Ferrihydrite to hematite

The transformation of ferrihydrite to hematite by dry heating involves a combination of dehydration/dehydroxylation and rearrangement processes leading to a gradual structural ordering within the ferrihydrite particles in the direction of the hematite structure. This transformation may or may not be facilitated by the postulated structural relationship between the two phases. EXAFS studies have shown, for example, that some face sharing between FeOg octahedra, characteristic of hematite, also exists in 6-line ferrihydrite (see chap. 2). 

Stanjek and Weidler (1992) and Weidler (1995) showed that 2- and 6-line ferrihy-drite behaved quite differently upon heating. During heating at 127 °C for 1180 h, the ratio of H20/Fe203 decreased from 2.64 to 1.23 for a 2-line ferrihydrite and from 1.57 to 0.85 for a 6-line ferrihydrite without much change in the X-ray diffractogram. 

340 °C) to 126 (at 672 °C) and then to 700 run (at 995 °C) and the MCLe increased from 47 to 220 nm and then to 10 (xm. At the same time, the occupancy of Fe sites rose from 11.2 to 11.5 and then to 11.7 per unit cell (full occupancy = 12) indicating that the amount of OH in the structure had fallen over this temperature range (Campbell et al. 2002). 

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Johnston, J. Lewis, D.G. 1983. A detailed study of the transformation of ferrihydrite to hematite in an aqueous medium at 92°C. Geochimica et Cosmochimica Acta, 47, 1823-1831. 

Fig. 1417 Transmission electron micrographs documenting the transformation of ferrihydrite to hematite (Fischer. Schwert-mann, 1975 with permission).

The laboratory derived model of hematite formation in soils via ferrihydrite has received general acceptance. So far, it is the only way to produce hematite at ambient temperatures and in the pH range of soils. Support from soil analysis, however, is meagre. Hematite is usually associated with other Fe oxides, mainly with goethite but not with ferrihydrite. There seems to be only one report of a ferrihydrite-hema-tite association (based on XRD and Mossbauer spectra) viz. in several andisols formed from basalt in the warm and moist climate of Hawaii (Parfitt et al., 1988). In this case, in addition to the low age of the soils, high release of Si may retard the transformation of ferrihydrite to hematite, whereas normally, the rate of transformation of ferrihydrite seems to be higher than that of ferrihydrite formation, so that this mineral does not persist. 

All fine grained Fc oxides lose adsorbed water at characteristic temperatures of between 100 and 200 °C. Structural OH in gocthite and lepido-crocite is lost at 250-400°C by the dehydroxylation reaction 2OH O + H2O. Even fine grained oxides such as hematite contain some OH in the structure (Stanjek Schwertmann, 1992) and this is driven off over a wide temperature range. For Fe oxides endothermic peaks result from the release of adsorbed or structural water, whereas exothermic peaks come from phase transformations (e.g. maghemite to hematite) or from recrystallization of smaller crystals into larger ones. An example of this is observed during the transformation of ferrihydrite to hematite. 

Glasauer, S., Hug, R, Weidler, RG and Gehring, A.U. (2000). Inhibition of sintering by Si during the conversion of Si-rich ferrihydrite to hematite. Clays Clay Miner, 48, 51-56. 

ABSTRACT The kinetics and mechanisms of the phase transformation of 2-line ferrihydrite to goethite and hematite are being assessed as a function of pH, temperature and Fe/As, Fe/Se, Fe/Mo molar ratios using batch experiments, BET analyses, XRD, and XANES. Initial results from XRD analyses show that ferrihydrite is stable at high pH ( 10) for up to seven days at 25°C, but considerable crystallization occurs at elevated temperatures. Specifically, XRD data show that ferrihydrite is transformed to a mixture of hematite and goethite at 50°C (-85% hematite and -15% goethite) and 75°C (-95% hematite and -5% goethite) after 24 hours and these ratios remain constant to the end of the experiments (seven days). 

Measured surface areas (11-point BET analyses) for pure phases such as ferrihydrite, goethite and hematite are in the range as proposed by Cornell Schwertmann (2003) (Table 1). Preliminary XRD analyses showed that temperature impacts the kinetics of phase transformation of ferrihydrite. Data indicated that after seven days, the rate of transformation from ferrihydrite to more crystalline forms, if it was occurring, was too slow to be measured at 25°C (Fig. 1). In contrast to the 25°C experiment, significant, transformations were observed at 50 (Fig. 2) and 75°C (Fig. 3) after 24... 

The structure derived from a Rietveld fit of a neutron diffraction pattern of a 6-line ferrihydrite which showed more and sharper lines (Fig. 2.9, lower) than an XRD pattern, was in agreement with the structure proposed by Drits et al. (1993) except that it was not necessary to assume the presence of hematite in order to produce a satisfactory fit (Jansen et al. 2002). The unit cell of the defect free phase had a = 0.29514(9) nm and c = 0.9414(9) nm and the average domain size derived from line broadening was 2.7(0.8) nm. Since forced hydrolysis of an Fe solution at elevated temperatures will ultimately lead to hematite, it is likely that incipient hematite formation may occur under certain synthesis conditions. Neither these studies nor Mbssbauer spectroscopy, which showed only a singular isomer shift at 4.2 K characteristic of Fe, supported the presence of " Fe (Cardile, 1988 Pankhurst Pollard, 1992). However, the presence, at the surface, of some Fe with lower (<6) coordination, perhaps as tetrahedra (Eggleton and Fitzpatrick, 1988) which may have become unsaturated on heating, has been suggested on the basis of XAFS results (Zhao et al. 1994). 

Biological reductive dissolution by Shewanella putrifaciens of Fe oxides in material from four Atlantic pleistocene sediments (ca. 1.5-41 g/kg Fe oxides) was compared with that of the synthetic analogues (ferrihydrite, goethite, hematite) (Zachara et al. 1998). In the presence of AQDS as an electron shuttle, the percentage of bio-reduc-tion of the three oxides was increased from 13.3 %(fh) 9.2%(gt) and 0.6%(hm) to 94.6% 32.8% and 9.9% with part of the Fe formed being precipitated as vivianite and siderite, but not as magnetite. The quinone was reduced to hydroquinone which in turn, and in agreement with thermodynamics, reduced the Fe as it had much better access to the oxide surface than did the bacteria themselves. 

Direct proof for the participation of free water in the transformation to hematite was recently presented by Bao and Koch (1999) the oxygen of the hematite formed from 2-line ferrihydrite in the presence of water with a 5 0 of-8.0%o had the same isotope ratio as this water, showing that the oxygen came predominately from the water present during the transformation and not from the ferrihydrite precursor. 

In general, foreign species in the system can have two different effects on the transformation of ferrihydrite to other Fe oxides they can either modify the rate of the transformation, usually by slowing the process, or change the composition (mainly the hematite/goethite ratio) and properties of the end product. Two principal mechanisms of interaction operate ... 

Fig.14.20 Effect of various clay minerals on the transformation of 2-line ferrihydrite to goethite and hematite at 25 °C and pH 5 after 16 yr as measured by the ratio of oxalate to dithionite soluble Fe (Feo/Fed) (Schwertmann et al.

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