Hematite metal substitution

Fig. 3.2 Fraction of various metals released versus Fe released during acid dissolution of synthetic metal-substituted magnetites (upper six plots Sidhu et al., 1978, with permission), goethites and hematites (lower plots Lim-Nunez dikes, 1987 with permission).

Wells, M.A. Gilkes, R.J. Anand, R.R. (1989) The formation of corundum and aluminous hematite by the thermal dehydroxylation of aluminous goethite. Clay Min. 24 513-530 Wells, M.A. Gilkes, R.J. Fitzpatrick, R.W. (2001) Properties and acid dissolution of metal-substituted hematites. Clays Clay Min. 49 60-72... 

Coordinative Environment. The coordinative environment of transition metal ions affects the thermodynamic driving force and reaction rate of ligand substitution and electron transfer reactions. FeIIIoH2+(aq) and hematite (a-Fe203) surface structures are shown in Figure 3 for the sake of comparison. Within the lattice of oxide/hydroxide minerals, the inner coordination spheres of metal centers are fully occupied by a regular array of O3- and/or 0H donor groups. At the mineral surface, however, one or more coordinative positions of each metal center are vacant (15). When oxide surfaces are introduced into aqueous solution, H2O and 0H molecules... 

Isomorphous substitution of iron oxides is important for several reasons. In the electronics industry, trace amounts (dopants) of elements such as Nb and Ge are incorporated in hematite to improve its semiconductor properties. Dopants are also added to assist the reduction of iron ores. In nature, iron oxides can act as sinks for potentially toxic M", M and M heavy metals. Investigation of the phenomenon of isomorphous substitution has also helped to establish a better understanding of the geochemical and environmental pathways followed by Al and various trace elements. Empirical relationships (e. g. Fe and V) are often found between the Fe oxide content of a weathered soil profile and the levels of various trace elements. Such relationships may indicate similarities in the geochemical behaviour of the elements and, particularly for Al/Fe, reflect the environment in which the oxides have formed (see chap. 16). 

Extensive replacement of Fe by transition metal cations and alkaline earth ions has been reported for b-FeOOH (Okamoto, 1968). Muller et al. (1979) found incorporation of up to 0.4 mol moF Ca solid solutions with the formula Fei xKxOi x(OH)i+x could be identified. Jimenez-Mateos et al. (1990) reported that Co and Mn, respectively, could replace up to 0.3 and 0.5 mol mol Fe. The unit cell parameters decreased in both cases with increasing substitution. These Mn- and Co-substituted 5-FeOOHs decomposed at 200 °C to give poorly crystalline, substituted hematites. 

For iron oxides, IR spectroscopy is useful as a means of identification. Hematite crystals in films that were too thin (<70nm) to be characterized by XRD were shown by IR to be oriented with the c-axis perpendicular to the surface of the film (Yubero et al. 2000). This technique also provides information about crystal morphology, degree of crystallinity and the extent of metal (especially Al) substitution because these properties can induce shifts in some of the IR absorption bands. It is also widely used both to obtain information about the vibrational state of adsorbed molecules (particularly anions) and hence the nature of surface complexes (see Chap. 11) and to investigate the nature of surface hydroxyl groups and adsorbed water (see Chap. 10). Typical IR spectra of the various iron oxides are depicted in Figure 7.1. Impurities arising either from the method of preparation or from adsorption of atmospheric compounds can produce distinct bands in the spectra of these oxides -namely at 1700 cm (oxalate), 1400 cm (nitrate) and 1300 and 1500 cm (carbonate). 

The hematite platelets show a predominantly metallic effect. Very thin particles with a thickness of 50 to 400 nm display a pale copper gloss, which is indicative of interference. The shade can be varied and the properties of the platelets can be controlled by doping. A1 or Mn are incorporated by substitution of Fe in the hematite lattice, and Si is incorporated interstitially [5.240]. Laminar iron oxide pigments are interesting because of their excellent fastness to light, outdoor exposure and their good mechanical stability. Main applications up to now are automotive lacquers and cosmetics. 

Secondary minerals either precipitate directly in the place of formation (and therewith are removed from flowing water) or continue migrating with water in suspended state. As a result, ground water loses first of all most weak acids and alkali (calcite, anhydrite, gypsum, etc.), and also heavy metals (gibbsite, goethite, hematite, etc.). The most common result of secondary minerogenesis is the substitution of one mineral for the other. 

Finally, as discussed in the next section, much effort has been placed on doping hematite with various elements to increase charge carrier conduction. These elements are typically present at 1 atom% or less, and while substitutional transition metal impurities have been predicted to introduce interband gap energy states [25], the same dopants have not been reported to significantly change the bandgap absorption energy or the absorptirMi coefficient [28—31],... 

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