Goethite morphology

For all three types of anisotropy the coercivity can be calculated (Tab. 7.7). For SD magnetite and maghemite, shape anisotropy, dominates over strain and crystal anisotropy, whereas for hematite and goethite, morphology has little influence on coer-civity. 

After 45 days of experiment, the precipitate showed an XRD pattern typical of a poorly ordered mineral, such as schwertmannite. Identification of this AMD mineral was confirmed by its distinctive spike morphology, observed by SEM-SE (Fig. 3). After 240 days, XRD detected the resolved reflection at d spacing of 4.18 A, which is characteristic of goethite. Therefore, the maintenance of the precipitate in contact with the solution enabled its evolution to a more crystalline state. The trend of decrease in pH that is observed by the end of the experiment reflects the production of acidity, associated with the transformation of schwertmannite into goethite as it was observed by Bigham etal. (1996) (eq. 1). 

The size and morphology of goethite crystals grown at pH 12 is related to the ageing history of the parent iron solutions (Atkinson et al., 1968). As the extent of hydro-... 

Goethite crystals produced by oxidation of Fe solutions at ambient temperature in neutral solution (Fig. 4.7 right) - a process likely to occur in nature - are usually much less developed and the crystals are smaller (MCLb 10 nm) than those obtained in alkaline Fe " solutions. If Al is taken up in the structure, these crystals become extremely small (MCL 5 nm) and show almost no particular habit. At higher pH (-12) the crystals are again acicular (MCL -30 nm) despite containing structural Al (Al/(Al-i-Fe) -0.3) they show internal disorder, however, and stars are frequent. This morphology is also observed for soil goethites (see Chap. 16). 

The commonest habits for hematite crystals are rhombohedral, platy and rounded (Fig. 4.19). The plates vary in thickness and can be round, hexagonal or of irregular shape. Under hydrothermal conditions, these three morphologies predominate successively as the temperature decreases (Rosier, 1983). The principal forms are given in Table 4.1. Hematite twins on the 001 and the 102 planes. The crystal structure of hematite has a less directional effect on crystal habit than does that of goethite and for this reason, the habit of hematite is readily modified. A variety of morphologies has been synthesized, but in most cases, the crystal faces that enclose the crystals have not been identified. 

Aluminium in the ferrihydrite system not only suppresses goethite in favour of hematite (see chap. 14) but also affects the morphology of hematite, probably by entering the structure. At temperatures of between 70 and 150 °C, a shift was noticed from rhombohedra to plates whose diameter and thickness were at a maximum at an Al/(Fe-i-Al) ratio of 0.05 (Schwertmann et al., 1979 Barron et al., 1984 Barron Torrent, 1984 Wolska Szajda, 1987). At higher levels of substitution, the plates became extremely thin and structural strain increased (Stanjek Schwertmann, 1992)... 

Fig. 12.17 Dissolution morphology of synthetic 210 faces, c) Monodomainic Al-goethite (Al/ goethite crystals after partial dissolution in 6 M (Fe-tAI) = 0.097 mol mol" ) with cavernous dis-HCI at 25 °C. a) Pure goethite with dissolution solution at crystal edges (Schwertmann, 1984 a, along domain boundaries, b) Pure goethite with with permission), diamond-shaped dissolution holes bounded by...

Hematite obtained at low temperatures retains the acicular morphology of the goethite precursor crystals, but at temperatures >600 °C, a sintering process leads to irregular particles of hematite. 

Barron,V Rendon, J.L. Torrent, J. Serna, C.J, (1984) Relation of infrared, crystallochemical, and morphological properties of Al-substi-tuted hematites. Clays Clay Min. 32 475-479 Barrow, J.J. Cox,V.C. (1992) The effects of pH and chloride concentration on mercury sorption. I. Goethite. J. Soil Sci. 43 437-450 Barrow, N. Madrid, L. Posner, A.M. (1981) A partial model for the rate of adsorption and desorption of phosphate by goethite. J. Soil Sci. 32 399-407... 

Mann, S. Cornell, R.M. Schwertmann, U. (1985) The influence of aluminium on iron oxides XII. High-resolution transmission electron microscopic (HRTEM) study of aluminous goethites. Clay Min. 20 255-262 Mann, S. Perry, C.C. Webb, J. Luke, B. Wil-liams, R.J.P. (1986) Structure, morphology, composition and organization of biogenic minerals in limpet teeth. Proc. R. Soc. Lond. 

Torrent, J. Schwertmann, U. (1987) Influence of hematite on the color of red beds. J. Sediment. Petrology 57 682-686 Torrent, J. (1987) Rapid and slow phosphate sorption by Mediterranean soils effect of iron oxides. Soil Sci. Soc. Am. J. 51 78-82 Torrent, J. (1991) Activation energy of the slow reaction between phosphate and goethites of different morphology. Aust. J. Soil Res. 29 69-74... 

Phosphate adsorption and desorption by goethites differing in crystal morphology. Soil Sd. Soc. Am. J. 54 1007-1012 Torrent, J. Guzman, R. Parra, M.A. (1982) Influence of relative humidity on the crystallization of Fe(III) oxides from ferrihydrite. Clays Clay Min. 30 337-340 Torrent, J. Schwertmann, U. Barron,V. 

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