Goethite transformation into hematite

Weibel, R. Grobety, B. (1999) Pseudomor-phous transformation of goethite needles into hematite in sediments of the triassic Skagerrak formation, Denmark. Clay Min. 

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-... 

Tindall, G.P. Muir, D.M. (1998) Effect of Eh on the rate and mechanism of the transformation of goethite into hematite in a high... 

Yapp. C.J. (2000) Climatic implications of surface domains in arrays of 5D and 5180 from hydroxyl minerals Goethite as an example. Geochim. Cosmochim. Acta 64 2009-2025 Yariv, S. Mendelovid, E. Villalba, R. (1980) Thermal transformation of goethite into hematite in alkali halide discs. J. Chem. Soc. Faraday Trans. I. 76 1442-1454 Yariv, S. Mendelovid, E. Villalba, R. Cohen, M. (1979) Transformation of goethite to maghemite in Csl discs. Nature 279 519-520... 

Over time, two-line ferrihydrite normally transforms into goethite or hematite in laboratory or natural environments (Rancourt et al., 2001, 839). However, extensive sorption of As(V) could delay the transformation (Ford, 2002). The crystallization of arsenic-bearing amorphous iron compounds often releases arsenic from the compounds (Welch et al., 2000, 599). In particular, while aging in seawater from Ambitle Island near Papua New Guinea, two-line ferrihydrites transformed into less arsenic-rich six-fine varieties. The arsenic released by the transformation of the ferrihydrites produced distinct crystals of claudetite (As203) (Rancourt et al., 2001, 838-839). 

Chukhrov et al. assign a special role to ferrihydrite (2.5 Fe203-4.5 H2O), which is believed to be the typical product of rapid oxidation of Fe + in slightly acid, neutral, and slightly alkaline solutions with the participation of iron bacteria. The oxidation process also is accelerated by the catalytic action of silica. In the course of time ferrihydrite spontaneously converts to hematite, but in solutions with Fe " ions ferrihydrite is transformed into stable goethite in the absence of significant amounts of oxygen. 

Iron oxides are most conveniently stored as dry powders. However, after prolonged storage in an air-dry state some metastable forms may transform into more stable ones. For example, ferrihydrite will gradually turn into hematite and goethite when kept in contact with the atmosphere, presumably owing to the presence of adsorbed non-stoichiometric water Fig. 2-1 shows an X-... 

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. 

Ferrihydrite obtained by hydrolysis of Fe2(S04)3 is very slowly transformed into goethite at pH 7 [20], After one year only 7% of the initially formed ferrihydrite was transformed, while for ferrihydrite obtained by hydrolysis of FeCls or Fe(N03)3 the degree of transformation into mixture of goethite and hematite was 30% at the same pH. On the other hand at pH 11 the transformation into goethite was almost complete after one year and the difference in the transformation kinetics between ferrihydrite obtained by hydrolysis of Fe2(S04>3, FeCU and Fe(N03)3 was less significant. At pH 8 10 substantial amount of hematite is present after a one year aging. These results show that in adsorption experiments with fresh precipitates we can deal with two different adsorbents in the beginning and in the end the experiment. 

Noncrystalline oxides, particularly ferrihydrite, are common in soils because the presence of soluble silica and organic matter tends to inhibit crystallization into more stable, better ordered oxides of Fe. Even so, ferrihydrite is considered to be unstable, gradually transforming to hematite in tropical or subtropical climates or to goethite in humid temperate climates. 

Samples from the site contained considerable amounts of freshly precipitated iron hydroxides. Their transformation into thermodynamically more stable minerals such as goethite or hematite has a very slow kinetics, thus ferrihydrite was chosen as the major adsorbing surface. The Diffuse Double Layer model (Dzombak and Morel, 1990) was selected to describe surface complexation. The respective intrinsic surface parameters and the reaction constants for the ions competing with uranium(VI) for sorption sites were taken from a database mainly based on Dzombak and Morel, 1990, with the urani-um(Vl) sorption parameters as determined by Dicke and Smith, 1996. The results, based on runs with 1000 varied parameter sets, are summarized in Table 5.2. 

The topotactic dehydration reaction [215] transforming goethite into hematite was studied in situ at room temperature and shown to... 

A characteristic of the iron oxide system is the variety of possible interconversions between the different phases. Under the appropriate conditions, almost every iron oxide can be converted into at least two others. Under oxic conditions, goethite and hematite are thermodynamically the most stable compounds in this system and are, therefore, the end members of many transformation routes. The transformations which take place between the iron oxides are summarized in Table 14.1. These interconversions have an important role in corrosion processes and in processes occurring in various natural environments including rocks, soils, lakes and biota. In the latter environments, they often modify the availability and environmental impact of adsorbed or occluded elements, for example, heavy metals. Interconversions are also utilized in industry, e.g. in the blast furnace and in pigment production, and in laboratory syntheses. 

Fig. 14.19 The effect of silicate (Si/Fe = 0.005) on the transformation of 2-line ferrihydrite into goethite and hematite at 70 °C (Cornell et al., 1987 with permission).

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. (1985) Effect of simple sugars on the alkaline transformation of ferrihydrite into goethite and hematite. Clays Clay Min. 33 219-227... 

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... 

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