Goethite acicular

Fig. 4.3 Common acicular, polydo-mainic goethite c stals terminated by (210) faces (see Schwertmann etal. 1985).

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). 

Epitaxial twins (Fig. 4.11) consist of a hematite centre with outgrowths of acicular goethite. As the structures of both goethite and hematite are based on an hep anion array, some of the interplanar spacings in the two compounds are similar and this fa-... 

Fig. 4.20 Synthetic hematites grown from ferrihydrite at temperatures <100°C (Schwertmann, unpubl.) a) Hexagonal plates grown at pH 7 and RT acicular crystals are goethite b) Laths grown at pH 11 and 80 °C in the presence of 2.5 10 M citrate (see Schwertmann et al., 1968). The fine granular material is unreacted ferrihydrite c) Framboids grown at pH 6 and 70°C in the presence of 2 10 M oxalate (see Fischer, ...

In this case, d (nm) is the diameter of spherical particles. Both sets of equations are only approximations, particularly for goethite, the particles of which are often acicular. However, they do enable an estimate of the rise in solubility, as particle size drops, to be obtained (see Fig. 8.3). There is little difference between the results calculated using the two sets of equations for particles >100 nm, but for 10 nm particles there is more than an order of magnitude difference between the two equations. The higher solubility of smaller particles may lead to their transformation to larger ones via solution, a process called Ostwald ripening. 

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. 

The crystals of soil Fe oxides are usually less well developed than those of synthetic ones. Goethite crystals from soils are, like synthetic ones, acicular (Fig. 16.7a, e) and show defects, micropores and serrated edges. Stars composed of spindles (Fig. 16.7 b)... 

Fig. 16.7 Electron micrographs of soil goethites. a) Acicular crystals from an Oxisol on peridotite. New Caledonia (Schwertmann, Latham, 1986 with permission), b) Starlike crystals from a redoximorphic paddy soil, China, c) Irregular crystals from an Ultisol on basalt, South Brazil (see also Schwertmann, Kampf,...

Dehydroxylation of goethite produces the ferrite reds - extremely colour fast and pure hematite. With low temperature calcination the acicular shape of the goethite precursor is retained, whereas high temperatures lead to a sintered product. 

Reaction at 85 °C of Fe " sulphate solution (buffered at pH 6 with sodium acetate) with hydroxylamine salts. The reaction is carried out under N2 and within 2 h, large clumps of acicular goethite, radiating from a central point, are formed (Ardizzone and Formaro, 1985). 

Needle iron stone See Goethite. . An acicular variety. 

Fig. 5-1. Electron micrographs of Al-substituted acicular goethites illustrating the decrease in crystal size with increasing degree of Al-substitution (given as Al/(AI+Fe) moEmoI). The goethites were produced by aging 2-Iine Al-contain-ing-ferrihydrites in 0.35-0.4 M KOH for 14 days at 70 °C (Cornell and Schwert-marm 1996 with permission)..

Fig. 5-3. High resolution electron micrograph of a thin section of acicular goethite crystals cut perpendicular to the crystallographic b-direction (needle axis). The lattice images (ca. 1 nm) seen in one crystal correspond to the a-di-mension of (he unit cell (0.9956 nm). The crystals are bounded by (101) faces. (Schwertmann, 1984 with permission). (Courtesy H. Vali).

Fig. 5-5. Three-dimensional Atomic Force Micrograph of acicular goethites with (001) faces in the plane of the paper (courtesy P. Weidlcr).

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