Goethite forms

High surface areas (80-150 m g ) are also reported for goethites formed by oxidation of Fe systems at pH 6-7 and room temperature, whereas those grown at pH 12 have lower areas of around 30 m g (Torrent et al., 1990). 

Goethite forms in aqueous media by direct precipitation from soluble Fe " species which are supplied by hydrolysis of Fe " solutions, by dissolution of a solid precursor, or by oxidation/hydrolysis of Fe" salt solutions. 

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). 

Erosional transport of iron stones may have led to a mechanical concentration of these spherical bodies in alluvial sediments or in marine depressions and caused their breakdown Trummererze). These deposits may be recemented by Fe oxides, predominantly goethite, formed in situ in the interstices. 

Fig. 18.5 Scanning electron micrograph of a tubercle from a corroded water pipe showing large hexagonal plates or prisms of green rust and small Fe " oxide crystals, probably lepidocrocite and goethite formed from oxidation of green rust (Bigham and Tuovinen, 1985, with permission, courtesy J. M. Bigham).

Schwertmann, U. Stanjek, H (1998) Stirring effects on properties of A1 goethite formed from ferrihydrite. Clays Clay Miner. 46 317-321... 

Under alkaline conditions, ferrihydrite dissolves to release soluble Fe species (Fe(OH)4) from which the less soluble goethite forms. 

Hematite forms competitively with akaganeite at reaction temperatures above 90 °C and goethite forms competitively if seed crystals of goethite are added (Atkinson et al., 1977). 

Cornell, R. M. and Giovanoli, R. (1985) Effect of solution conditions on the proportion and morphology of goethite formed from ferrihydrite. Clays Clay Min. 33 424-432. 

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