Akaganeite structure

Aicagan ite. can incorporate up to 0.06 mol mol Cu and much smaller amounts of Cr, Mn, Co, Ni and Zn in the structure (Inouye et al., 1974 Holm, 1985 Buch-wald Clarke, 1989). Incorporation of Al, Cr and Ga has also been reported (Lorenz Kempe, 1987). Cornell (1992) produced akaganeite from acid Si-containing Fe " solutions and found by congruent dissolution that up to 0.04 mol mol Si could be incorporated. The Si species were probably located in the tunnels (0.5 nm ) of the akaganeite structure. 

Further candidates for the tunnel position in the schwertmannite structure is selenate. It seems that the oxyanions of hexavalent elements (S, Se, Cr) can be accomodated in the akaganeite structure whereas tliose of pentavalent ones (P, As) may only adsorb on the surface (Waychunas et al. 1995). 

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. 

Schwertmannite, Fei60is(0H)y(S04)z nHyO, has the same basic structure as akaganeite, but contains sulphate instead of chloride ions. This recently recognized mineral frequently occurs in nature as an oxidation product of pyrite and can be... 

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). 

Akaganeite (named after the Akagane mine in Japan) is isostructural with hollan-dite. Compounds with this structure have a tetragonal or monoclinic unit cell. Bernal et al. (1959) and Keller (1970) both concluded that the unit cell of akaganeite was tetragonal with a = 1.000 nm and c = 0.3023 nm. The structural refinement of a natural sample using XRD and Rietveld analysis indicated, however, that the unit cell is monoclinic with a = 1.060 nm, b = 0.3039 nm, c = 1.0513 nm and p = 90.24° (Post Buchwald, 1991). There are eight formula units per unit cell. 

Fig. 2.6 Structure of akaganeite. a) Arrangement of octahedral double chains in tunnels with chloride ions in the centre ofthe tunnels, b) Ball-and-stick model with unit cell outlined.

Somatoids are often twinned on the (322) plane to give star-shaped or x-shaped twins (Eig. 4.15 a). Incorporation of low levels of Si in the structure promotes twinning with 0.04 mol mol Si, akaganeite was almost 100% twinned (Cornell, 1992). These crystals have a visibly roughened surface. Increasing citrate concentration during forced hydrolysis at 100 °C and pH 1 reduced the length of the somatoids from... 

An example involving the akaganeite hematite transformation is shown in Figure 14.8 where the average pore diameter increased from 1.1 nm at 150 °C to 3.7 nm at 350 °C and then to > 15 nm at 500 °C. The t-plot method using H2O as an adsorbate has also been used to investigate the location of H2O in the tunnel structure of akaganeite (Naono et al., 1993). 

The crystals of akaganeite are not microporous. Micropores observed by TEM are considered to be due to irradiation in the electron beam (Galbrait et al., 1979 Naono et al., 1982). Open ended, cylindrical, interparticular micropores have been reported these arose as a result of alignment of the rod-like crystals into parallel arrays (Paterson and Tait, 1977). Akaganeite does possess a potential structural microporosity arising from the presence of 0.21-0.24 nm across tunnels in the structure. At room... 

In the antiferromagnetic state, the spins are oriented parallel to the c-axis. Moss-bauer studies have indicated that the number and type of subsites in the magnetic structure may be influenced by the halide concentration, the nature of the halide and the level of excess protons vhich balance the halide charge. When chloride is present in the structure, there are tv o different Fe " sites, vhereas for fluoride-containing akaganeite, the number of non-equivalent cation sites may be greater. 

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). 

Fig. 13.4 The structural embryo of akaganeite (Schneider Schwyn, 1987, with permission).

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... 

Gonzalez-Calbet, J.M. Alario-Franco, M.A. (1982) A thermodynamic and electron microscopic study of the decomposition of akaga-neite. Thermochim. Acta 58 45-51 Gonzalez-Calbet, J.M., Alario-Franco, M.A. Gayoso-Andrade, M. (1981) The porous structure of synthetic akaganeite. J. inorg. nucl. Chem. 43 257-264... 

Paterson, E. Tait, J.M. (1977) Nitrogen adsorption on synthetic akaganeite and its structural implications. Clay Min. 12 345-352 Paterson, E. Swaffield, R. Clark, D.R. (1982) Thermal decomposition of synthetic akaganeite (P-FeOOH). Thermochim. Acta 54 201-211... 

Post, J. E. Buchwald, V. F. (1991) Crystal structure refinement of akaganeite. 

Fig. 1-1. Structural models of goethite, akaganeite, lepidocrocite, feroxyhyte, hematite and magnetite. Small, open circles indicate H atoms (from H. Stanjek, in Cornell and Schwertmann, 1996, with permission).

Akaganeite cannot be prepared at pHs above 5 because the OH ion is far more competitive than the chloride ion for structural sites. Akaganeite displays two morphologies somatoids or cigar shaped crystals and much smaller rod-like crystals. Samples of somatoids frequently contain a proportion of twinned crystals, whereas the rod-like crystals are never twinned. 

Akaganeite may also be precipitated from Fe fluoride solutions (Bemal et al. 1959 Naono et al. 1993), but not from bromide or iodide solutions the latter ions are too large to fit into the tunnels in the structure. 

Paterson, E. and Tbit, J. M. (1977) Nitrogen adsorption on synthetic akaganeite and its structural implications. Clay Min. 12 345-352. 

Schwertmannite, is a common nanoparticle-product of neutralization of sulfuric acid-rich solutions (Bigham et al. 1994). The original structural analysis indicated that sulfate was contained within tunnels similar to those found in akaganeite (FeOOH). However, recent work by Waychunas et al. (2001) suggests that this is a defective, nanoporous phase and that sulfate occupies inner and outer sphere positions on the surface, and probably on the internal surfaces of defect regions within the structure. 

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