Akaganeite hematite

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). 

Chen, M., Jiang, J., Zhou, X. and Diao, G. (2008) Preparation of akaganeite nanorods and their transformation tosphere shape hematite. Journal for Nanoscience and Nanotechnology, 8, 3942-3948. 

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...

To the naked eye, goethite and akaganeite appear yellow-brown, lepidocrocite orange and hematite usually red (Plate 6.1). Feroxyhyte and ferrihydrite are dark reddish brown, maghemite brown to brownish red and magnetite and wiistite are black. 

Based on these results, and in spite of considerable variation in the band positions of samples of the same Fe oxide, at least 80 % of the pure akaganeite, feroxyhyte, ferrihydrite, hematite and lepidocrocite samples could be correctly classified by Scheinost et al. (1998) by discriminant functions based on the above four bands (Fig.7.3 right) Magnetite could be identified by its band at 1500 nm, but for goethite. 

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). 

Gt goethite Lp lepidocrocite Ak akaganeite Fh ferrihydrite Hm hematite Mt magnetite Mh maghemite Wu wiistite... 

Decomposition of akaganeite starts at 150 °C and complete conversion to hematite is achieved at ca. 500 °C. This is not a topotactic transformation it involves a complete breakdown of the bcc anion packing of akaganeite followed by reconstruction of the hep anion array of hematite. Initially, the product is in the form of elongated, porous... 

There is some uncertainty about whether akaganeite transforms directly to hematite. Some authors (Bernal et al., 1959 Dezsi et al., 1967 Morales et al., 1984) con-... 

At temperatures of up to 70 °C, akaganeite grown by hydrolysis of FeCl3 is stable for months in the acidic mother liquor (Cornell, 1992). If, however, the system is seeded with goethite or hematite, the akaganeite gradually transforms into these com-... 

Fig. 14.13 Extent of transformation at 70 °C of akaganeite to goethite and hematite versus time, a) rod-shaped akaganeite in M KOH b-d) spindle-shaped akaganeite b) M KOH c) 0.1 M KOH d) 0.1 M KOH -h Mn -" (Mn/(Fe-hMn) = 0.1) (Cornell Giovanoli, 1990,1991 with permission).

Fig. 15.8 Well ciystalline euhedral platy hematite (a), poorly crystalline spherical Si-containing hematite together with ring-like layer Fe-silicates ( ) (b), and akaganeite (c) from the Atlantis Deep, Red Sea, (Photo H.-Ch. Bartscherer) (Schwertmann etal., unpubl.)...

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