The Formation of Hematite from Amorphous Ir On(iii)hydr Oxide

-The formation of hematite from amorphous Fe(III)hydroxide in aqueous systems at pH 6 and 70~ both with and without oxalate, was followed by kinetic measurements, electron microscopy, i.r. spectroscopy and thermal analysis. In the absence of oxalate, small amorphous particles coalesce into aggregates which eventually become single crystals of hematite. When oxalate is present, crystal growth is much faster and does not proceed through the intermediate stage of aggregation. Aggregates, when formed, consist of groups of single crystals. It is suggested that oxalate accelerates the nucleation of hematite crystals by acting as a template, the Fe-Fe distance in Fe oxalate ions being similar to that in hematite. I N T R O D U C T I O N Among the iron oxides occurring in soils and sediments, hematite and goethite are by far the most common. Although hematite rarely occurs in soils of temperate climates, mixtures of both forms are often found in warmer regions. Thermodynamic evidence, although conflicting, indicates a difference in stability of the two forms, so that only one or the other should exist at equilibrium (Mohr et al., 1972). Langmuir (1971) stressed the importance of particle size to explain the conflicting results but admitted sluggish kinetics as a reason for extremely slow mutual transformation. Since many of the soils in which both oxides occur are very old, the conversion of goethite into hematite and vice versa at ambient temperature must be extremely slow indeed. Furthermore, long term laboratory experiments have shown that the ratio of goethite to hematite formed at room temperature from amorphous Fe(III)hydroxide in water does not change with time (Schwertmann, unpublished results). Apparently these two crystalline forms in soils are not inter-related genetically by a simple hydrationdehydration type of reaction (c~ Fe20 3 + H2 0 ~2 ~ FeOOH), but are formed by separate pathways from a common source. Numerous laboratory experiments have shown that amorphous or para-crystalline Fe(III)hydroxide is a suitable source. Environmental conditions play a decisive role in determining which of the two forms are produced (Schwertmann, 1969), probably because of their influence on the kinetics of the alternative reactions. However, to clarify this, it is necessary to know more about the relevant reaction mechanisms. Fischer (1971) found that goethite is produced from amorphous Fe(III)hydroxide by a reconstructive transformation involving dissolution and reprecipitation (nomenclature after Mackay, 1960) but very little is known about the mechanism of formation of hematite. Feitknecht and Michaelis (1962) believe it to be an in<CM. 23"1 c ternal dehydration and crystallization, but do not describe the process in detail. The aim of the present work is to elucidate the mechanism of hematite formation. Since only hematite is formed in the presence of oxalate at pH values around 7 (Schwertmann, 1969, 1970), these conditions were chosen for study.