Lepidocrocite structure

Gehring, A.U. (1985) A microchemical study of iron ooids. Eclogae Geol. Helv. 78 451-457 Gehring, A.U. Karthein, R. Reller, A. (1990) Activated state in the lepidocrocite structure during thermal treatment. Naturwissenschaf-ten 77 177-179... 

Orange iron oxide with the lepidocrocite structure (y-FeOOH) is obtained if dilute solutions of the iron(II) salt are precipitated with sodium hydroxide solution or other alkalis until almost neutral. The suspension is then heated for a short period, rapidly cooled, and oxidized [3.22], [3.23],... 

The synthesis of Al-substituted lepidocrocite needs special precautions because A1 may promote goethite in a Fe system. However, poorly crystalline lepidocrocites with up to 12 mol% A1 in the stmcture have been produced by oxidation of mixed FeCl2/Al(N03)3 solutions with C02-free air at pH 8 in a NH3/NH4Cl-buffer (0.2 M NH3 0.2 M NH4CI 1 19) with the pH being kept constant by adding M NH3 dropwise (Schwertmann and Wolska, 1990 Wolska et al. 1994). Note that A1 can only be incorporated into the lepidocrocite structure, if the pH of the system is close to 8. 

Enhanced Reactivity of Fe(III) formed at Surfaces. Another way to keep the Fe(III) hydroxide formed from oxygenation of Fe(II) from extensive polymerization, is to oxidize (02) the adsorbed Fe(II). Apparently the Fe(III) formed on the surface (or part of it), plausibly because of a different coordinative arrangement of the adsorbed ions, does not readily polymerize fully to a "cross-linked" three-dimensional structure and is thus more reactive than freshly formed lepidocrocite. 

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. 

Fig. 2.3 Basic structural units and Fe-Fe distances (in nm) for hematite, goethite, akaganeite and lepidocrocite and their associated radial distribution functions as obtained from EXAFS spectra. The first peak in the radial distribution...

Lepidocrocite (Greek lepidos = scale, flake and krokoeis = saffron-coloured) is iso-structural with boehmite (Tab. 2.1). Unlike goethite and akaganeite which have a turmel structure, lepidocrocite is a layered compound. The orthorhombic unit cell contains four formula units and has the edge lengths, a = 1.2520(6) nm h = 0.3873(2) nm and c = 0.3071(6) nm (Ewing, 1935 Oles et al., 1970 Christensen Norlund-Christensen, 1978). 

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. 2.5 Structure of lepidocrocite. a) Cubic close packed anion arrangement and distribution of cations over the octahedral interstices. Projection on (001). Octahedral arrangement and unit cell outlined, b) Projection of anion close packing on (010). Octahedral arrangement and unit cell outlined, c) Projection of anion close packing on (001). Dashed circles represent Fe in the next lo A/er layer, d) Arrangement of oc-...

Lepidocrocite is paramagnetic at room temperature. The Neel temperature of 77 K is much lower than that of the other iron oxides and is the result of the layer-like structure of this mineral. The sheets of Fe(0,0H)6 octahedra are linked by weak hydrogen bonds, hence magnetic interactions are relatively weak. The saturation hyperfine field is also lower than for any other iron oxide (Tab. 6.2). In the antiferromagnetic state, the spins are ordered parallel to the c-axis with spins in alternate layers having opposite signs. A decrease of T by 5 K was observed for Al-lepidocrocites with an Al/(Fe-i-Al) ratio of 0.1 (De Grave et al., 1995). 

Like X-ray diffraction patterns, neutron and electron diffraction patterns provide averaged information about the structure of a compound. Details of these techniques are given in works by Hirsch et al. (1965) and West (1988). Neutron diffraction involves interaction of neutrons with the nuclei of the atoms. As the neutrons are scattered relatively evenly by all the atoms in the compound, they serve to indicate the positions of the protons in an oxide hydroxide. This technique has been applied to elucidation of the structure and/or magnetic properties of goethite (Szytula et al., 1968 Forsyth et al., 1968), akaganeite (Szytula et al., 1970), lepidocrocite (Oles et al., 1970 Christensen Norlund-Christensen, 1978), hematite (Samuelson Shirane, 1970 Fernet et al., 1984) and wiistite (Roth, 1960 Cheetham et al., 1971 Battle Cheetham, 1979). A neutron diffractogram of a 6-line ferrihydrite was recently produced by Jansen et al. (2002) and has helped to refine its structure (see chap. 2). 

Leland and Bard (1987) found that the different iron oxides induced photooxidation of oxalate and sulphite at rates that varied by up to two orders of magnitude. For oxalate, the rate was greater for maghemite than for hematite, but this order was reversed for sulphite. Lepidocrocite (layer structure) induced faster oxidation of both compounds that did the other polymorphs of FeOOH (tunnel structures) the authors considered that the rate differences were probably associated with structural differences between the adsorbents. 

Processings of iron oxides at room temperature. II. Mechanochemical reaction effects on the structure and surface of pure, synthetic lepidocrocite. Mat. Res. Bull. 17 1017-1023... 

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