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  • 1
    Language: English
    In: Science (New York, N.Y.), 31 July 2015, Vol.349(6247), pp.aaa6760
    Description: Field and laboratory observations show that crystals commonly form by the addition and attachment of particles that range from multi-ion complexes to fully formed nanoparticles. The particles involved in these nonclassical pathways to crystallization are diverse, in contrast to classical models that consider only the addition of monomeric chemical species. We review progress toward understanding crystal growth by particle-attachment processes and show that multiple pathways result from the interplay of free-energy landscapes and reaction dynamics. Much remains unknown about the fundamental aspects, particularly the relationships between solution structure, interfacial forces, and particle motion. Developing a predictive description that connects molecular details to ensemble behavior will require revisiting long-standing interpretations of crystal formation in synthetic systems, biominerals, and patterns of mineralization in natural environments.
    Keywords: Crystallization ; Mathematical Models ; Attachment ; Pathways ; Crystal Growth ; Dynamical Systems ; Crystals ; Dynamics ; Miscellaneous Sciences (So) ; (An);
    ISSN: 00368075
    E-ISSN: 1095-9203
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  • 2
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 September 2014, Vol.140, pp.708-719
    Description: Amorphous ferric arsenate (AFA, FeAsO · H O) is an important As precipitate in a range of oxic As-rich environments, especially acidic sulfide-bearing mine wastes. Its structure has been proposed to consist of small polymers of single corner-sharing FeO octahedra ( ∼3.6 Å) to which arsenate is attached as a monodentate binuclear C complex (‘chain model’). Here, we analyzed the structure of AFA and analogously prepared amorphous ferric phosphates (AFP, FePO · H O) by a combination of high-energy total X-ray scattering, Fe K-edge X-ray absorption spectroscopy, and Fe Mössbauer spectroscopy. Pair distribution function (PDF) analysis of total X-ray scattering data revealed that the coherently scattering domain size of AFA and AFP is about 8 Å. The PDFs of AFA lacked Fe–Fe pair correlations at ∼3.6 Å indicative of single corner-sharing FeO octahedra, which strongly supports a local scorodite (FeAsO ·2H O) structure. Likewise, the PDFs and Fe K-edge extended X-ray absorption fine structure data of AFP were consistent with a local strengite (FePO ·2H O) structure of isolated FeO octahedra being corner-linked to PO tetrahedra ( = 3.25(1) Å). Mössbauer spectroscopy analyses of AFA and AFP indicated a strong superparamagnetism. While AFA only showed a weak onset of magnetic hyperfine splitting at 5 K, magnetic ordering of AFP was completely absent at this temperature. Mössbauer spectroscopy may thus offer a convenient way to identify and quantify AFA and AFP in mineral mixtures containing poorly crystalline Fe(III)-oxyhydroxides. In summary, our results imply a close structural relationship between AFA and AFP and suggest that these amorphous materials serve as templates for the formation of scorodite and strengite (phosphosiderite) in strongly acidic low-temperature environments.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 3
    Language: English
    In: Precambrian Research, July 2015, Vol.263, pp.142-156
    Description: Paleontological inferences, molecular clocks, and biomarker fossils indicate sponges evolved in the Cryogenian, but Precambrian sponge fossils are rare, poorly substantiated, and controversial. Spicule-like microstructures (SLMs) hosted in phosphatized fossils from the Ediacaran Doushantuo Formation (∼635–551 Ma) at Weng’an of South China have been interpreted as cylindrical siliceous monaxons, and their hosting fossils as the oldest demosponges in the fossil record. In order to assess their veracity as the oldest spiculate demosponges, we utilize a suite of nanoscale analytical techniques—including scanning electron microscopy, synchrotron X-ray fluorescence mapping, X-ray absorption near edge structure (XANES) spectroscopy, focused ion beam electron microscopy, and transmission electron microscopy—to evaluate the ultrastructures and elemental, chemical, and mineralogical compositions of the SLMs. Our data decisively shows that the SLMs are carbonaceous in composition and rectangular in transverse sections, and therefore, are not cylindrical siliceous spicules. Instead, the SLMs may be microbial strands, axial filaments of early hexactinellids, or acicular crystals molded by organic matter. Regardless, our new data invalidate the oldest and only Precambrian demosponges with mineralized spicules. These results indicate that interpretations of Precambrian sponge fossils should be scrutinized with compositional, mineralogical, and ultrastructural data collected using analytical techniques. In addition, our conclusions affirm that no unequivocal biomineralizing sponges occur below the Ediacaran–Cambrian boundary. If hexactinellids and demosponges did diverge in the Cryogenian as suggested by molecular clocks and biomarkers, they either evolved biomineralization long after their divergence or their biomineralized spicules were never preserved until after the Ediacaran–Cambrian boundary. In either case, the dearth of biomineralizing sponge fossils in the Precambrian and their abundance in the early Cambrian must reflect a geobiologically significant aspect of the Precambrian–Phanerozoic transition.
    Keywords: Biomineralization ; Doushantuo Formation ; Ediacaran Period ; Spicules ; Sponges ; Geology
    ISSN: 0301-9268
    E-ISSN: 1872-7433
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  • 4
    Language: English
    In: Proc. Natl. Acad. Sci. USA, 19 November 2010, Vol.107((7) ; 02, 2010)
    Description: 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.
    Keywords: Environmental Sciences ; Aging ; Ambient Temperature ; Citrates ; Defects ; Distribution Functions ; Electrons ; Ferrimagnetism ; Ferritin ; Hematite ; Magnetism ; Particle Size ; Precursor ; Relaxation ; Soils ; Spin ; Stability ; Structural Models ; Transformations ; Weathering ; Sciences (General) ; Environmental Sciences
    ISSN: 0027-8424
    E-ISSN: 1091-6490
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  • 5
    Language: English
    In: Geochimica et Cosmochimica Acta, Sept 1, 2014, Vol.140, p.708(12)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2014.05.040 Byline: Christian Mikutta, Christian Schroder, F. Marc Michel Abstract: Amorphous ferric arsenate (AFA, FeAsO.sub.4*xH.sub.2O) is an important As precipitate in a range of oxic As-rich environments, especially acidic sulfide-bearing mine wastes. Its structure has been proposed to consist of small polymers of single corner-sharing FeO.sub.6 octahedra (r.sub.Fe-Fe [approximately equal to]3.6A) to which arsenate is attached as a monodentate binuclear.sup.2C complex ('chain model'). Here, we analyzed the structure of AFA and analogously prepared amorphous ferric phosphates (AFP, FePO.sub.4*xH.sub.2O) by a combination of high-energy total X-ray scattering, Fe K-edge X-ray absorption spectroscopy, and.sup.57Fe Mossbauer spectroscopy. Pair distribution function (PDF) analysis of total X-ray scattering data revealed that the coherently scattering domain size of AFA and AFP is about 8A. The PDFs of AFA lacked Fe-Fe pair correlations at r [approximately equal to]3.6A indicative of single corner-sharing FeO.sub.6 octahedra, which strongly supports a local scorodite (FeAsO.sub.4*2H.sub.2O) structure. Likewise, the PDFs and Fe K-edge extended X-ray absorption fine structure data of AFP were consistent with a local strengite (FePO.sub.4*2H.sub.2O) structure of isolated FeO.sub.6 octahedra being corner-linked to PO.sub.4 tetrahedra (r.sub.Fe-P =3.25(1)A). Mossbauer spectroscopy analyses of AFA and AFP indicated a strong superparamagnetism. While AFA only showed a weak onset of magnetic hyperfine splitting at 5K, magnetic ordering of AFP was completely absent at this temperature. Mossbauer spectroscopy may thus offer a convenient way to identify and quantify AFA and AFP in mineral mixtures containing poorly crystalline Fe(III)-oxyhydroxides. In summary, our results imply a close structural relationship between AFA and AFP and suggest that these amorphous materials serve as templates for the formation of scorodite and strengite (phosphosiderite) in strongly acidic low-temperature environments. Article History: Received 17 January 2014; Accepted 24 May 2014 Article Note: (miscellaneous) Associate editor: Jean Francois Boily
    Keywords: Phosphates -- Analysis ; Spectroscopy -- Analysis ; Iron Compounds -- Analysis
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 6
    Language: English
    In: Geochimica et Cosmochimica Acta, Sept 1, 2012, Vol.92, p.275(17)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2012.06.010 Byline: A. Cristina Cismasu (a), F. Marc Michel (a)(b), Jonathan F. Stebbins (a), Clement Levard (a), Gordon E. Brown (a)(b)(c) Abstract: The association of Al with ferrihydrite (Fh) may have a considerable effect on the composition, structure, and surface properties of Fh nanoparticles, and thus impact its reactivity and interaction with pollutant species. Aluminous Fh is abundant in natural environments, but the mode of association of Al with this nanomineral is not yet fully understood. Al.sup.3+ speciation may vary from true chemical substitution for Fe.sup.3+, to adsorption or surface precipitation, and/or to formation of a mixture of two (or more) individual nanoscale phases. The conditions of formation (i.e. slow vs. rapid precipitation) may also affect the nature of Fh nanoparticles in terms of their crystallinity, phase purity, and Al speciation. In this study we used a variety of laboratory (TEM, NMR, ICP-AES) and synchrotron-based techniques (X-ray total scattering and PDF analysis, scanning transmission X-ray microscopy, Al K-edge XANES spectroscopy) to characterize two synthetic Al-bearing Fh series formed at different precipitation rates in the presence of 5-40mol% Al. We find that Al is dominantly octahedrally coordinated in the synthetic Fh samples and that up to 20-30mol% Al substitutes for Fe in the Fh structure, regardless of the synthesis method we used. Formation of separate aluminous phases (e.g., gibbsite) was most significant at Al concentrations above 30mol% Al in slowly precipitated samples. However, small amounts (〈6% of total Al) of Al-hydroxide phases were also detected by NMR spectroscopy in samples with lower Al content (as low as 15mol% Al), particularly in the Fh series that was precipitated slowly. Furthermore, it appears that the amount of Al incorporated in Fh is not affected by the synthesis methods we used and is more likely controlled by the accumulated strain caused by Al substitution in the Fh lattice. Given the prevalence of naturally occurring aluminous ferrihydrite, assumptions about ferrihydrite reactivity in natural environments should consider the impact of Al substitution on reduction potential, Fe bioavailability, as well as sorption reactions. Author Affiliation: (a) Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA (b) Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., MS 69, Menlo Park, CA 94025, USA (c) Department of Photon Science, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., MS 69, Menlo Park, CA 94025, USA Article History: Received 6 September 2011; Accepted 9 June 2012 Article Note: (miscellaneous) Associate editor: Christopher Kim
    Keywords: Precipitation (Meteorology) -- Analysis ; Adsorption -- Analysis ; Nanotechnology -- Analysis ; Iron Oxides -- Analysis
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 7
    Language: English
    In: Geochimica et Cosmochimica Acta, Oct 15, 2013, Vol.119, p.46(15)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2013.05.040 Byline: A. Cristina Cismasu, Clement Levard, F. Marc Michel, Gordon E. Brown Abstract: Naturally occurring ferrihydrite often contains impurities such as Al and Si, which can impact its chemical reactivity with respect to metal(loid) adsorption and (in)organic or microbially induced reductive dissolution. However, the surface composition of impure ferrihydrites is not well constrained, and this hinders our understanding of the factors controlling the surface reactivity of these nanophases. In this study, we conducted Zn(II) adsorption experiments combined with Zn K-edge X-ray absorption spectroscopy measurements on pure ferrihydrite (Fh) and Al- or Si-bearing ferrihydrites containing 10 and 20mol% Al or Si (referred to as 10AlFh, 20AlFh and 10SiFh, 20SiFh) to evaluate Zn(II) uptake in relation to Zn(II) speciation at their surfaces. Overall, Zn(II) uptake at the surface of AlFh is similar to that of pure Fh, and based on Zn K-edge EXAFS data, Zn(II) speciation at the surface of Fh and AlFh also appears similar. Binuclear bidentate.sup.IVZn-.sup.VIFe complexes (at [approximately equal to]3.46A (.sup.2C[1]) and [approximately equal to]3.25A (.sup.2C[2])) were identified at low Zn(II) surface coverages from Zn K-edge EXAFS fits. With increasing Zn(II) surface coverage, the number of second-neighbor Fe ions decreased, which was interpreted as indicating the formation of.sup.IVZn polymers at the ferrihydrite surface, and a deviation from Langmuir uptake behavior. Zn(II) uptake at the surface of SiFh samples was more significant than at Fh and AlFh surfaces, and was attributed to the formation of outer-sphere complexes (on average 24% of sorbed Zn). Although similar Zn-Fe/Zn distances were obtained for the Zn-sorbed SiFh samples, the number of Fe second neighbors was lower in comparison with Fh. The decrease in second-neighbor Fe is most pronounced for sample 20SiFh, suggesting that the amount of reactive surface Fe sites diminishes with increasing Si content. Although our EXAFS results shown here do not provide evidence for the existence of Zn-Al or Zn-Si complexes, their presence is not excluded for Zn-sorbed AlFh or SiFh. The results of this study indicate that Zn(II) interaction with Fh is influenced by the type of impurities associated with this nanomineral, particularly in the case of Si-bearing Fh, and this may have implications for our understanding of metal(loid) mobility in natural systems. Article History: Received 3 August 2012; Accepted 29 May 2013 Article Note: (miscellaneous) Associate editor: Owen Duckworth
    Keywords: Zinc Compounds -- Analysis ; Iron Oxides -- Analysis ; Adsorption -- Analysis ; Spectroscopy -- Analysis
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 8
    Language: English
    In: Geochimica et Cosmochimica Acta, Sept 15, 2015, Vol.165, p.44(18)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2015.05.024 Byline: Elizabeth D. Swanner, Wenfang Wu, Ronny Schoenberg, James Byrne, F. Marc Michel, Yongxin Pan, Andreas Kappler Abstract: Much interest exists in finding mineralogical, organic, morphological, or isotopic biosignatures for Fe(II)-oxidizing bacteria (FeOB) that are retained in Fe-rich sediments, which could indicate the activity of these organisms in Fe-rich seawater, more common in the Precambrian Era. To date, the effort to establish a clear Fe isotopic signature in Fe minerals produced by Fe(II)-oxidizing metabolisms has been thwarted by the large kinetic fractionation incurred as freshly oxidized aqueous Fe(III) rapidly precipitates as Fe(III) (oxyhydr)oxide minerals at near neutral pH. The Fe(III) (oxyhydr)oxide minerals resulting from abiotic Fe(II) oxidation are isotopically heavy compared to the Fe(II) precursor and are not clearly distinguishable from minerals formed by FeOB isotopically. However, in marine hydrothermal systems and Fe(II)-rich springs the minerals formed are often isotopically lighter than expected considering the fraction of Fe(II) that has been oxidized and experimentally-determined fractionation factors. We measured the Fe isotopic composition of aqueous Fe (Fe.sub.aq) and the final Fe mineral (Fe.sub.ppt) produced in batch experiment using the marine Fe(II)-oxidizing phototroph Rhodovulum iodosum. The [delta].sup.56Fe.sub.aq data are best described by a kinetic fractionation model, while the evolution of [delta].sup.56Fe.sub.ppt appears to be controlled by a separate fractionation process. We propose that soluble Fe(III), and Fe(II) and Fe(III) extracted from the Fe.sub.ppt may act as intermediates between Fe(II) oxidation and Fe(III) precipitation. Based on.sup.57Fe Mossbauer spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray total scattering, we suggests these Fe phases, collectively Fe(II/III).sub.interm, may consist of organic-ligand bound, sorbed, and/or colloidal Fe(II) and Fe(III) mineral phases that are isotopically lighter than the final Fe(III) mineral product. Similar intermediate phases, formed in response to organic carbon produced by FeOB and inorganic ligands (e.g., SiO.sub.4.sup.4- or PO.sub.4.sup.3-), may form in many natural Fe(II)-oxidizing environments. We propose that the formation of these intermediates is likely to occur in organic-rich systems, and thus may have controlled the ultimate isotopic composition of Fe minerals in systems where Fe(II) was being oxidized by or in the presence of microbes in Earth's past. Article History: Received 18 August 2014; Accepted 10 May 2015 Article Note: (miscellaneous) Associate editor: Andrew Ross Bowie
    Keywords: Seawater – Physiological Aspects ; Seawater – Analysis ; Hydrothermal Systems (Geology) – Physiological Aspects ; Hydrothermal Systems (Geology) – Analysis ; Oxidation-Reduction Reactions – Physiological Aspects ; Oxidation-Reduction Reactions – Analysis ; Precipitation (Meteorology) – Physiological Aspects ; Precipitation (Meteorology) – Analysis
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 9
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 September 2012, Vol.92, pp.275-291
    Description: The association of Al with ferrihydrite (Fh) may have a considerable effect on the composition, structure, and surface properties of Fh nanoparticles, and thus impact its reactivity and interaction with pollutant species. Aluminous Fh is abundant in natural environments, but the mode of association of Al with this nanomineral is not yet fully understood. Al speciation may vary from true chemical substitution for Fe , to adsorption or surface precipitation, and/or to formation of a mixture of two (or more) individual nanoscale phases. The conditions of formation (i.e. slow vs. rapid precipitation) may also affect the nature of Fh nanoparticles in terms of their crystallinity, phase purity, and Al speciation. In this study we used a variety of laboratory (TEM, NMR, ICP-AES) and synchrotron-based techniques (X-ray total scattering and PDF analysis, scanning transmission X-ray microscopy, Al K-edge XANES spectroscopy) to characterize two synthetic Al-bearing Fh series formed at different precipitation rates in the presence of 5–40 mol% Al. We find that Al is dominantly octahedrally coordinated in the synthetic Fh samples and that up to 20–30 mol% Al substitutes for Fe in the Fh structure, regardless of the synthesis method we used. Formation of separate aluminous phases (e.g., gibbsite) was most significant at Al concentrations above 30 mol% Al in slowly precipitated samples. However, small amounts (〈6% of total Al) of Al-hydroxide phases were also detected by NMR spectroscopy in samples with lower Al content (as low as 15 mol% Al), particularly in the Fh series that was precipitated slowly. Furthermore, it appears that the amount of Al incorporated in Fh is not affected by the synthesis methods we used and is more likely controlled by the accumulated strain caused by Al substitution in the Fh lattice. Given the prevalence of naturally occurring aluminous ferrihydrite, assumptions about ferrihydrite reactivity in natural environments should consider the impact of Al substitution on reduction potential, Fe bioavailability, as well as sorption reactions.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 10
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 October 2013, Vol.119, pp.46-60
    Description: Naturally occurring ferrihydrite often contains impurities such as Al and Si, which can impact its chemical reactivity with respect to metal(loid) adsorption and (in)organic or microbially induced reductive dissolution. However, the surface composition of impure ferrihydrites is not well constrained, and this hinders our understanding of the factors controlling the surface reactivity of these nanophases. In this study, we conducted Zn(II) adsorption experiments combined with Zn K-edge X-ray absorption spectroscopy measurements on pure ferrihydrite (Fh) and Al- or Si-bearing ferrihydrites containing 10 and 20 mol% Al or Si (referred to as 10AlFh, 20AlFh and 10SiFh, 20SiFh) to evaluate Zn(II) uptake in relation to Zn(II) speciation at their surfaces. Overall, Zn(II) uptake at the surface of AlFh is similar to that of pure Fh, and based on Zn K-edge EXAFS data, Zn(II) speciation at the surface of Fh and AlFh also appears similar. Binuclear bidentate Zn– Fe complexes (at ∼3.46 Å ( C[1]) and ∼3.25 Å ( C[2])) were identified at low Zn(II) surface coverages from Zn K-edge EXAFS fits. With increasing Zn(II) surface coverage, the number of second-neighbor Fe ions decreased, which was interpreted as indicating the formation of Zn polymers at the ferrihydrite surface, and a deviation from Langmuir uptake behavior. Zn(II) uptake at the surface of SiFh samples was more significant than at Fh and AlFh surfaces, and was attributed to the formation of outer-sphere complexes (on average 24% of sorbed Zn). Although similar Zn–Fe/Zn distances were obtained for the Zn-sorbed SiFh samples, the number of Fe second neighbors was lower in comparison with Fh. The decrease in second-neighbor Fe is most pronounced for sample 20SiFh, suggesting that the amount of reactive surface Fe sites diminishes with increasing Si content. Although our EXAFS results shown here do not provide evidence for the existence of Zn–Al or Zn–Si complexes, their presence is not excluded for Zn-sorbed AlFh or SiFh. The results of this study indicate that Zn(II) interaction with Fh is influenced by the type of impurities associated with this nanomineral, particularly in the case of Si-bearing Fh, and this may have implications for our understanding of metal(loid) mobility in natural systems.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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