Hematite formation

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). 

The mechanism by which hematite is formed from ferrihydrite in an aqueous system, appears more complicated than that by which goethite forms. If hematite crystals are added to the system they do not function as seeds for hematite formation but induce epitaxial growth of goethite instead (Atkinson et al. 1968 Cornell Giovanoli, 1985). 

A number of observations help to understand the mechanism of hematite formation from ferrihydrite in aqueous systems i. e. under conditions essentially different from those for solid-state transformation by dry heating (see 14.2.6). Air-dry storage of ferrihydrite containing 100-150g H20/kg of water (found by weight loss) at room temperature for 20.4 years in closed vessels led to partial transformation to fairly well crystalline hematite with a little goethite (Schwertmann et al., 1999). In contrast, no hematite was formed from ferrihydrite if the content of adsorbed water was substantially reduced (Stanjek and Weidler, 1992 Weidler, 1997) as seen from the following examples ... 

Although titanium retards the transformation of ferrihydrite (pH 6-11), it enhances the formation of goethite over hematite (Fitzpatrick Le Roux, 1976 Fitzpatrick et al., 1978). The opposite was found for trivalent chromium (Schwertmann et al., 1989) and vanadium (Schwertmann Pfab, 1994) besides retarding the transformation, higher concentrations of both ions led to enhanced hematite formation. 

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. 

Combes, J.M. Manceau, A. Galas, G. (1990) Formation of ferric oxides from aqueous solutions A polyhedral approach by X-ray absorption spectroscopy. 11. Hematite formation from ferric gels. Geochim. Cosmochim. Acta 54 1083-1091... 

Schwertmarm, U. Murad, E. Schulze, D.G. (1982a) Is there holocene reddening (hematite formation) in soils of axeric temperate areas Geoderma 27 209—223... 

Iron extraction values show that iron speciation varies significantly between layers in the cave (Fig. 6A). Values for amorphous, total, and ferrous iron range from 2.4 to 84 pmol/g. Extraction results indicate a significant amount of goethite in the lower layers of the sequence as determined by total minus ammonium-oxalate extractable iron (52 pmol/g in the bottom yellow layer) (Schwertmann and Taylor, 1977). The upper layers have total iron values represented almost entirely by ammonium-oxalate extractable iron (83 pmol/g in the red layer and 59 pmol/g in the top orange layer) suggestive of ferrihydrite (a necessary precursor to hematite formation). The ferrihydrite in the upper layers is indicative of formation by rapid oxidation of ferrous iron (Schwertmann, 1993). The black layer contains the only cave sediment with a significant amount (42 pmol/g) of extracted ferrous iron. 

Both goethite and hematite can form from ferrihydrite. Under alkaline conditions, hematite formation is less likely the higher the pH and the lower the temperature (Schwertmann and Murad, 1983 Cornell and Gio-vanoli, 1985). At < 70 C, a pH >12 is usually sufficient to avoid hematite (Lewis and Schwertmann, 1980). 

Two-line ferrihydrite, for use as a catalyst, can also be synthesized by thermal decomposition of iron penta carbonyl, Fe(CO)5, in a stream of moist air at 500°C (Kosowski, 1993 Zhao et al.l993). It is a free flowing, reddish brown powder with a much lower bulk density than freeze-dried ferrihydrite prepared as above, probably because of much weaker aggregation. Small changes in the reaction conditions (water content of the air, duration of heating) may induce hematite formation. An analogous recipe involved slow thennal decomposition of trinuclear aceto-hy-droxy Fe "-nitrate for 20-40 hr in air in an attempt to simulate the red pigment on the Martian surface (Morris et al. 1991). 

Because of the similar thermodynamic stability of goethite and hematite, formation of hematite competes with nucleation of goethite which proceeds via dissolution of the ferrihydrite precursor. The lower the temperature, the more likely it is that goethite will form. Goethite nucleation can therefore be inhibited by preheating the oven and also all solutions before they are combined. The presence of chloride should be avoided because it promotes the formation of akaganeite (P-FeOOH). However, chloride concentrations below 0.02 M can be tolerated at temperatures of around 100 °C. 

Combes J-M, Manceau A, Calas G (1986) Study of the local structure in poorly ordered precursors of iron oxyhydroxides. J de Physique (Colloque C8, Vol. 2) 697-C8/701 Combes J-M, Manceau A, Calas G (1989a) XAS study of the evolution of local order around iron(III) in the solution to gel to iron oxide (a-Fe203) transformation. PhysicaB (Amsterdam) 158 419-420 Combes J-M, Manceau A, Calas G (1990) Formation of ferric oxides from aqueous solutions A polyhedral approach by X-ray absorption spectroscopy II. Hematite formation from ferric gels. Geochim Cosmochim Acta 54 1083-1091... 

When the pure iron oxide particles were calcined, indications of hematite formation were already present at 450°C/6h, but well-crystallized a-Fe203 was obtained at 600°C/6h. In case of the two doped oxides, no control of atmosphere was required for calcination up to 600°C/25 min, and the products were of the spinel structure. Above 600°C (800°C and above for 3-5 h), hematite appeared as an additional phase. The particles obtained after calcination were rather large (overall span for the three species 2Q-150 nm), showing significant growth from the as-prepared stage. As an example, the Fe203 particles increased in size from 10-30 nm before calcination to 40-85 nm at 600°C/6 h. 

Torrent, J., Barron, V. Liu, Q. (2006). Magnetic enhancement is linked to and precedes hematite formation in aerobic soil. Geophysical Research Letters, 33, L02401. 

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