Akaganeite transformation

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-... 

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. 

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). 

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... 

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).

Bauer et ah, 1986). It is also a significant corrosion product of Fe alloy phases on Antarctic meteorites where its formation is induced by the chloride ions coming from airborne seaspray and/or volcanic activity (Buchwald and Clarke, 1989). In these meteorites, akaganeite is located adjacent to the corroding surface and beneath a layer of goethite/spinel into which it eventually transforms. 

Cornell, R.M. Giovanoli, R. (1990) Transformation of akaganeite into goethite and hematite in alkaline media. Clays Clay Min. 38 469-476... 

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... 

Akaganeite may transform into either goethite or hematite at high enough temperature if left in the mother liquor long enough. However, at 70 °C neither transformation nor any Ostwald ripening was observed after a 5 month period. 

Forced hydrolysis of Fe solutions involves hydrolysing Fe(N03)3, Fe(C104)3 or FeCls solutions at a temperature close to 100 °C under strongly acidic conditions (pH 1-2). It is believed that, if hematite is prepared from Fe(N03)3 or Fe(C104)3, it forms from Fe hydroxy species (Johnston and Lewis, 1983, 1986), whereas, if hematite is prepared from FeCls, akaganeite may be an intermediate product which then transforms to hematite via solution (Hamada and Matijevic, 1981). 

Hydrothermal transformation of various Fe oxides. Ferrihydrite (2-line), lepidocrocite, akaganeite and goethite (if poorly crystalline) can be converted to large (1-3 am) hexagonal plates of hematite if kept under water in a teflon bomb at 180 °C for 10 days. 

In weathering, steel rust formed on atmospheric corrosion in different environments is composed of crystalline compounds like haematite, magnetite and oxyhydroxides of iron like goethite, akaganeite, lepidocrocite and feroxyhite apart from amorphous ferric oxyhydroxide rust. These rust constituents transform to one another during wet-dry cycles of atmospheric exposure [17].Various phases of corrosion products formed in progressive exposure to atmosphere are given in Table 1.1 [6, 18]. 

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