Goethite domains

Fig. 4.8 High resolution electron micrograph of two goethite domains and their interdomai-nic zone. The lattice fringes at 0.5 nm correspond to the (200) spacing (courtesy S. Mann, Bristol).

Stumm and Sigg (1979) predicted the coagulation stability domains of goethite based on surface chemistry and surface potential estimates (Fig. 7.9). 

Fig. 4.6 High resolution electron micrograph of natural goethite a) Diamond-shaped cross sections of domains running along [010] and bounded by 101 faces. Lattice fringes correspond to the c -parameter. b) Higher magnification shows the a fringes (ca. 1 nm) and structural distortions. (Smith Eggleton, 1983 with permission courtesy R.A. Eggleton).

Conditions which promote multi-domainic goethites are high ionic strength (either [KOH] or salt) and also low synthesis temperature (<40°C). In alkaline solutions, multi-domainic character decreases and domain width increases as Al substitution increases to Al/(Fe-i-Al) of 0.15, whereas at Al/( Al-nFe) >0.15 single domain crystals result (Schulze Schwertmaim, 1984 Mann et al., 1985). Multidomainic goethites can recrystallize to single domain crystals as a result of hydrothermal treatment at 125-180 °C (Fig. 4.9) (Schwertmann et al., 1985). 

Pores may be present as structural features (e. g. between domains) or as a result of aggregation of particles. They may also be the result of partial dehydroxylation (oxide hydroxides) or dissolution. Although the shapes of pores can be quite variable, there are some definite, basic forms. The commonest of these are 1) slit shaped, the walls of which may or may not be parallel 2) ink bottle which are closed upon all sides but one from which a narrow neck opens and 3) cylindrical. Upon partial dissolution, pores bounded by well-defined crystal planes (e. g. 102 in goethite) develop (Chap. 12). 

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...

A high resolution electron microscopy examination of domain boundaries in synthetic goethite. J. Chem. Soc. Faraday Trans. 

Yapp. C.J. (2000) Climatic implications of surface domains in arrays of 5D and 5180 from hydroxyl minerals Goethite as an example. Geochim. Cosmochim. Acta 64 2009-2025 Yariv, S. Mendelovid, E. Villalba, R. (1980) Thermal transformation of goethite into hematite in alkali halide discs. J. Chem. Soc. Faraday Trans. I. 76 1442-1454 Yariv, S. Mendelovid, E. Villalba, R. Cohen, M. (1979) Transformation of goethite to maghemite in Csl discs. Nature 279 519-520... 

This method produces ca. 9 g goethite with a surface area of ca. 20 m / g. The crystals are acicular and consist of several domains along the needle (a-)axis. An electron micrograph is shown in Fig. 5-2 a, an X-ray diffractogram in Fig. 5-6 and an IR spectrum in Fig. 5-7. The crystals are bounded mainly by 101 faces (Fig. 5-3). 

Cornell, R. M. and Giovanoli, R. (1986) Factors that govern the formation of multi-domainic goethites. Clays Clay Min. 34 557-564. 

When samples of goethite of differing crystallinities are compared, the titration curves are found to reflect these diflFerences by increased spread of the curves at pH values distant from the pzc [55]. These are described in this model by increases in the capacitance terms [55]. The observed behaviour probably reflects increased penetration of protons into pores between domains. In this case, we might think of the changes as reflecting changes in surface roughness and thus in the mean distance between planes. That this should be reflected in a capacitance term in not entirely inappropriate. 

Most of the published studies on the reaction of ions with iron oxides have used samples of oxides which have been exposed to various extents to carbon dioxide. Models fitted to data from such studies do not reflect fundamental characteristics of the surface but are affected by the degree of exposure to carbon dioxide. In the case of goethite, most of the samples have consisted of multi-domainal crystals. It is only when such crystals have been healed by hydrothermal treatment that the true adsorption reactions of the surface can... 

Figure 13. Plot of log Pj, vs. pH domain of colloid stability of goethite...

Two-step adsorption kinetics of metd cations at iron oxides has been observed for several metals (26-28) and mainly attributed to interaction of the metal cations with different sites of the metal oxide surface (29). Diffusion of cations into micro- or macropores can be ruled out for synthetic goethites (30) due to an essential lack of porosity as determined by t-plot analysis (31). However, diffusion processes may be important for other oxides, especially amorphous iron oxides or oxides with certain coatings and oxide surfaces which have been subject to substantial dissolution processes. Thus, other processes than diffusion must have caused the two different kinetic domains of Fe(II) sorption to iron oxide surfaces. 

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