Denis G. Rancourt

Nanogoethite is the dominant reactive oxyhydroxide phase in lake and marine sediments

Iron oxides affect many elemental cycles in aquatic sediments via numerous redox reactions and their large sorption capacities for phosphate and trace elements. The reactive ferric oxides and oxyhydroxides are usually quantified by operationally defined selective chemical extractions that are not mineral specific. We have used cryogenic 57Fe Mossbauer spectroscopy to show that the reactive iron oxyhydroxide phase in a large variety of lacustrine and marine environments is nanophase goethite (α-FeOOH), rather than the assumed surface-complex–stabilized, two-line ferrihydrite and accompanying mixture of clay and oxyhydroxide Fe-bearing phases. This result implies that the kinetic and stability parameters of the type of nanogoethite that we observe to be present in sediments should be first determined and then used in models of early diagenesis. The identity and characteristics of the reactive phase will also set constraints on the mechanisms of its authigenesis.

Interplay of surface conditions, particle size, stoichiometry, cell parameters, and magnetism in synthetic hematite-like materials

We have studied several synthetic hematite-like materials, produced via different reactions using various hydrothermal conditions and various temperatures of annealing in air, by bulk elemental analysis, weight loss measurements, scanning electron microscopy, powder X-ray diffraction, Mössbauer spectroscopy, and SQUID magnetometry. We conclude that hematite-like materials cannot be related to pure stoichiometric hematite via a single stoichiometric or physical parameter and that at least two degrees of freedom are required. This is most clearly seen when we introduce a plot of the cell parameter c versus the cell parameter a on which hematite-like materials do not fall on a single line but occupy an entire region that is bounded by hydrohematite-hematite and protohematite-hematite lines. A Morin transition boundary on this c-a plot separates a region where Morin transitions occur from a larger region where Morin transitions do not occur down to 4.2 K. Previous claims that particle size is the dominant factor controlling the Morin transition are understood in terms of correlations between stoichiometry and particle size that are produced at synthesis. Changing contents of incorporated molecular water and structural hydroxyls with associated cation vacancies have different characteristic effects on the crystal structure and move the sample coordinates in different directions on a c-a plot. It is also shown that an accessory sulphate content is adsorbed on the individual hematite crystallites and is not structurally incorporated. Mössbauer spectroscopy is used, as usual, to identify and characterize the spin structure. In addition, hyperfine field distributions from room temperature spectra, extracted by a new method, give a sensitive measure of sample conditions but not a unique one since several factors affect the extracted distributions in similar ways.

Mineralogy of a natural As-rich hydrous ferric oxide coprecipitate formed by mixing of hydrothermal fluid and seawater: Implications regarding surface complexation and color banding in ferrihydrite deposits

Abstract We characterized the most As-rich natural hydrous ferric oxide (HFO) material ever reported using powder X-ray diffraction (pXRD), transmission electron microscopy (TEM), X-ray fluorescence spectroscopy (XRF), light element analysis using gas chromatography (GC), visible-infrared (vis-IR) diffuse reflectivity, 57Fe Mössbauer spectroscopy, and superconducting quantum interference device (SQUID) magnetometry. We find that the natural As-HFO material is very similar to synthetic coprecipitated As-HFO materials, but is significantly different from all known natural and synthetic As-free HFO materials and ferrihydrite samples. The pXRD patterns show systematic differences with patterns for 2-line ferrihydrite, that are interpreted as evidence for significant populations of oxygen-coordinated Fe-As pairs. Observations by TEM, combined with energy dispersive spectroscopy (EDS) microanalysis, show agglomerations of nanophase primary particles and no evidence for other Fe- or As-bearing phases. Mössbauer spectroscopy shows octahedrally coordinated Fe3+, with a large fraction (~20%) of the octahedral Fe environments that are significantly distorted by the presence of As, compared to the Fe local environments in As-free ferrihydrite and HFO samples. The loss on ignition (LOI) is quantitatively consistent with OH + H2O, measured by GC, which, in turn, is consistent with ~1 nm diameter primary particles having all their surface cations (Fe3+, As5+, Si4+, C4+) coordinated on the free surface side by OH- and OH2. The banding into adjacent yellowish and reddish layers that occurs in the As-HFO deposits was studied by performing mineralogical analyses of the separated adjacent layers of two couplets of yellowish and reddish material. The yellowish samples were found not to contain secondary crystalline phases (as did the reddish samples, in small amounts) and to be relatively As-rich, C- and Si-poor. The observed anticorrelations between As and Si and between As and inorganic C suggest that natural HFOs, which usually contain significant molar amounts of Si, may not be as efficient at surface complexing As (and P) as their Si and C-free synthetic counterparts, unless formed by co-precipitation with the As (or P). The yellowish and reddish layers were also clearly resolved by both Mössbauer spectroscopy and magnetometry. Complexation of arsenate onto the HFO core was found to significantly increase the average quadrupole splitting (QS) obtained from Mössbauer spectroscopy by an amount that could not be explained by other chemical differences and that is consistent with an ~1 nm diameter particle size and somewhat smaller HFO core. The Munsell hue YR index (5-10 YR) was found to be strongly correlated to the average QS, thereby establishing that the color differences, corresponding to the measured shifts of the main visible band edge, are due to the local distortions in the [6]Fe3+ environments that are induced by As complexation, via their influence on the relevant ligand field transitions. SQUID magnetometry allows the following observations. (1) The superparamagnetic to superferromagnetic transitions occur at 25 K and lower in As-HFO, compared to 55 K in synthetic 2-line ferrihydrite, suggesting a smaller magnetic primary particle (or core) size for As-HFO and inter-particle magnetic interaction reduction by surface complexed As, Si, and C. (2) The ratio of supermoment magnitude to magnetic particle size (m2/n, where m is the net number of Fe3+ atomic moments per supermoment and n is the number of Fe3+ cations per particle or HFO core) decreases with increasing As content in the sequence synthetic-HFO > reddish-As-HFO > yellowish-As-HFO. (3) The magnetic susceptibility magnitudes for As-HFO and synthetic 2-line ferrihydrite differ by a factor of 10 and suggest different supermoment formation mechanisms (m2/n < 1 vs. m2/n > 1, respectively) related to differences in intra-particle cationic and anionic disorder and magnetic particle size.

Constraints on structural models of ferrihydrite as a nanocrystalline material

Abstract Recently, Michel et al. (2007a) have presented a structure for ferrihydrite that we show to be incorrect. We do this by comparing (1) the sample form factor without adjustable parameters from powder X-ray diffraction data using a recently developed method with (2) exactly simulated (Debyesum method) theoretical sample form factors for the recently proposed structure (including vacancy, particle size and shape, and positional disorder effects). Michel et al. (2007a) used pair distribution functions (PDFs) extracted from synchrotron diffraction data fitted with calculated PDFs using adjustable scale and peak shape parameters. The PDF method gives consistent short-range (coordination sphere) correlations but under-emphasizes intermediate-range correlations that represent more stringent constraints on the structure. Main characteristic diffraction peaks of six-line ferrihydrite (lines 2, 3, and 4) are not reproduced by the proposed structural model. We expect our method to offer rigorous tests of proposed structures of any nanocrystalline materials.

Mössbauer spectroscopy of minerals: III. Octahedral-site Fe2+ quadrupole splitting distributions in the phlogopite-annite series

We develop the methodology of quadrupole splitting distribution (QSD) analysis by evaluating the influences of absorber thickness, absorber texture, and the asumed Lorentzian width on the extracted QSD. We then present the first study to describe the Mössbauer spectra of members of a mineral family in terms of QSDs. The Fe2+ QSD and its characteristics (average QS, peak QS, skewness, and standard deviation) show gradual trends with changing Fe/(Fe+Mg) in a synthetic Al-deficient phlogopite-annite series. Al-deficient natural samples of phlogopite and annite show similar behaviour. The Fe2+ QSDs can be interpreted as population distributions of local distortion environments (LDEs) and, as such, contain much information that will become more accessible as theoretical calculations linking particular LDEs to corresponding QS values are developed.

Low-spin γ-FeNi(γLS) proposed as a new mineral in FeNi-bearing meteorites: epitaxial intergrowth of γLS and tetrataenite as a possible equilibrium state at ∼20–40 at% Ni

Abstract We argue that the so-called paramagnetic phase seen by Mossbauer spectroscopy in taenite lamella from octahedrite meteorites, ataxite meteorites, the metal particles of FeNi-bearing chondrite meteorites, and synthetic particle-irradiated FeNi alloys is a low-spin γ-FeNi phase ( γ LS ), related to the close packed low-spin phases seen in the pressure-temperature phase diagrams of both metallic Fe and synthetic FeNi alloys and many other Fe-alloy systems. At a given composition, this γ LS phase is quite distinct from the ordinary (high-spin) γ-phase ( γ HS ) in that it has a different electronic structure associated with very different magnetic properties (small-moment antiferromagnetism versus large-moment ferromagnetism) and a lower lattice parameter. It should be considered a new mineral for which we suggest the name antitaenite. We further propose that in the meteorites γ LS always occurs in a fine epitaxial intergrowth with tetrataenite (atomically ordered FeNi). This resolves outstanding difficulties in meteoritic and particle-irradiated FeNi metallurgy.

Octahedral-site Fe2+ quadrupole splitting distributions from Mössbauer spectroscopy along the (OH, F)-annite join

We have performed a detailed Mössbauer study of synthetic annites on the (OH, F)-join. Recently developed data treatment and spectral analysis methods were used to extract true intrinsic Fe2+ quadrupole splitting distributions (QSDs) that represent the most information that can be resolved from the spectra. The overall room temperature (RT) QSDs can be consistently interpreted in terms of four QSD contributions (or populations) centered at: QSHH∼2.55 mm/s for Fe2+O4(OH)2 octahedra (cis and trans not resolved), QSHF ∼ 2.35 mm/s for Fe2+O4(OH)F octahedra (cis and trans not resolved), QScFF∼2.15 mm/s for cis-Fe2+O4F2 octahedra, and QStFF∼ 1.5 mm/s for trans-Fe2+O4F2 octahedra. Each such contribution has a width (σΔ ∼ 0.2 mm/s) caused by distortions of the octahedra. Minor contributions due to Fe2+O5(OH) and Fe2+O5F octahedra probably also contribute to the overall Fe2+ QSDs. The ferric iron spectral components were also characterized. Here, two distinct types of octahedral Fe3+ contributions are seen and interpreted as being due mainly to Fe3+O5OH and Fe3+O5F octahedra, respectively. Tetrahedral Fe3+ is seen only in the OH-annite end-member and the total Fe3+ content drops significantly on addition of F.

MECHANISMS AND CRYSTAL CHEMISTRY OF OXIDATION IN ANNITE: RESOLVING THE HYDROGEN-LOSS AND VACANCY REACTIONS

A synthetic octahedral-site-vacancy-free annite sample and its progressive oxidation, induced by heating in air, were studied by powder X-ray diffraction (pXRD), Mössbauer spectroscopy, nuclear reaction analysis (NRA), Raman spectroscopy, X-ray fluorescence (XRF) spectroscopy, gas chromatography (GC), thermogravimetric analysis (TGA), differential thermal analysis (DTA), scanning electron microscopy (SEM), and size-fraction separation methods. For a set heating time and as temperature is increased, the sample first evolves along an annite-oxyannite join, until all H is lost via the oxybiotite reaction (Fe2+ + OH− ⇌ Fe3+ + O2− + H↑). It then evolves along an oxyannite-ferrioxyannite join, where ideal ferrioxyannite, KFe3+8/3□1/3AlSi3O12, is defined as the product resulting from complete oxidation of ideal oxyannite, KFe3+2Fe2+AlSi3O12, via the vacancy mechanism (3 Fe2+ ⇌ 2 Fe3+ + [6]□ + Fe↑). A pillaring collapse transition is observed as a collapse of c near the point where

Magnetism of synthetic and natural annite mica: ground state and nature of excitations in an exchange-wise two-dimensional easy-plane ferromagnet with disorder

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