Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism

F. Marc Michel; Vidal Barrón; José Torrent; María P. Morales; Carlos J. Serna; Jean-François Boily; Qingsong Liu; Andrea Ambrosini; A. Cristina Cismasu; Gordon E. Brown

doi:10.1073/pnas.0910170107

New Research In

Edited* by W. G. Ernst, Stanford University, Stanford, CA, and approved December 30, 2009 (received for review September 4, 2009)

Abstract

The natural nanomineral ferrihydrite is an important component of many environmental and soil systems and has been implicated as the inorganic core of ferritin in biological systems. Knowledge of its basic structure, composition, and extent of structural disorder is essential for understanding its reactivity, stability, and magnetic behavior, as well as changes in these properties during aging. Here we investigate compositional, structural, and magnetic changes that occur upon aging of “2-line” ferrihydrite in the presence of adsorbed citrate at elevated temperature. Whereas aging under these conditions ultimately results in the formation of hematite, analysis of the atomic pair distribution function and complementary physicochemical and magnetic data indicate formation of an intermediate ferrihydrite phase of larger particle size with few defects, more structural relaxation and electron spin ordering, and pronounced ferrimagnetism relative to its disordered ferrihydrite precursor. Our results represent an important conceptual advance in understanding the nature of structural disorder in ferrihydrite and its relation to the magnetic structure and also serve to validate a controversial, recently proposed structural model for this phase. In addition, the pathway we identify for forming ferrimagnetic ferrihydrite potentially explains the magnetic enhancement that typically precedes formation of hematite in aerobic soil and weathering environments. Such magnetic enhancement has been attributed to the formation of poorly understood, nano-sized ferrimagnets from a ferrihydrite precursor. Whereas elevated temperatures drive the transformation on timescales feasible for laboratory studies, our results also suggest that ferrimagnetic ferrihydrite could form naturally at ambient temperature given sufficient time.

The structural and physical properties of ferrihydrite, an exclusively nano-sized ferric oxyhydroxide, are of importance in explaining its chemical reactivity and wide variety of occurrences. In both pristine and contaminated soils and sediments, ferrihydrite acts as a natural filter of inorganic contaminants through sorption reactions, thus affecting their transport and fate in the environment. Biomineralization of ferrihydrite as the inorganic iron core in ferritin—the protein mainly involved in iron storage and homeostasis in the human body—also occurs in a vast number of organisms (1). Bloom-forming marine diatoms, for example, use ferritin for enhanced iron storage (2), which suggests that ferrihydrite may also have underlying importance in primary productivity in the world’s oceans.

A well-known example of a nanomineral (3), ferrihydrite has no known crystalline counterpart formed in the laboratory or found in nature. As such, the basic crystal structure (4–7) and physical properties of ferrihydrite [e.g., density, composition (7, 8), and magnetic properties (9–14)] have remained controversial. A variety of structural models have been proposed for ferrihydrite (see (4) for review) but all have proven difficult to confirm unequivocally by conventional crystallographic techniques. The most recent model, and arguably the most complete, was derived from analysis of the pair distribution function of x-ray total scattering data that suggested that the ferrihydrite structure, although disordered, could be described using a single structural phase (6). However, this model has since received criticism for giving a composition that is anomalously H-poor (7) as well as not fully satisfying the observed diffraction features (7), calculated bond valence sums (5), or the measured density (7, 8) of ferrihydrite. In addition, ferrihydrite, whether natural or synthetic, is generally found to be antiferromagnetic with superparamagnetic behavior at ambient temperature [see (15) and references therein]. Several studies also suggested an additional ferromagnetic-like component attributed to the presence of uncompensated surficial spins (16, 17), but this has been difficult to confirm due to the uncertainty regarding the relationships between magnetic behavior and basic crystal structure. The metastability of ferrihydrite, particularly at elevated temperatures, has led to added ambiguity regarding the ordering temperature (i.e., Néel or Curie) because direct measurement is not feasible (9). Nonetheless, it is generally accepted that ferrihydrite’s metastability is related in part to inherent structural disorder, although the nature and extent of this disorder remain poorly defined.

Understanding the relationship between the structure and magnetic properties of ferrihydrite, as well as changes in both as a function of aging, is of particular importance because it provides independent constraints on ferrihydrite structure and compositional variations with aging. Being antiferromagnetic at ambient temperature (15, 18) and with only a weak ferrimagnetic-like component, ferrihydrite is normally not considered in the interpretation of magnetic enhancement in soils on Earth (19) or Mars (20), nor is it considered useful in the tailoring of functional ferrimagnetic nanomaterials, in contrast with the ferrimagnets magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) (21, 22). However, as will be shown below, ferrihydrite aged at different temperatures in the presence of selected anions undergoes a significant magnetic enhancement corresponding to the formation of an intermediate phase preceding its transformation into hematite (α-Fe₂O₃) (11, 23–25), and this phase may play an important role in the magnetic enhancement of aerobic soils. Additionally, such understanding may lead to new insights about the formation of biogenic magnetic phases in organisms. For example, ferritin-derived ferrihydrite in humans, with varying degrees of structural order, has been related to different neurodegenerative diseases (26). In this regard, biogenic ferrimagnetic crystals, identified as magnetite, that have been found in human brain tissues (27–29) may originate from biodegradation of a ferrihydrite precursor.

Here we have synthesized a series of ferrihydrite samples with differing magnetic properties and structural order that allowed us to determine compositional and structural changes occurring with aging and to explain the enhanced magnetic properties mentioned above (see Methods Summary and SI Appendix). We used the atomic pair distribution function (PDF) derived from synchrotron high-energy x-ray total scattering to discriminate between competing structural models and detect subtle structural changes in the transformation of ferrihydrite to hematite. Both real- and reciprocal-space fitting of the total scattering show that the precursor ferrihydrite (disordered with antiferromagnetic-like behavior) undergoes a series of changes that lead to an ordered ferrimagnetic phase (ferrifh) with significantly larger particle size (10–12 nm), far fewer cation vacancies, and less lattice strain than its disordered precursor. The detailed characterization results represent a critical advance in our understanding of ferrihydrite because we are now able to validate the structural model proposed by Michel et al. (6) and show how prior inconsistencies in its diffraction characteristics, calculated bond valence sums, density, and composition can be explained.

Results & Discussion

Ferrihydrite Structural Variations with Aging.

As shown in Fig. 1A, the initial ferrihydrite (fh, t = 0 h) is characterized by a small number of poorly resolved Bragg maxima and a significant underlying diffuse scattering component typical of so-called 2-line ferrihydrite (30). The presence of citrate (at a molar ratio with respect to Fe of 3%) slows the transformation of ferrihydrite to hematite while promoting the formation of the intermediate ferrifh phase (see SI Appendix). After 14 h aging time at 175 °C in the presence of citrate, the structural transformation to hematite is complete. During the first 8 h of aging, corresponding to the period of significant magnetic enhancement (Fig. 1B), an extended set of diffraction maxima develop from regions formerly dominated by diffuse scattering (see SI Appendix) with no indication of a structural transformation prior to that involving the formation of hematite. These changes signal the formation of the ferrifh phase.

Fig. 1.

X-ray scattering and corresponding magnetic enhancement. (A) X-ray total scattering intensities collected as a function of sample aging time (h) at 175 °C with citrate. Transformation to hematite (Arrows) begins at t = 8 h (indicated by *) and is complete by 14 h. (B) Formation of a strongly magnetic intermediate phase indicated by increases in room temperature magnetic susceptibility (χ) and saturation magnetization (M_s) at low temperature (5 K), with both reaching maxima at approximately 11 h.

The transition from fh to ferrifh is accompanied by changes in particle size and composition. Relative to the precursor fh, the average particle size of the nonhematite fraction, as indicated by transmission electron microscopy, increases by approximately 200% (Fig. 2A) and corresponds to an approximately 50% decrease in specific surface area (Fig. 2B, see SI Appendix). In addition, density (Fig. 2B) and total Fe (Fig. 2C) of the solid phase during the initial 8 h aging period increased by 15% and 18%, respectively, and are related inversely to the amount of water lost (OH^-, H₂O) as determined by thermogravimetric analyses (TGA) (Fig. 2C). Continuous weight losses, corresponding to the removal of loosely bound or surface-adsorbed water (< 125 °C) and dehydroxylation of more strongly bound structural water (> 125 °C), are observed by TGA. For example, fh undergoes weight losses of 7% and 20% when heated to 125 °C and 1000 °C, respectively. After 8 h of aging, the losses are 2% and 8%, respectively, and the final hematite product has a total weight loss of only approximately 2% (see SI Appendix).

Fig. 2.

Changes in particle size, physicochemical properties, and phase abundances as a function of aging. (A) Average particle size from TEM analysis. Bars indicate standard deviations. (B) Density (Open Diamonds) from pycnometry and specific surface area (Light Shade) from N₂-BET as a function of aging. (C) Total Fe (Solid Squares) of solid phase and hydration weight losses by TGA (Shaded). Medium and dark shaded regions indicate weight losses after heating to 1,000 °C and 125 °C, respectively. Note that data for t = 14 h are representative of single-phase hematite. (D) Percent phase abundances of precursor ferrihydrite (fh), ferrimagnetic ferrihydrite (ferrifh), and hematite (hm) from chemometric analysis.

The Fourier transform of the normalized total scattering (a portion of which is shown in Fig. 1A) results in the PDF, or G(r) function (see Methods Summary), and gives the probability of finding an atom at a given distance r from another atom (31). The real-space distribution of atom-atom correlations in the PDFs also shows variations with aging (Fig. 3A, see SI Appendix) but with the following important advantage over the analysis of reciprocal-space scattering: The PDF is sensitive to variations in short- (r < ∼ 5 Å) and intermediate-range (r < ∼ 15–20 Å) order (31). Indeed, real-space analysis of the PDFs allows phase identification and estimation of phase abundances, provides quantitative structural information, and, in certain cases, helps to discriminate between competing structural models (see SI Appendix).

Fig. 3.

Real- and reciprocal-space analyses. (A) Experimental PDFs for samples with aging times t = 0 h (Lower) to 14 h (Upper). (B) Individual PDFs for fh, ferrifh, and hm obtained by the MCR method for the three phases (Circles). Calculated fits for ferrihydrite or hematite overlain (Red) with difference plots below (Gray). (C) Comparison of the reciprocal-space scattering data at t = 8 h (Circles) with calculated diffraction profiles (Lines) based on fits of the PDF for ferrifh with models for (i) ferrihydrite, (ii) maghemite, and (iii) magnetite (∗= hematite).

Chemometric analysis provides a quantitative means of determining the number of phases consistent with the PDF data (see Methods Summary). The factor indicator (IND) function provides compelling evidence for the presence of only three distinct solid phases in our samples. A linear combination of the three dominant orthonormal basis vectors extracted from a singular value decomposition of the 13 PDFs, in fact, represents 99.8% of the variance of the PDF data. The multivariate curve resolution (MCR) method was used to rotate the three dominant vectors into real chemical space. These gave rise to individual PDFs for the fh, hematite, and an intermediate phase that can be ascribed to ferrifh (Fig. 3B). There is a dramatic decrease in the concentration of the precursor ferrihydrite phase down to 14% by 8 h of aging time, with a concomitant increase in ferrifh, followed by hematite formation (Fig. 2B). In contrast, in the absence of citrate, aging of fh results in transformation to hematite within 2 hr, with very little of the ferrifh intermediate observed (23).

The three PDFs representing the three phases were fit with only two structural models: (i) The single-phase model of ferrihydrite (6), or (ii) the known hematite structure (Fig. 3B, see SI Appendix). Surprisingly, the PDFs for both fh and ferrifh could be satisfactorily fit by the structural model of ferrihydrite proposed by Michel et al. (6). Additionally, the extracted structural parameters satisfy Pauling bond valence sums (see SI Appendix) and provide a remarkable reproduction of diffraction maxima for the sample aged for 8 h, thus providing strong evidence that the basic structure of the intermediate phase is indeed that of ferrihydrite (Fig. 3C, i). Alternative structural models, such as maghemite and magnetite, are clearly inconsistent with the diffraction data (Fig. 3C, ii and iii). Moreover, the basic ferrihydrite structure persists until rapid transformation to hematite occurs at t = > 11 h.

Variations in the intensities of particular correlations in the PDFs reflect subtle changes in the short- and intermediate-range order of fh that are attributable, in part, to an ordering phenomenon involving structural relaxation and the filling of partially vacant cation sites (Fig. 4A). Our previous study of disordered ferrihydrite (6) suggested the presence of vacancies in both the octahedral (Fe2) and tetrahedral (Fe3) sites, but complete filling of the octahedral (Fe1) site. During aging, filling of these cation vacancies is indicated by intensity increases of specific pair correlations (Fig. 4B and C, see SI Appendix). Refinement of the Fe site occupancy values during fitting of the experimental PDFs is consistent with these intensity increases and indicates that cation sites in the ferrihydrite structure are only fully occupied for samples with aging times ≥8 h (Fig. 4, Inset). Relative to this low vacancy form of ferrihydrite and when considered along with the observed changes in total Fe, the number of vacancies in the Fe2 and Fe3 sites in fh are 45–50% each, which is higher than previously estimated (6). One plausible mechanism for charge-balancing this number of cation vacancies in the ferrihydrite structure is the incorporation of three protons as hydroxyls into the vacant Fe³⁺ sites. Evidence for such proton substitutions for Si⁴⁺ has been found from neutron diffraction of D-substituted hydrogarnets (32). Our TGA results indicate a level of hydration in fh consistent with an average total hydration of approximately 7.4 protons per unit cell, with 5.4 protons being an amount sufficient to satisfy the approximately 45–50% vacancies in the Fe2 and Fe3 sites and 2 as hydroxyls on the O1 site (see next section and SI Appendix).

Fig. 4.

Evidence for Fe vacancies in ferrihydrite. (A) Intensity increases for specific correlations (Red Arrows) during initial 8 h aging time are attributed to filling of vacant Fe sites. Shifts in other correlations (Blue *) are due to the decreasing a-dimension of the unit cell during the fh → ferrifh transition. (B) Element-specific and (C) site-specific partials calculated from the refined structure of ordered ferrifh (Fig. 3B). (Inset) Refined occupancies (%) from PDF analysis normalized for total Fe for three Fe sites in ferrihydrite during aging.

Size-dependent structural relaxation in ferrihydrite is indicated by systematic and anisotropic variations in lattice dimensions occurring during the transition from fh to ferrifh, consistent with a reduction in strain (Fig. 5). These trends are observed in both real- and reciprocal-space fitting (see SI Appendix) and, during the initial 8 h aging, correspond to an approximately 1% increase in unit cell volume (see SI Appendix). If we consider the lattice parameter changes in ferrihydrite as a function of size, the data indicate that the sample with the smallest particles, i.e., that with the highest surface-to-volume ratio, is the most strained (Fig. 5, SI Appendix).

Fig. 5.

Evidence for strain in ferrihydrite. Size-dependent anisotropic changes in the c- (Circles) and a- (Diamond) lattice parameters indicate strain in ferrihydrite. Zero strain (Dashed Gray Line) corresponds to a theoretical crystalline ferrihydrite.

Composition and Density of Ferrihydrite.

The high defect concentrations in the form of cation vacancies in fh result in substantial deviations between measured and predicted compositions and densities. By combining detailed structural information with measured total Fe and hydration losses from TGA, we are now able to propose a composition for disordered fh of Fe_8.2O_8.5(OH)_7.4 + 3H₂O (see SI Appendix), which differs significantly from previously suggested compositions (15), as well as that of ordered ferrifh (Fe₁₀O₁₄(OH)₂ + ∼ H₂O). The compositional changes result in a large density increase of ferrifh (4.85–4.9 g cm^-3) relative to its disordered precursor (4.0–4.3 g cm^-3) (8, 33) that is consistent with the filling of cation vacancies in the basic structural model of ferrihydrite (6) (SI Appendix). As will be shown below, the large magnetic enhancement in the ferrifh intermediate can be explained by an ordering of the electron spin moments of iron in this low-defect structure.

Origin of Ferrimagnetism in Ordered Ferrihydrite.

M_s and χ of the initial ferrihydrite are typical of 2-line ferrihydrite and signify the initial presence of a parasitic, weakly ferrimagnetic phase. Smooth increases in χ and M_s (at 5 K) up to 58.5 A m² kg^-1 (for t = 11 h; see SI Appendix) during aging are concomitant with particle growth and specific changes in composition, keeping the basic structure of ferrihydrite.

A prior study using external-field ⁵⁷Fe Mössbauer spectroscopy indicated the presence of tetrahedral Fe(III) in a magnetically enhanced intermediate of ferrihydrite (24). Note that the authors of that study assigned one tetrahedral Fe plus another octahedral Fe to “hydromaghemite” and one-third of the octahedral Fe to a residual “6-line ferrihydrite.” These three Fe types are accounted for by the model proposed by Michel et al. (6). The evidence presented here further supports the idea that ferrimagnetic behavior in ferrihydrite can result from the arrangement of magnetic moments at tetrahedral and octahedral Fe sites rather than from a new phase.

M_s values of 20.4 and 58.5 A m² kg^-1 correspond to 3 and per formula unit for samples at t = 0 and 11 h, respectively. We explain this increase by proposing the existence of a ferrimagnetic structure in ferrifh consisting of magnetic moments in opposite directions and a net magnetization resulting from uncompensated electron spins probably at the octahedral positions (Fig. 6, see SI Appendix). Based on the proposed structure of ferrihydrite (6), if magnetic moments in all Fe3 (tetrahedral or A) and Fe2 (octahedral or B) sites are aligned with 4 spins in one direction and 6 spins opposing in Fe1 (octahedral or B) sites, as has been observed for hexagonal ferrites (34), a moment of is obtained (Fig. 6A). Alternatively, if magnetic moments in A and B sites are aligned antiparallel, as in magnetite, a resulting moment of would be expected (see SI Appendix). The nanometer size of the particles could account for a further reduction in the magnetization due to spin canting down to the experimental values measured for the ordered ferrihydrite sample prepared in this study (). Recent density functional theory calculations provide additional evidence that the model shown (Fig. 6) corresponds to the magnetic groundstate (35).

Fig. 6.

Possible electron spin orientations and magnetic moments in ordered ferrihydrite. Based on the Michel et al. 2007 structure of ferrihydrite (6), 6 Fe³⁺ with spins in one direction and 4 Fe³⁺ opposing results in a magnetic moment of . Alternatively, aligning the spins of the Fe3 (tetrahedral) and Fe2 (octahedral) sites antiparallel to one another results in a magnetic moment of .

Magnetic properties show strong size dependence. For example, M_s decreases linearly for magnetite/maghemite nanoparticles with decreasing crystallite size (77 and 12 A m² kg^-1 for particles of 13.5 and 4 nm in diameter, respectively) due to surface and internal spin canting (cation vacancy order-disorder) (36). The high field irreversibility observed for samples at t = 0 and 1 h, which is closely related to the existence of a certain degree of magnetic disorder, supports this assumption. Whether the origin of this magnetic disorder is related to the lack of crystallinity in the samples and/or the existence of some degree of cation vacancy disorder or is just a result of the reduced symmetry and uncompensated magnetic interactions of the surface spins is difficult to determine (36). Moreover, filling A sites in a sample aged from t = 0 to t = 11 h reinforces A–B interactions and the alignment of B moments, leading to an increase in the saturation magnetization. It can be concluded that M_s is therefore a structure-sensitive property for nanometer-sized particles and that it depends on the degree of cation site occupancy and surface and internal spin canting.

Although both the ordered ferrifh and ferrimagnetic magnetite and maghemite have strong magnetism, they are essentially different in several key aspects. First, temperature-dependent magnetic measurements (37) revealed a ferrihydrite phase with a Curie temperature (T_C = 13 °C) that is close to the value estimated by Berquó et al. (9) for Si-ferrihydrite. Due to the low T_C, the ambient-temperature M_s of the ordered ferrihydrite is highly reduced. Second, the coercivity of the ordered ferrihydrite at 5 K is much higher (52.5 mT) than that of magnetite and maghemite (typically only several tens of mT assuming that shape anisotropy is dominant). Therefore, the ordered ferrihydrite is characterized by both high coercivity (antiferromagnetism) and high magnetism (ferrimagnetism). Moreover, with increasing ordering, the antiferromagnetic-like behavior is gradually masked by the ferrimagnetism.

Previous studies have attributed the ferromagnetic-like moment of ferrihydrite to the presence of surface-uncompensated spins. Thus the ordered ferrihydrite (ferrifh) should be dominantly antiferromagnetic because the effect of the surface-uncompensated spins is significant only for finer grained ferrihydrite particles (fh) (e.g., 2–3 nm). In contrast, our results show that ferrifh is ferrimagnetic, and the disordered surface spins decrease rather than increase the bulk M_s for fh.

Does Ferrimagnetic Ferrihydrite Occur in Nature?

A temperature-dependent magnetic study (37) suggested the presence of an intermediate maghemite-like phase with a T_C of approximately 400° C. The present study, however, confirmed that the intermediate phase is essentially ordered ferrihydrite (ferrifh). The ferrihydrite → ordered ferrimagnetic ferrihydrite → hematite (fh → ferrifh → hm) transformation model is parsimonious and consistent with observations on the mineralogical and magnetic properties of soils from different geographic areas, as discussed in detail by Torrent et al. (38). Thus, the coexistence of ferrimagnets and hematite in soils suggests that magnetic enhancement and hematite formation are concomitant, whereas there is a general absence of ferrimagnets in soils dominated by formation of goethite (α-FeOOH) (38, 39). The alternative hypothesis that magnetic enhancement is preceded by microbially mediated Fe reduction (40, 41) - and Fe²⁺ in the soil solution reacts with a Fe³⁺ hydroxide phase to form magnetite (later oxidized to maghemite)—now appears unlikely because reduction of Fe³⁺ to Fe²⁺ requires the soil to be water-saturated for significant periods. Under these conditions, however, hematite is not formed (42), and the magnetic enhancement is therefore low also due to the absence of ferrimagnets.

Magnetic measurements indicate that the grain size of the ferrimagnets in soils lies mostly near the superparamagnetic (SP)—single domain (SD) threshold (approximately 20–25 nm), with little differences between soils from different areas (43, 44). This observation suggests a common origin of nano-sized soil ferrimagnets over a wide range of climatic conditions and initial input of natural magnetic phases. Although magnetic extracts of soils are commonly dominated by coarse-grained natural ferrimagnets that make characterization of nano-sized ferrimagnets in soils difficult by conventional methods (45), transmission electron microscope images revealed the presence of 10–50 nm magnetic particles in soils from the Russian Steppes (41). Fine-grained ferrimagnets in soils thus encompass the size range of ferrifh found in the present study.

Additional permissive evidence for the presence of ferrifh in magnetically enhanced soils is provided by the magnetic enhancement observed in the laboratory during the ambient-temperature aging of ferrihydrite in the presence of selected anions over hundreds of days (23), which could be due to the formation of an ordered ferrihydrite intermediate, in light of results from the present study. Therefore, a similar transformation in soils at ambient temperature is also possible, particularly if ligands such as citrate and phosphate are present. Although the magnetic data for our samples do not exclude the presence of maghemite per se (see SI Appendix), we find no x-ray diffraction evidence in this study [or in similar experiments at 25–175 °C (23)] for its formation. This finding is surprising given the qualitative similarities between the structures of ferrihydrite (6) and maghemite (15) and the fact that the stabilities of these phases have the following order: ferrihydrite < maghemite < hematite (46). Our findings may also be important for understanding other transformation pathways such as disordered ferrihydrite → goethite (47, 48).

Summary and Conclusions

This study has shown that there is no major difference in the overall structural topology between the disordered and ordered forms of ferrihydrite, and thus we are now able to confirm that the basic structure proposed for ferrihydrite (6) is verified in that it can reproduce both the real-space PDF and reciprocal-space diffraction data. We also propose a new composition for disordered ferrihydrite (Fe_8.2O_8.5(OH)_7.4 + 3H₂O), derived from a combination of TGA and total Fe measurements and constrained by the crystal structure, which is no longer H-poor. In addition, we can explain the anomalously low measured density of disordered ferrihydrite as being due to cation vacancies. Specifically, the nature of disorder in 2-line ferrihydrite can be understood as consisting, in part, of a random distribution of iron vacancies in two specific cation sites that are likely charge-balanced by the incorporation of structural protons. Moreover, the filling of cation vacancies during the transition from disordered ferrihydrite to ferrimagnetic ferrihydrite leads to an ordering of the electron spin moments of iron and corresponding magnetic enhancement.

Although the findings presented here represent a significant advance in confirming the general topology of the disordered ferrihydrite structure, a full description of the positions of all atoms is not yet available. For example, the surface structure of ferrihydrite is virtually unexplored and may exhibit varying degrees of restructuring relative to the particle interior. Even though we have assumed a periodic structural model with cation vacancies that reproduces the diffraction data, the surface structure of disordered ferrihydrite is unlikely to be truly periodic, based on observed changes in cell parameters and strain with structural ordering and increasing particle size. Additionally, the positions and suggested charge-balancing role of protons in the ferrihydrite structure are yet to be confirmed. In light of these unknowns, a significant future challenge will be determining positions of atoms, including protons, at both the particle surfaces and in the interior structural framework. Perhaps, in situ temperature-dependent studies, for example using proton NMR spectroscopy or neutron scattering, will allow the separation of these contributions and provide further insight into these remaining questions.

Our results reveal the relationship between crystal structure and magnetic structure in ordered ferrihydrite, and we also identify a pathway that may explain the magnetic enhancement in aerobic soils that is associated with the formation of nano-sized ferrimagnets from a ferrihydrite precursor. Although the existence of ordered ferrimagnetic ferrihydrite in soils or in ferritin in biological systems has not yet been verified, confirmation of this phase in these systems is a challenging goal stimulated by the present study.

Methods

Suspensions of 2-line ferrihydrite were prepared by precipitating 0.01M ferric nitrate with 1M potassium hydroxide to a final pH of 7. The initial solution contained citrate in a molar ratio citrate/Fe of 3% and was aged at 175 °C in individual Teflon-lined vessels and under aerobic, dark conditions for periods ranging from 0–14 h.

High-energy x-ray total scattering data were collected at beamline 11-ID-B (approximately 90 keV, λ = 0.13702(4) Å) at the Advanced Photon Source (APS), Argonne National Laboratory (ANL). PDFs were calculated from the Fourier transform of the reduced structure function truncated at 28 ± 0.5 Å^-1. For additional details regarding PDF analysis, and physical, chemical, and magnetic characterization, see SI Methods.

Chemometric analyses included an evaluation of the dimensionality of the PDFs using the factor indicator function (IND) (49) from the results of a singular value decomposition (SVD) of the 13 PDFs. The data matrix A_n×13 (n rows of atom-atom distances and 13 columns for each equilibration period) was represented as where U is a matrix of orthogonal (basis) output vectors of unit length, S is the vector and the transposed (T) vector V is a matrix of input basis vectors. The IND function identifies the number (k) of chemically relevant vectors reproducing A. The remaining vectors are associated with extractable errors (E). The resulting A_net matrix can be calculated with . A multivariate curve resolution (MCR) analysis (50) was carried out on the resulting A_net matrix. The basis vectors U were rotated into real chemical space, and the concentration profiles of the different components were simultaneously resolved starting from initial estimates obtained from evolving factor analysis (49). All calculations were carried out in the computational language of MATLAB (The Mathworks, Inc.).

MCR analysis is different from principal component analysis (PCA) because the eigenvectors of PCA are abstract whereas the vectors of MCR have a chemical meaning. MCR builds from the results of PCA or SVD (we used the latter) by rotating abstract PCA/SVD eigenvectors into real chemical space. This vector rotation procedure generates a concentration profile for each component which, in our case, corresponds to the reaction times of the ferrihydrite. The MCR method was therefore essential in extracting the intermediate ferrihydrite phase.

Acknowledgments

We thank Dr. Peter J. Chupas and Evan Maxey of the APS for assistance with x-ray data collection. This work was supported in part through the Stanford Environmental Molecular Science Institute (National Science Foundation Grant CHE-0431425) and, in part, from the U.S. Department of Energy (DOE), Office of Biological and Environmental Research, Environmental Remediation Sciences Program and National Science Foundation Grant EF-0830093 (Center for Environmental Implications of NanoTechnology) (F.M.M., A.C.C., and G.E.B., Jr.). This work was partly funded by Spain’s Ministry of Science and Innovation and European Regional Development Funds [Project MAT2008-01489 (M.P.M. and C.J.S.) and Project AGL2006-C03-02 (V.B. and J.T.)]. Support was provided by the 100 Talent Program of the Chinese Academy of Sciences and by National Nature Science Foundation of China Grant 40821091 (Q.S.L.). We are grateful for access to the APS-ANL which is supported by the U.S. DOE, Office of Basic Energy Sciences under Contract DE-AC02-06CH11357.

Footnotes

¹To whom correspondence should be addressed. E-mail: fmichel{at}stanford.edu.
Author contributions: F.M.M. and V.B. designed research; F.M.M., V.B., J.T., M.P.M., Q.L., A.A., and A.C.C. performed research; F.M.M., J.T., M.P.M., C.J.S., J.-F.B., Q.L., and A.A. analyzed data; F.M.M., V.B., J.T., M.P.M., C.J.S., J.-F.B., Q.L., and G.E.B.J. wrote the paper; and V.B. and J.-F.B. contributed new reagents/analytic tools.
The authors declare no conflict of interest.
This Direct Submission article had a prearranged editor.
This article contains supporting information online at www.pnas.org/cgi/content/full/0910170107/DCSupplemental.

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2. et al.
(2007) High crystallinity Si-ferrihydrite: An insight into its Néel temperature and size dependence of magnetic properties. J Geophys Res-Sol Ea 112:B02102.
OpenUrl CrossRef
↵
1. Berquó TS,
2. et al.
(2009) Effects of magnetic interactions in antiferromagnetic ferrihydrite particles. J Phys-Condens Mat 21:176005.
OpenUrl
↵
1. Cabello E,
2. et al.
(2009) Magnetic enhancement during the crystallization of ferrihydrite at 25 and 50 °C. Clays Clay Miner 57:46–53.
OpenUrl Abstract/FREE Full Text
↵
1. Coey JMD,
2. Readman PW
(1973) New spin structure in an amorphous ferric gel. Nature 246:476–478.
OpenUrl CrossRef
↵
1. Pankhurst QA,
2. Pollard RJ
(1992) Structural and magnetic-properties of ferrihydrite. Clays Clay Miner 40:268–272.
OpenUrl Abstract
↵
1. Pannalal SJ,
2. et al.
(2005) Room-temperature magnetic properties of ferrihydrite: A potential magnetic remanence carrier? Earth Planet Sci Lett 236:856–870.
OpenUrl CrossRef
↵
1. Cornell RM,
2. Schwertmann U
(2003) The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses (Wiley–VCH, Weinheim).
↵
1. Guyodo Y,
2. et al.
(2006) Magnetic properties of synthetic six-line ferrihydrite nanoparticles. Phys Earth Planet In 154:222–233.
OpenUrl CrossRef
↵
1. Zergenyi RS,
2. et al.
(2000) Low-temperature magnetic behavior of ferrihydrite. J Geophys Res-Sol Ea 105:8297–8303.
OpenUrl CrossRef
↵
1. Gider S,
2. et al.
(1995) Classical and quantum magnetic phenomena in natural and artificial ferritin proteins. Science 268:77–80.
OpenUrl Abstract/FREE Full Text
↵
1. Mullins CE
(1977) Magnetic-susceptibility of soil and its significance in soil science—Review. J Soil Sci 28:223–246.
OpenUrl CrossRef
↵
1. Morris RV,
2. et al.
(2006) Mossbauer mineralogy of rock, soil, and dust at Meridiani Planum, Mars: Opportunity’s journey across sulfate-rich outcrop, basaltic sand and dust, and hematite lag deposits. J Geophys Res-Planet 111:E12S15.
OpenUrl CrossRef
↵
1. Raj K,
2. Moskowitz R
(1990) Commercial applications of ferrofluids. J Magn Magn Mater 85:233–245.
OpenUrl CrossRef
↵
1. Tartaj P,
2. et al.
(2003) The preparation of magnetic nanoparticles for applications in biomedicine. J Phys D Appl Phys 36:R182–R197.
OpenUrl CrossRef
↵
1. Barrón V,
2. Torrent J
(2002) Evidence for a simple pathway to maghemite in Earth and Mars soils. Geochim Cosmochim Acta 66:2801–2806.
OpenUrl CrossRef
↵
1. Barrón V,
2. Torrent J,
3. de Grave E
(2003) Hydromaghemite, an intermediate in the hydrothermal transformation of 2-line ferrihydrite into hematite. Am Mineral 88:1679–1688.
OpenUrl Abstract/FREE Full Text
↵
1. Popov VV,
2. Gorbunov AI
(2006) Effect of organic acids on the hydrothermal crystallization of Fe(OH)₃. Inorg Mater 42:769–776.
OpenUrl CrossRef
↵
1. Bartzokis G,
2. et al.
(2007) Brain ferritin iron may influence age- and gender-related risks of neurodegeneration. Neurobiol Aging 28:414–423.
OpenUrl CrossRef PubMed
↵
1. Brem F,
2. et al.
(2005) Characterization of iron compounds in tumour tissue from temporal lobe epilepsy patients using low temperature magnetic methods. Biometals 18:191–197.
OpenUrl CrossRef PubMed
↵
1. Gossuin Y,
2. et al.
(2005) Looking for biogenic magnetite in brain ferritin using NMR relaxometry. NMR Biomed 18:469–472.
OpenUrl CrossRef PubMed
↵
1. Kirschvink JL,
2. Kobayashikirschvink A,
3. Woodford BJ
(1992) Magnetite biomineralization in the human brain. Proc Natl Acad Sci USA 89:7683–7687.
OpenUrl Abstract/FREE Full Text
↵
1. Michel FM,
2. et al.
(2007) Similarities in 2-and 6-line ferrihydrite based on pair distribution function analysis of x-ray total scattering. Chem Mater 19:1489–1496.
OpenUrl CrossRef
↵
1. Egami T,
2. Billinge SJL
(2003) Underneath the Bragg Peaks: Structural Analysis of Complex Materials (Elsevier, Oxford).
↵
1. Foreman DW
(1968) Neutron and x-ray diffraction study of Ca₃Al₂(O₄D₄)₃ a garnetoid. J Chem Phys 48:3037–3041.
OpenUrl CrossRef
↵
1. Towe KM,
2. Bradley WF
(1967) Mineralogical constitution of colloidal hydrous ferric oxides. J Colloid Interf Sci 24:384–392.
OpenUrl CrossRef
↵
1. Cullity BD
(1972) Introduction to Magnetic Materials (Addison–Wesley, Reading).
↵
1. Pinney N,
2. et al.
(2009) Density functional theory study of ferrihydrite and related Fe-oxyhydroxides. Chem Mater, DOI: 10.1021/cm9023875.
↵
1. Morales MP,
2. et al.
(1999) Surface and internal spin canting in gamma-Fe₂O₃ nanoparticles. Chem Mater 11:3058–3064.
OpenUrl CrossRef
↵
1. Liu QS,
2. et al.
(2008) Magnetism of intermediate hydromaghemite in the transformation of 2-line ferrihydrite into hematite and its paleoenvironmental implications. J Geophys Res-Sol Ea 113:B01103.
OpenUrl CrossRef
↵
1. Torrent J,
2. Barrón V,
3. Liu QS
(2006) Magnetic enhancement is linked to and precedes hematite formation in aerobic soil. Geophys Res Lett 33:L02401.
OpenUrl CrossRef
↵
1. Torrent J,
2. Liu QS,
3. Bloemendal J,
4. Barrón V
(2007) Magnetic enhancement and iron oxides in the Upper Luochuan Loess-paleosol sequence, Chinese Loess plateau. Soil Sci Soc Am J 71:1570–1578.
OpenUrl CrossRef
↵
1. Dearing JA,
2. et al.
(1996) Magnetic susceptibility of soil: An evaluation of conflicting theories using a national dataset. Geophys J Int 127:728–734.
OpenUrl CrossRef
↵
1. Maher BA,
2. Alekseev A,
3. Alekseeva T
(2003) Magnetic mineralogy of soils across the Russian Steppe: Climatic dependence of pedogenic magnetite formation. Paleaogeogr Palaeocl 201:321–341.
OpenUrl CrossRef
↵
1. Schwertmann U
(1985) The effect of pedogenic environments on iron oxide minerals. Adv Soil S 1:171–200.
OpenUrl
↵
1. Geiss CE,
2. Zanner CW
(2006) How abundant is pedogenic magnetite? Abundance and grain size estimates for loessic soils based on rock magnetic analyses. J Geophys Res-Sol Ea 111:B12S21O.
OpenUrl
↵
1. Liu QS,
2. et al.
(2005) Determination of primary magnetic minerals of a weathered metapelite xenolith from Zhouba region, North China, by combining thermomagnetic runs and low-temperature measurements. Chinese J Geophys-Ch 48:876–881.
OpenUrl
↵
1. Chen TH,
2. et al.
(2005) Characteristics and genesis of maghemite in Chinese loess and paleosols: Mechanism for magnetic susceptibility enhancement in paleosols. Earth Planet Sci Lett 240:790–802.
OpenUrl CrossRef
↵
1. Navrotsky A,
2. Mazeina L,
3. Majzlan J
(2008) Size-driven structural and thermodynamic complexity in iron oxides. Science 319:1635–1638.
OpenUrl Abstract/FREE Full Text
↵
1. Burleson DJ,
2. Penn RL
(2006) Two-step growth of goethite from ferrihydrite. Langmuir 22:402–409.
OpenUrl CrossRef PubMed
↵
1. Penn RL,
2. Erbs JJ,
3. Gulliver DM
(2006) Controlled growth of alpha-FeOOH nanorods by exploiting-oriented aggregation. J Cryst Growth 293:1–4.
OpenUrl CrossRef
↵
1. Malinowski ER
(2002) Factor Analysis in Chemistry (Wiley, New York).
↵
1. Jaumot J,
2. Gargallo R,
3. de Juan A,
4. Tauler R
(2005) A graphical user-friendly interface for MCR-ALS: A new tool for multivariate curve resolution in MATLAB. Chemom Intell Lab Syst 76:101–110.
OpenUrl CrossRef

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Proceedings of the National Academy of Sciences: 107 (7)

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Article Classifications

Physical Sciences
Geology

[1] ↵
Chasteen ND,
Harrison PM
(1999) Mineralization in ferritin: An efficient means of iron storage. J Struct Biol 126:182–194.
OpenUrl CrossRef PubMed

[2] Chasteen ND,

[3] Harrison PM

[4] ↵
Marchetti A,
et al.
(2009) Ferritin is used for iron storage in bloom-forming marine pennate diatoms. Nature 457:467–470.
OpenUrl CrossRef PubMed

[5] Marchetti A,

[6] et al.

[7] ↵
Hochella Jr MF,
et al.
(2008) Nanominerals, mineral nanoparticles, and Earth systems. Science 319:1631–1635.
OpenUrl Abstract/FREE Full Text

[8] Hochella Jr MF,

[9] et al.

[10] ↵
Jambor JL,
Dutrizac JE
(1998) Occurrence and constitution of natural and synthetic ferrihydrite, a widespread iron oxyhydroxide. Chem Rev 98:2549–2585.
OpenUrl CrossRef PubMed

[11] Jambor JL,

[12] Dutrizac JE

[13] ↵
Manceau A
(2009) Evaluation of the structural model for ferrihydrite derived from real-space modelling of high-energy x-ray diffraction data. Clay Miner 44:19–34.
OpenUrl Abstract/FREE Full Text

[14] Manceau A

[15] ↵
Michel FM,
et al.
(2007) The structure of ferrihydrite, a nanocrystalline material. Science 316:1726–1729.
OpenUrl Abstract/FREE Full Text

[16] Michel FM,

[17] et al.

[18] ↵
Rancourt DG,
Meunier JF
(2008) Constraints on structural models of ferrihydrite as a nanocrystalline material. Am Mineral 93:1412–1417.
OpenUrl Abstract/FREE Full Text

[19] Rancourt DG,

[20] Meunier JF

[21] ↵
Hiemstra T,
Van Riemsdijk WH
(2009) A surface structural model for ferrihydrite I: Sites related to primary charge, molar mass, and mass density. Geochim Cosmochim Acta 73:4423–4436.
OpenUrl CrossRef

[22] Hiemstra T,

[23] Van Riemsdijk WH

[24] ↵
Berquó TS,
et al.
(2007) High crystallinity Si-ferrihydrite: An insight into its Néel temperature and size dependence of magnetic properties. J Geophys Res-Sol Ea 112:B02102.
OpenUrl CrossRef

[25] Berquó TS,

[26] et al.

[27] ↵
Berquó TS,
et al.
(2009) Effects of magnetic interactions in antiferromagnetic ferrihydrite particles. J Phys-Condens Mat 21:176005.
OpenUrl

[28] Berquó TS,

[29] et al.

[30] ↵
Cabello E,
et al.
(2009) Magnetic enhancement during the crystallization of ferrihydrite at 25 and 50 °C. Clays Clay Miner 57:46–53.
OpenUrl Abstract/FREE Full Text

[31] Cabello E,

[32] et al.

[33] ↵
Coey JMD,
Readman PW
(1973) New spin structure in an amorphous ferric gel. Nature 246:476–478.
OpenUrl CrossRef

[34] Coey JMD,

[35] Readman PW

[36] ↵
Pankhurst QA,
Pollard RJ
(1992) Structural and magnetic-properties of ferrihydrite. Clays Clay Miner 40:268–272.
OpenUrl Abstract

[37] Pankhurst QA,

[38] Pollard RJ

[39] ↵
Pannalal SJ,
et al.
(2005) Room-temperature magnetic properties of ferrihydrite: A potential magnetic remanence carrier? Earth Planet Sci Lett 236:856–870.
OpenUrl CrossRef

[40] Pannalal SJ,

[41] et al.

[42] ↵
Cornell RM,
Schwertmann U
(2003) The Iron Oxides: Structure, Properties, Reactions, Occurrences and Uses (Wiley–VCH, Weinheim).

[43] Cornell RM,

[44] Schwertmann U

[45] ↵
Guyodo Y,
et al.
(2006) Magnetic properties of synthetic six-line ferrihydrite nanoparticles. Phys Earth Planet In 154:222–233.
OpenUrl CrossRef

[46] Guyodo Y,

[47] et al.

[48] ↵
Zergenyi RS,
et al.
(2000) Low-temperature magnetic behavior of ferrihydrite. J Geophys Res-Sol Ea 105:8297–8303.
OpenUrl CrossRef

[49] Zergenyi RS,

[50] et al.

[51] ↵
Gider S,
et al.
(1995) Classical and quantum magnetic phenomena in natural and artificial ferritin proteins. Science 268:77–80.
OpenUrl Abstract/FREE Full Text

[52] Gider S,

[53] et al.

[54] ↵
Mullins CE
(1977) Magnetic-susceptibility of soil and its significance in soil science—Review. J Soil Sci 28:223–246.
OpenUrl CrossRef

[55] Mullins CE

[56] ↵
Morris RV,
et al.
(2006) Mossbauer mineralogy of rock, soil, and dust at Meridiani Planum, Mars: Opportunity’s journey across sulfate-rich outcrop, basaltic sand and dust, and hematite lag deposits. J Geophys Res-Planet 111:E12S15.
OpenUrl CrossRef

[57] Morris RV,

[58] et al.

[59] ↵
Raj K,
Moskowitz R
(1990) Commercial applications of ferrofluids. J Magn Magn Mater 85:233–245.
OpenUrl CrossRef

[60] Raj K,

[61] Moskowitz R

[62] ↵
Tartaj P,
et al.
(2003) The preparation of magnetic nanoparticles for applications in biomedicine. J Phys D Appl Phys 36:R182–R197.
OpenUrl CrossRef

[63] Tartaj P,

[64] et al.

[65] ↵
Barrón V,
Torrent J
(2002) Evidence for a simple pathway to maghemite in Earth and Mars soils. Geochim Cosmochim Acta 66:2801–2806.
OpenUrl CrossRef

[66] Barrón V,

[67] Torrent J

[68] ↵
Barrón V,
Torrent J,
de Grave E
(2003) Hydromaghemite, an intermediate in the hydrothermal transformation of 2-line ferrihydrite into hematite. Am Mineral 88:1679–1688.
OpenUrl Abstract/FREE Full Text

[69] Barrón V,

[70] Torrent J,

[71] de Grave E

[72] ↵
Popov VV,
Gorbunov AI
(2006) Effect of organic acids on the hydrothermal crystallization of Fe(OH)₃. Inorg Mater 42:769–776.
OpenUrl CrossRef

[73] Popov VV,

[74] Gorbunov AI

[75] ↵
Bartzokis G,
et al.
(2007) Brain ferritin iron may influence age- and gender-related risks of neurodegeneration. Neurobiol Aging 28:414–423.
OpenUrl CrossRef PubMed

[76] Bartzokis G,

[77] et al.

[78] ↵
Brem F,
et al.
(2005) Characterization of iron compounds in tumour tissue from temporal lobe epilepsy patients using low temperature magnetic methods. Biometals 18:191–197.
OpenUrl CrossRef PubMed

[79] Brem F,

[80] et al.

[81] ↵
Gossuin Y,
et al.
(2005) Looking for biogenic magnetite in brain ferritin using NMR relaxometry. NMR Biomed 18:469–472.
OpenUrl CrossRef PubMed

[82] Gossuin Y,

[83] et al.

[84] ↵
Kirschvink JL,
Kobayashikirschvink A,
Woodford BJ
(1992) Magnetite biomineralization in the human brain. Proc Natl Acad Sci USA 89:7683–7687.
OpenUrl Abstract/FREE Full Text

[85] Kirschvink JL,

[86] Kobayashikirschvink A,

[87] Woodford BJ

[88] ↵
Michel FM,
et al.
(2007) Similarities in 2-and 6-line ferrihydrite based on pair distribution function analysis of x-ray total scattering. Chem Mater 19:1489–1496.
OpenUrl CrossRef

[89] Michel FM,

[90] et al.

[91] ↵
Egami T,
Billinge SJL
(2003) Underneath the Bragg Peaks: Structural Analysis of Complex Materials (Elsevier, Oxford).

[92] Egami T,

[93] Billinge SJL

[94] ↵
Foreman DW
(1968) Neutron and x-ray diffraction study of Ca₃Al₂(O₄D₄)₃ a garnetoid. J Chem Phys 48:3037–3041.
OpenUrl CrossRef

[95] Foreman DW

[96] ↵
Towe KM,
Bradley WF
(1967) Mineralogical constitution of colloidal hydrous ferric oxides. J Colloid Interf Sci 24:384–392.
OpenUrl CrossRef

[97] Towe KM,

[98] Bradley WF

[99] ↵
Cullity BD
(1972) Introduction to Magnetic Materials (Addison–Wesley, Reading).

[100] Cullity BD

[101] ↵
Pinney N,
et al.
(2009) Density functional theory study of ferrihydrite and related Fe-oxyhydroxides. Chem Mater, DOI: 10.1021/cm9023875.

[102] Pinney N,

[103] et al.

[104] ↵
Morales MP,
et al.
(1999) Surface and internal spin canting in gamma-Fe₂O₃ nanoparticles. Chem Mater 11:3058–3064.
OpenUrl CrossRef

[105] Morales MP,

[106] et al.

[107] ↵
Liu QS,
et al.
(2008) Magnetism of intermediate hydromaghemite in the transformation of 2-line ferrihydrite into hematite and its paleoenvironmental implications. J Geophys Res-Sol Ea 113:B01103.
OpenUrl CrossRef

[108] Liu QS,

[109] et al.

[110] ↵
Torrent J,
Barrón V,
Liu QS
(2006) Magnetic enhancement is linked to and precedes hematite formation in aerobic soil. Geophys Res Lett 33:L02401.
OpenUrl CrossRef

[111] Torrent J,

[112] Barrón V,

[113] Liu QS

[114] ↵
Torrent J,
Liu QS,
Bloemendal J,
Barrón V
(2007) Magnetic enhancement and iron oxides in the Upper Luochuan Loess-paleosol sequence, Chinese Loess plateau. Soil Sci Soc Am J 71:1570–1578.
OpenUrl CrossRef

[115] Torrent J,

[116] Liu QS,

[117] Bloemendal J,

[118] Barrón V

[119] ↵
Dearing JA,
et al.
(1996) Magnetic susceptibility of soil: An evaluation of conflicting theories using a national dataset. Geophys J Int 127:728–734.
OpenUrl CrossRef

[120] Dearing JA,

[121] et al.

[122] ↵
Maher BA,
Alekseev A,
Alekseeva T
(2003) Magnetic mineralogy of soils across the Russian Steppe: Climatic dependence of pedogenic magnetite formation. Paleaogeogr Palaeocl 201:321–341.
OpenUrl CrossRef

[123] Maher BA,

[124] Alekseev A,

[125] Alekseeva T

[126] ↵
Schwertmann U
(1985) The effect of pedogenic environments on iron oxide minerals. Adv Soil S 1:171–200.
OpenUrl

[127] Schwertmann U

[128] ↵
Geiss CE,
Zanner CW
(2006) How abundant is pedogenic magnetite? Abundance and grain size estimates for loessic soils based on rock magnetic analyses. J Geophys Res-Sol Ea 111:B12S21O.
OpenUrl

[129] Geiss CE,

[130] Zanner CW

[131] ↵
Liu QS,
et al.
(2005) Determination of primary magnetic minerals of a weathered metapelite xenolith from Zhouba region, North China, by combining thermomagnetic runs and low-temperature measurements. Chinese J Geophys-Ch 48:876–881.
OpenUrl

[132] Liu QS,

[133] et al.

[134] ↵
Chen TH,
et al.
(2005) Characteristics and genesis of maghemite in Chinese loess and paleosols: Mechanism for magnetic susceptibility enhancement in paleosols. Earth Planet Sci Lett 240:790–802.
OpenUrl CrossRef

[135] Chen TH,

[136] et al.

[137] ↵
Navrotsky A,
Mazeina L,
Majzlan J
(2008) Size-driven structural and thermodynamic complexity in iron oxides. Science 319:1635–1638.
OpenUrl Abstract/FREE Full Text

[138] Navrotsky A,

[139] Mazeina L,

[140] Majzlan J

[141] ↵
Burleson DJ,
Penn RL
(2006) Two-step growth of goethite from ferrihydrite. Langmuir 22:402–409.
OpenUrl CrossRef PubMed

[142] Burleson DJ,

[143] Penn RL

[144] ↵
Penn RL,
Erbs JJ,
Gulliver DM
(2006) Controlled growth of alpha-FeOOH nanorods by exploiting-oriented aggregation. J Cryst Growth 293:1–4.
OpenUrl CrossRef

[145] Penn RL,

[146] Erbs JJ,

[147] Gulliver DM

[148] ↵
Malinowski ER
(2002) Factor Analysis in Chemistry (Wiley, New York).

[149] Malinowski ER

[150] ↵
Jaumot J,
Gargallo R,
de Juan A,
Tauler R
(2005) A graphical user-friendly interface for MCR-ALS: A new tool for multivariate curve resolution in MATLAB. Chemom Intell Lab Syst 76:101–110.
OpenUrl CrossRef

[151] Jaumot J,

[152] Gargallo R,

[153] de Juan A,

[154] Tauler R

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Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism

Abstract

Results & Discussion

Ferrihydrite Structural Variations with Aging.

Composition and Density of Ferrihydrite.

Origin of Ferrimagnetism in Ordered Ferrihydrite.

Does Ferrimagnetic Ferrihydrite Occur in Nature?

Summary and Conclusions

Methods

Acknowledgments

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References

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Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism

Abstract

Results & Discussion

Ferrihydrite Structural Variations with Aging.

Composition and Density of Ferrihydrite.

Origin of Ferrimagnetism in Ordered Ferrihydrite.

Does Ferrimagnetic Ferrihydrite Occur in Nature?

Summary and Conclusions

Methods

Acknowledgments

Footnotes

References

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