–As mine, the collection was repeated in different seasons to study mineralogical and geochemical variations of precipitates. The sampling was done in 1999–2002 from the ditches and drainage ponds of the tailings and waste rock piles that are susceptible to seasonal changes. Mineralogy of the precipitates was evaluated by X-ray diffraction (XRD) and infrared spectroscopy (IR), and precipitate geochemistry was examined by selective extractions. Schwertmannite (Fe₈O₈(OH)₆SO₄) was the most typical Fe hydroxide mineral found. Goethite was almost as common as schwertmannite, was often poorly ordered, and contained up to 10 wt.% of SO₄. Goethite and schwertmannite were commonly found as mixtures, and they occurred in similar pH and SO₄ concentrations. Ferrihydrite (nominally Fe₅HO₈ · 4H₂O) was typically found in areas not influenced by acid mine drainage, and also in acid mine waters with high organic matter or As content. Jarosite (KFe₃(SO₄)₂(OH)₆) was found only in one site. In addition, some gypsum (CaSO₄ · 2H₂O) and aluminous sulphate precipitates (presumably basaluminite, Al₄(SO₄)(OH)₁₀ · 5H₂O) were identified. Selective extractions showed that acid extracts Fe_tot/S_tot-ratios of schwertmannite and goethite samples were similar, but the ratio of oxalate-extractable to total Fe, Fe_ox/Fe_tot, of goethite samples were lower than those of the schwertmannite samples. Only Al, Si and As were bound to precipitates in substantial amounts, up to several wt.%. In schwertmannites and goethites, Al, Cu, Co, Mn and Zn were mostly structural, substituting for Fe in an Fe oxyhydroxide structure or bound to surface adsorption sites in pores limited by diffusion. In ferrihydrites, heavy metals were also partly bound in adsorbed form dissolving in acid ammonium acetate. Ferrihydrites and goethites were more enriched in Co, Mn and Zn than schwertmannites, but schwertmannites and ferrihydrites were more enriched in As than goethites. Mineralogical and geochemical evidence showed that in the spring, after the snowmelt, the acid mine drainage precipitates were predominantly schwertmannite, and were partly transformed during warm summer months to goethite. The phase transformation of precipitates was followed by a decrease in pH values and increase in SO₄ concentrations of waters. Adsorbed As retarded the phase transformation.     

Study of Al-substituted hematites, prepared from thermal treatment of lepidocrocite

A study of the characteristics of the Morin transition in aluminous hematites, α-(Fe_1?xAl_x)₂O₃, produced from thermally transformed lepidocrocites, is reported. Six compositions with Al contents between 0.2 and 10 at% have been considered. It is argued that these samples present the advantage that they contain smaller amounts of hydroxyl and water as compared to hematites obtained by other preparation methods. The samples were characterised by a variety of conventional techniques, including thermal analyses, X-ray diffraction, FTIR, TEM/EDX, BET surface-area measurements and diffuse reflectance spectroscopy. All results indicate that the Al is structurally incorporated in the hematite lattices. Transmission Mössbauer spectra were recorded at various temperatures between 80?K and room temperature in order to precisely determine the Morin-transition region and the spin structure in both the low-temperature antiferromagnetic and weakly ferromagnetic states. It was found that the Morin-transition temperatures are markedly higher as compared to similar hematites made from aluminous goethites and that a transition phenomenon persists to an Al substitution of up to at least 10 at%. This different behaviour is ascribed to lower concentrations of structural hydroxyl groups in these lepidocrocite-based hematites.     

Goethite, calcite, and organic matter from Permian and Triassic soils: carbon isotopes and CO2 concentrations

Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO₂ components—atmospheric CO₂ and CO₂ from in situ oxidation of organic matter. The δ¹³C values measured for the Fe(CO₃)OH component in solid solution in these Permian goethites are −13.5‰ for the Lower Leonardian (∼283 Ma BP) paleosol (MCGoeth) and −13.9‰ for the Upper Leonardian (∼270 Ma BP) paleosol (SAP). These goethites contain the most ¹³C-rich Fe(CO₃)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO₂ components. δ¹³C measured for 43 organic matter samples in the Lower Leonardian (Waggoner Ranch Fm.) has an average value of −20.3 ± 1.1‰ (1s). The average value yields a calculated Early Permian atmospheric Pco₂ value of about 1 × PAL, but the scatter in the measured δ¹³C values of organic matter permits a calculated maximum Pco₂ of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO₃)OH in MCGoeth and SAP correspond to soil CO₂ concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO₂ concentrations are similar to modern soils in warm, wet environments.The average δ¹³C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth (Waggoner Ranch Fm.) range from −6.5‰ to −4.4‰, with a mean δ¹³C value for all profiles of −5.4‰. Thus, the value of Δ¹³C between the pedogenic calcite data set and MCGoeth is 8.1 (±0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric Pco₂ and organic matter δ¹³C values were the same for both paleosol types. Furthermore, the atmospheric Pco₂ calculated for the Early Permian from the average measured carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1 × PAL, with a maximum calculated atmospheric Pco₂ (permitted by one standard deviation of the organic matter δ¹³C value) of ∼5 × PAL.If, however, measured average δ¹³C values of the plant organic matter are more positive than the original soil organic matter as a result of diagenetic loss of ¹³C-depleted, labile organic compounds, calculated Permian atmospheric Pco₂ using these ¹³C-enriched organic values would underestimate the actual atmospheric Pco₂ using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO₂ concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO₂.The Fe(CO₃)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in ¹³C relative to Fe(CO₃)OH in goethites from soils in which there are mixtures of two isotopic CO₂ components. Field-relationships and the δ¹³C value (−1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO₂ components at the time of goethite crystallization. The three components were probably atmospheric CO₂, CO₂ from in situ oxidation of organic matter and CO₂ from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO₂ components, as recorded by Fe(CO₃)OH in goethite, has been described in modern soil, this is the first example from a documented paleosol. Its preservation affirms the need for careful, case-by-case assessment of ancient paleosols to establish that goethite in any particular soil is likely to be a valid proxy of atmospheric Pco₂.     

Arsenate and chromate incorporation in schwertmannite

High concentrations of Cr (up to 812 ppm) and As (up to 6740 ppm) were detected in precipitates of the mineral schwertmannite in areas influenced by acid mine drainage. Schwertmannite may act as well as a natural filter for these elements in water as well as their source by releasing the previously bound elements during its dissolution or mineral-transformation. The mechanisms of uptake and potential release for the species arsenate and chromate were investigated by performing synthesis and stability experiments with schwertmannite.Schwertmannite, synthesized in solutions containing arsenate in addition to sulphate, was enriched by up to 10.3 wt% arsenate without detectable structural changes as demonstrated by powder X-ray diffraction (XRD). In contrast to arsenate, a total substitution of sulphate by chromate was possible in sulphate-free solutions. Thereby, the chromate content in schwertmannite could reach 15.3 wt%.To determine the release of oxyanions from schwertmannite over time, synthetic schwertmannite samples containing varying amounts of sulphate, chromate and arsenate were kept at a stable pH of either 2 or 4 over 1 year in suspension. At several time intervals Fe and the oxyanions were measured in solution and alterations of the solid part were observed by XRD and Fourier-Transform infrared (FT-IR) spectroscopy. At pH 2 schwertmannite partly dissolved and the total release of arsenate (24%) was low in contrast to chromate (35.4–57.5%) and sulphate (67–76%). Accordingly, the ionic activity product (log IAP) of arsenated schwertmannite was lowest (13.5), followed by the log IAP for chromated schwertmannite (16.2–18.5) and the log IAP for regular (=non-substituted) schwertmannite (18). At pH 4 schwertmannite transformed to goethite, an effect which occurred at the fastest rate for regular schwertmannite (=arsenate- and chromate-free), followed by chromate and arsenate containing schwertmannite. Both chromate and more evidently arsenate have a stabilizing effect on the schwertmannite structure, because they retarded the dissolution and transformation reactions.These kinetic investigations as well as crystallographic considerations demonstrated that the strength of the Fe(III) complexes with the anions controls the formation process and the stability of schwertmannite: with increasing affinity of the oxyanions to form complexes with Fe(III), the strength of the resulting binding and thus the stability and substitution preference increases.     

Aluminous goethite: A mössbauer study

Mössbauer measurements at 300 K, 77 K and 4.2 K and X-ray data are presented for synthetic aluminous goethites (α Fe_1?x Al _x OOH) in two series containing up to 15 mole percent aluminium (hydrothermal preparation) and 19 mole percent aluminium (low-temperature preparation). The Mössbauer spectra for specimens at 300 K and 77 K display broadened and relaxed line-shapes with the relaxation rate increasing with aluminium substitution, whereas all the 4.2 K spectra can be described by a single magnetically split spectrum. At 4.2 K the magnitude of this splitting is 505 kOe for pure goethite and it decreases by 0.52 kOe per mole percent aluminium substitution. The absolute value of the recoil-free fraction f at 4.2 K has been measured for pure goethite and for aluminous goethites containing 7, 15 and 19 mole percent aluminium; it increases from f=0.69±0.02 to f=0.89±0.02 in this range. The increase is attributed to a stiffening of the goethite lattice as it contracts to accommodate the smaller aluminium ion. At 300 K f is found to decrease from f=0.65±0.05 for pure goethite to f=0.50±0.03 for goethite with 19 mole percent aluminium.     

Paleoenvironmental implications of concentration and C/C ratios of Fe(CO3)OH in goethite from a mid-latitude Cenomanian laterite in southwestern Minnesota

A mid-Cretaceous (∼95 Ma) laterite in southwestern Minnesota contains pisolites that consist primarily of gibbsite, quartz, and kaolinite with smaller amounts of goethite (α-FeOOH) and hematite. The presence of minor berthierine (an Fe(II) sheet silicate) suggests that this Cenomanian laterite experienced some degree of low temperature reductive diagenesis during its burial history. The prospects for extracting useful paleoenvironmental information from the pisolitic goethite were explored by studying measured mole fraction (X_m) and δ¹³C_m values of the Fe(CO₃)OH component in solid solution in the goethite using the method of incremental vacuum dehydration-decarbonation.Data arrays that occupy distinctly different domains in plots of δ¹³C_m vs. 1/X_m suggest the existence of two generations of goethite in the pisolites. The apparently younger generation of goethite (“generation 2”) evolves CO₂ at 170 °C, while the older generation (“generation 1”) evolves CO₂ at 220 °C. The distribution of the data suggests that generation 2 goethite is a proxy for mixing of CO₂ from three distinct CO₂ sources in an oxidative environment which post-dated the reductive diagenesis. The small amount of generation 1 goethite seems to have persisted through the reductive diagenesis, and nine of the generation 1 goethite data appear to define a proxy, two-endmember, soil CO₂ mixing line. Such two-component mixing is consistent with expectations for a highly evolved, carbonate-free laterite (i.e., the pre-diagenetic Cenomanian weathering system). The δ¹³C_m values of these nine data points range from −23.1‰ to −13.7‰, whereas X_m values range from 0.0007 to 0.0222. Linear regression of these nine data yields a slope of 0.0064, which corresponds to an ancient tropospheric CO₂ concentration of about 1900 ppmV.Isotopic data from pisolitic kaolinite indicate a paleotemperature of about 24 °C at a paleolatitude of ∼40°N. This is substantially warmer than modern continental temperatures at such latitudes and is consistent with published indications of a generally warmer mid-Cretaceous climate. Moreover, the correspondence of a warmer mid-Cretaceous climate with the inferred, relatively high concentration of Cenomanian tropospheric CO₂ (∼1900 ppmV) is consistent with the idea that variations of atmospheric CO₂ have a relation to climate change. The results of this study emphasize the importance of careful evaluation of incremental dehydration-decarbonation data from natural goethites to assess the possibility that more than one generation of goethite is present in a sample. However, the results also indicate that the carbon isotope information recorded in admixed goethite generations may be sorted out and used in paleoenvironmental interpretations.     

Sorption and transport of aqueous FeII in a goethite-coated sand column under anoxic conditions

Experiments were conducted under static batch and dynamic flow conditions to evaluate the sorption of Fe^II onto three goethites (G1, G2 and G3) having different crystal habits, morphologies and surface properties. Results reveal that G1 exhibited the highest Fe^II sorption extent and lowest kinetic rate constant, which may result from higher surface site density, surface roughness and edge surface faces. Surface complexation modeling parameters derived from batch experiments were combined with hydrodynamic parameters to simulate breakthrough curves in goethite-coated sand packed columns. The total sorbed amount of Fe^II at complete breakthrough was in agreement with that expected from the batch experiments, except for G1. Sorption breakthrough predictions that make use of surface complexation parameters accurately predicted Fe^II mobility in G2 and G3 columns, but poorly in G1 column. Experiments at various flow rates in G1 columns represented different amounts of Fe^II sorbed at complete breakthrough, thereby underscoring the impact of kinetic sorption. Moreover, Fe dissolution/re-precipitation or Fe^II-induced transformation of goethite was suspected at the lowest flow rate in the G1 column. The influence of goethite phase specific reactivity on Fe^II sorption under batch versus advective–dispersive flow is herein demonstrated. These findings have strong implications to assess transport of Fe^II and environmental contaminants both in natural and engineered systems.     

The preparation and characterization of vanadium-substituted goethite: The importance of temperature

Despite a close geo-chemical association between vanadium (V) and iron (Fe) in natural environments, there is little research on the substitution of V in goethite. To assess the effect of temperature on V-substitution in goethite, a series of V-substituted goethite were prepared under varying synthesis temperatures, and analysed using wet chemical and multi-spectroscopic techniques. Vanadium substitution was inversely related to synthesis temperatures and was hindered by the oxidation of V³⁺ to V^4+/5+ as indicated by X-ray absorption near-edge spectroscopy. The presence of V (V⁵⁺ > V⁴⁺ > V³⁺) at high temperature hindered the nucleation of goethite and crystal growth along particular faces resulting in large-sized and twinned crystals as shown by transmission electron microscopy. The large-sized goethite crystals released more Fe (mmoles) per unit surface area during proton-promoted dissolution than the smaller-sized crystals, which could be due to distorted V^4+/5+ local coordination environments in the mineral structure. The dissolution studies showed a heterogeneous distribution of V and/or crystal defects in goethite crystals. The results show that low synthesis temperatures preserved the oxidation state of V³⁺, which has ionic radius and hydrolytic properties similar to Fe³⁺, and hence resulted in as much as 13.3 mol per cent substitution. The structural stability of the goethite decreased upon V-substitution in order; V³⁺ > V⁴⁺ > V⁵⁺. This research provides important information about the interaction between temperature, V incorporation, and crystal structure properties of goethite for V sequestration and other potentially toxic metal cations.     

Impacts of hydrous manganese oxide on the retention and lability of dissolved organic matter

Minerals constitute a primary ecosystem control on organic C decomposition in soils, and therefore on greenhouse gas fluxes to the atmosphere. Secondary minerals, in particular, Fe and Al (oxyhydr)oxides—collectively referred to as “oxides” hereafter—are prominent protectors of organic C against microbial decomposition through sorption and complexation reactions. However, the impacts of Mn oxides on organic C retention and lability in soils are poorly understood. Here we show that hydrous Mn oxide (HMO), a poorly crystalline δ-MnO₂, has a greater maximum sorption capacity for dissolved organic matter (DOM) derived from a deciduous forest composite O_i, O_e, and O_a horizon leachate (“O horizon leachate” hereafter) than does goethite under acidic (pH 5) conditions. Nonetheless, goethite has a stronger sorption capacity for DOM at low initial C:(Mn or Fe) molar ratios compared to HMO, probably due to ligand exchange with carboxylate groups as revealed by attenuated total reflectance-Fourier transform infrared spectroscopy. X-ray photoelectron spectroscopy and scanning transmission X-ray microscopy–near-edge X-ray absorption fine structure spectroscopy coupled with Mn mass balance calculations reveal that DOM sorption onto HMO induces partial Mn reductive dissolution and Mn reduction of the residual HMO. X-ray photoelectron spectroscopy further shows increasing Mn(II) concentrations are correlated with increasing oxidized C (C=O) content (r = 0.78, P < 0.0006) on the DOM–HMO complexes. We posit that DOM is the more probable reductant of HMO, as Mn(II)-induced HMO dissolution does not alter the Mn speciation of the residual HMO at pH 5. At a lower C loading (2 × 10² μg C m^?2), DOM desorption—assessed by 0.1 M NaH₂PO₄ extraction—is lower for HMO than for goethite, whereas the extent of desorption is the same at a higher C loading (4 × 10² μg C m^?2). No significant differences are observed in the impacts of HMO and goethite on the biodegradability of the DOM remaining in solution after DOM sorption reaches steady state. Overall, HMO shows a relatively strong capacity to sorb DOM and resist phosphate-induced desorption, but DOM–HMO complexes may be more vulnerable to reductive dissolution than DOM–goethite complexes.     

Mineralogy and geochemistry of laterites from the Morro dos Seis Lagos Nb (Ti,REE) deposit (Amazonas,Brazil)

The Morro dos Seis Lagos niobium deposit (2897.9 Mt at 2.81 wt% Nb₂O₅) is associated with laterites formed by the weathering of siderite carbonatite. This iron-rich lateritic profile (>100 m in thickness) is divided into six textural and compositional types, which from the top to the base of the sequence is: (1) pisolitic laterite, (2) fragmented laterite, (3) mottled laterite, (4) purple laterite, (5) manganiferous laterite, and (6) brown laterite. All the laterites are composed mainly of goethite (predominant in the lower and upper varieties) and hematite (predominant in the intermediate types, formed from goethite dehydroxylation). The upper laterites were reworked, resulting in goethite formation. In the manganiferous laterite (10 m thick), the manganese oxides (mainly hollandite, with associated cerianite) occur as veins or irregular masses, formed in a late event during the development of the lateritic profile, precipitated from a solution with higher oxidation potential than that for Fe oxides, closer to the water table. Siderite is the source for the Mn. The main Nb ore mineral is Nb-rich rutile (with 11.26–22.23 wt% Nb₂O₅), which occurs in all of the laterites and formed at expense of a former secondary pyrochlore, together with Ce-pyrochlore (last pyrochore before final breakdown), Nb-rich goethite and minor cerianite. The paragenesis results of lateritization have been extremely intense. Minor Nb-rich brookite formed from Nb-rich rutile occurs as broken spherules with an “oolitic” (or Liesegang ring structure). Nb-rich rutile and Nb-rich brookite incorporate Nb following the [Fe³⁺ + (Nb, Ta) for 2Ti] substitution and both contain up to 2 wt% WO₃. The laterites have an average Nb₂O₅ content of 2.91 wt% and average TiO₂ 5.00 wt% in the upper parts of the sequence. Average CeO₂ concentration increases with increasing depth, from 0.12 wt% in the pisolitic type to 3.50 wt% in the brown laterite. HREE concentration is very low.     

A model for O/O variations in CO2 evolved from goethite during the solid-state α-FeOOH to α-Fe2O3 phase transition

“Plateau” δ¹⁸O values of CO₂ that evolved from the Fe(CO₃)OH component during isothermal vacuum dehydrations (200-230 °C) of 18 natural goethites range from 8.2 to 28.1‰. In contrast, the measured δ¹⁸O values of the goethite structural oxygen range from −11.3 to 1.7‰. The results of this study indicate that the apparent oxygen isotope fractionation factor (¹⁸α_app) between plateau CO₂ and initial goethite is systematically related to the rate of isothermal vacuum dehydration. The nonlinear correlation and the magnitudes of the ¹⁸α_app values are predicted by a relatively simple mass balance model with the following assumptions: (1) the rate of isothermal vacuum dehydration of goethite (for the interval from 0 to ∼60 to 80% loss of structural hydroxyl hydrogen) can be reasonably well represented by first-order kinetics and (2) isotopic exchange between evolving H₂O vapor and solid occurs only in successive, local transition states. The generally good correspondence between the model predictions and the experimental data seems to validate these assumptions. Thus, the ¹⁸O/¹⁶O ratios of the evolved CO₂ can act as probes into the transient processes operating at the molecular level during the solid-state goethite-to-hematite phase transition. For example, the activation energy for the rate constant associated with the transition state, oxygen isotopic exchange between solid and H₂O vapor, is tentatively estimated as 28 ± 11 KJ/mol. Such knowledge may be of consequence in understanding the significance of ¹⁸O/¹⁶O ratios in hematites from some natural environments (e.g., Mars?).Kinetic data and δ¹⁸O values of CO₂ are routinely obtained in the course of measurements of the abundance and δ¹³C values of the Fe(CO₃)OH in goethite. The observed correlation between ¹⁸α_app and dehydration rates suggests that plateau δ¹⁸O values of evolved CO₂ may provide complementary estimates of the δ¹⁸O values of total goethite structural oxygen (O, OH, CO₂) with an overall precision of about ±1‰. However, because of isotopic exchange during the dehydration process, δ¹⁸O values of the evolved CO₂ do not reflect the original δ¹⁸O values of the CO₂ that was occluded as Fe(CO₃)OH in goethite.