FeOOH structures

A common feature of the dehydroxylation of all iron oxide hydroxides is the initial development of microporosity due to the expulsion of water. This is followed, at higher temperatures, by the coalescence of these micropores to mesopores (see Chap. 5). Pore formation is accompanied by a rise in sample surface area. At temperatures higher than ca. 600 °C, the product sinters and the surface area drops considerably. During dehydroxylation, hydroxo-bonds are replaced by oxo-bonds and face sharing between octahedra (absent in the FeOOH structures see Chap. 2) develops and leads to a denser structure. As only one half of the interstices are filled with cations, some movement of Fe atoms during the transformation is required to achieve the two thirds occupancy found in hematite. 

Cornell, R.M. (1991) Simultaneous incorporation of Mn, Ni and Co in the goethite (a-FeOOH) structure. Clay Min. 26 427-430 Cornell, R.M. (1992) Preparation and properties of Si substituted akaganeite (P-FeOOH). Z. Pflanzenemahr. Bodenk. 155 449-453 Cornell, R.M. Giovanoli, R. Schindler, P.W. (1987) Effect of silicate species on the transformation of ferrihydrite into goethite and hematite in alkaline media. Clays Clay Min. 35 12-28... 

Properties total iron 58.1%, 3-FeOOH structure confirmed by XRD [661], specific surface area 51 iiT/g (original and stored) [568], 51.6 mVg [1319], wheat-grain shape [661], TEM image available [661]. 

Natural FeOOH, Structure and Origin Unknown Water-washed, and dried at 110°C. 

A minor part of Fe(III) ions (7 %) can be only assigned to y-FeOOH structure. 

Electrokinetically driven iron mineralization originates when Fe(III) combines with OH" ions produced at the cathode to form insoluble ferric hydroxides [Fe(OH)3(s)], hematite ( -Fe203) andgoethite (FeOOH) (e.g., Faulkner, Hopkinson, and Cundy, 2005 Mukhopadhyay, Sundquist, and Schmitz, 2007). SEM observations of soil samples taken from the anodic zones of the experimental cells reveals a ubiquitous association between the iron minerals and subsidiary quantities of chromium. Cr(III) can substitute for Fe(III) in the FeOOH structure (Eary and Rai, 1988), and Cr(VI) reduction by Fe " leads to the development of solids at nearneutral pH showing mixed iron/chromium solid solution of the form Fe Cri x(OH)3 (Eary and Rai, 1988 Fendorf and Li, 1996). Such conditions would have been met at the interface between the acidic and alkali portions of the experimental cells (Fig. 8.5). When Fe(III) is produced solely from the stochiometric reaction with chromate, the value of x is 0.75 (Batchelor et al, 1998) ... 

D. Chambaete, E. De Grave, On the Neel temperature of -FeOOH structural dependence and its Implications. J. Magn. Magn. Mater. 42, 263-268 (1984)... 

The verification of the presence of hydrogen in the film has proved more controversial, primarily because many of the structural investigations have been carried out after the film has been dried in vacuo. An example of the problems here is the fact that electron diffraction, which has to be carried out in vacuo, reveals a relatively well-crystallised spinel lattice whose origin may be the comparatively high sample heating encountered in the electron beam. Moreover, the use of in situ techniques, such as Mossbauer and X-ray absorption spectroscopy, clearly reveals marked differences between the spectra of the films in situ and the spectra of the same films ex situ as well as the spectra of y-Fe203 and y-FeOOH standards. These differences are most naturally ascribed to hydration of the spinel forms. 

The quadrupole splitting of the heat treated FePc/XC-72 electrode measured ex situ, prior to the electrochemical experiments, was larger than that found in situ. Smaller values for A have been reported for certain ferric hydroxide gels and for small particles of FeOOH (Table II), and thus the effect associated with the immersion of the specimen in the electrolyte is most probably related to the incorporation of water into the oxide structure. For this reason, the material observed in situ at this potential will be referred to hereafter as FeOOH(hydrated), without implying any specific stoichiometry. 

Titration calorimetry and cylindrical internal reflection-Fourier transform infrared (CIR-FTIR) spectroscopy are two techniques which have seldom been applied to study reactions at the solid-liquid interface. In this paper, we describe these two techniques and their application to the investigation of salicylate ion adsorption in aqueous goethite (a-FeOOH) suspensions from pH 4 to 7. Evidence suggests that salicylate adsorbs on goethite by forming a chelate structure in which each salicylate ion replaces two hydroxyls attached to a single iron atom at the surface. 

Oxide composition and lattice structure influences the coordin-ative environment of surface sites, and should have an impact on rates of ligand substitution. Hematite (Fe203), goethite (a-FeOOH), and lepidocrocite (y-FeOOH), for example, are all Fe(III) oxide/ hydroxides, but may exhibit different rates of surface chemical... 

The orange coloured lepidocrocite, y-FeOOH, is named after its platy crystal shape (lepidos scale) and its orange colour (krokus = saffron). It occurs in rocks, soils, biota and rust and is often an oxidation product of Fe ". It has the boehmite (y-AlOOH) structure which is based on cubic close packing (ccp) of anions. 

Akaganeite, P-FeOOH, is named after the Akagane mine in Japan where it was first discovered (Mackay, 1962). It occurs rarely in nature and is found mainly in Cl-rich environments such as hot brines and in rust in marine environments. Unlike the other FeOOH polymorphs, it has a structure based on body centered cubic packing of anions (bcp) (hollandite structure) and contains a low level of either chloride or fluoride ions. It has a brown to bright yellow colour. 

FeOOH (synthetic) and its poorly crystalline mineral form, feroxyhyte (5 -FeOOH), are reddish-brown, ferrimagnetic compounds. Their structures are based on hep anion arrays and differ in the ordering of the cations. Feroxyhyte was first described by Chukhrov et al. in 1976 and occurs (rarely) in various surface environments. 

The sheets are held together solely by hydrogen bonds (Fig. 2.5 d, e). Deuteration of the bulk OH in lepidocrocite is facilitated by the layer structure ease of deuteration of FeOOH follows the order lepidocrocite > goethite > akaganeite (Ishikawa et al., 1986). 

Fig. 2.8 Structure of high pressure FeOOH. a) Octahedral chains, b) Ball-and-stick model with unit cell outlined, (a, b Stanjek, unpubl.)...

As expected for a material with cubic sites, the RT spectrum of stoichiometric wiis-tite consists of a single line. The spectrum of the non-stoichiometric material shows an asymmetrically broadened doublet with contributions from Fe " resonance and from two quadrupole split Fe" doublets. The component peaks which contribute to this spectrum have not been fully resolved owing to the range of Fe" environments in the structure (i. e. the variation in Fe content and vacancy level). At 77 K the Mossbauer spectrum consists of a broad doublet with contributions from Fe" and Fe ". Fe(OH)2 shows a sextet corresponding to a Bhf of 16.6 T at 20 K (Miyamoto, 1976 Genin et al. 1986) and to -20 Tat 4K (Refait et al. 1999). The Tn is at 34 K (Miyamoto et al., 1967). The spectrum of high pressure FeOOH at room temperature consists of a sextet (Peret et al. 1973). 

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