Goethite crystal morphology

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). 

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...

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. 

The main chapters (4-14) are concerned with the preparative methods. For the majority of oxides particularly hematite and goethite more than one preparative method is described. Properties such as crystal morphology and surface area frequently depend on preparative conditions and a selection of methods is presented to enable a range of oxides with specific characteristics to be produced. The production of so-called monodis-perse Fe oxides, i. e. products with a rather narrow particle size distribution, is also included. 

Fig. 5-4, Crystal morphology of goethites. Cornell and Schwertmann, 1996 with permission.

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. 

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. 

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. 

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