Other iron oxides

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. 

Aicagan ite. can incorporate up to 0.06 mol mol Cu and much smaller amounts of Cr, Mn, Co, Ni and Zn in the structure (Inouye et al., 1974 Holm, 1985 Buch-wald Clarke, 1989). Incorporation of Al, Cr and Ga has also been reported (Lorenz Kempe, 1987). Cornell (1992) produced akaganeite from acid Si-containing Fe solutions and found by congruent dissolution that up to 0.04 mol mol Si could be incorporated. The Si species were probably located in the tunnels (0.5 nm ) of the akaganeite structure. 

Numerous coprecipitates of ferrikydrite with different cations (and anions) have been synthesized and exist in nature, but so far, no definite proof of structural incorporation has been produced, probably because of the very low particle size and crystallinity of the (2-line) ferrihydrite which makes the distinction between a position at 

A small amount of Cr could be incorporated in wiistite at 1350 °C (Bogdandy Engell, 1971) and MgO and MnO were completely miscible with FeO the mixed phases are important in the reduction of iron ores. Wiistite can be doped with small amounts of Mn, Mg, Ca and 10 g kg Si or Al to promote reduction (Moukassi et al., 1984). In green rust Fe has been replaced by Ni (Refait Genin, 1997) and by Mg (Refait et al. 2001). 

Detailed information about crystal growth is available in the books by Sunagawa (1987) and Mullin (1993). A very brief summary of the topic is provided here. Crystal growth from solution involves a number of steps. They are  

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Other iron oxidizers include Sphaerotilus, Crenothrix, and Lep-tothrix species. Each bacterium is filamentous. 

In contrast, the reddish-brown jerrihydrite (often wrongly termed amorphous iron oxide or hydrous ferric oxide (HFO) ) is widespread in surface environments. It was first described by Chukhrov et al. in 1973. Unlike the other iron oxides it exists exclusively as nano-crystals and unless stabilized in some way, transforms with time into the more stable iron oxides. Ferrihydrite is, thus, an important precursor of more stable and better crystalline Fe oxides. Structurally ferrihydrite consists of hep anions and is a mixture of defect-free, and defective structural units.The composition, especially with respect to OH and H2O, seems to be variable. A preliminary formula, often used, is FesOgH H2O. 

Wustite, FeO, is the other iron oxide which contains only divalent Fe. It is usually non-stoichiometric (O-defident). The structure is similar to that of NaCl and is based on ccp anion packing. Wustite is black. It is an important intermediate in the reduction of iron ores. 

Structural relationships exist between certain planes in the hematite structure and those in other iron oxides, namely magnetite and goethite (Tab. 2.6). There is, for example, a relationship between the (111) plane of magnetite and (001) plane of hema-... 

Magnetite differs from most other iron oxides in that it contains both divalent and trivalent iron. Its formula is written as Y[XY]04 where X = Fe , Y = Fe " and the brackets denote octahedral sites (M sites). Eight tetrahedral sites (T sites) are distributed between Fe" and Fe", i.e. the trivalent ions occupy both tetrahedral and octahedral sites. The structure consists of octahedral and mixed tetrahedral/octahedral layers stacked along [111] (Fig. 2.13a). Figure 2.13b shows the sequence of Fe- and O-layers and a section of this structure with three octahedra and two tetrahedra is depicted in Figure 2.13 c. 

Lepidocrocite is paramagnetic at room temperature. The Neel temperature of 77 K is much lower than that of the other iron oxides and is the result of the layer-like structure of this mineral. The sheets of Fe(0,0H)6 octahedra are linked by weak hydrogen bonds, hence magnetic interactions are relatively weak. The saturation hyperfine field is also lower than for any other iron oxide (Tab. 6.2). In the antiferromagnetic state, the spins are ordered parallel to the c-axis with spins in alternate layers having opposite signs. A decrease of T by 5 K was observed for Al-lepidocrocites with an Al/(Fe-i-Al) ratio of 0.1 (De Grave et al., 1995). 

The high pressure form of FeOOH is more compact than any other iron oxide hydroxide, hence it has a higher than usual Neel temperature of 470 K. At room temperature, high pressure FeOOH is antiferromagnetic with a collinear spin arrangement parallel to the c-axis (Fernet et al., 1973). High-pressure FeOOH is completely miscible with CrOOH. Substitution with Cr reduces T to the extent that with 80%... 

The magnetite is considered to form from a ferrihydrite precursor by interaction of this phase with dissolved Fe" ions (Kirschvink Lowenstam, 1979 Lowenstam, 1981 Nesson Lowenstam, 1985). The same mechanism operates for inorganic synthesis at around pH 7 (see chap. 13). Most probably the other iron oxides in the teeth form by a similar mechanism, but under conditions of slightly lower pH and/ or higher redox potential. The separation of these minerals in time and space suggests local variations in growth conditions. 

H.V.Jr. Adams, J.B. (1993) Pigmenting agents in Martian soils Inferences from spectral, Mossbauer, and magnetic properties of nanophase and other iron oxides in Hawaiian palagonitic soil PN-9. Geochim. Cosmo-chim. Acta 57 4597-4609 Morris, R.V Golden, D.C. ShelfepTD. ... 

Since the discovery in 1951 that magnetoplumbite, BaFei20i9 is a hard, semiconducting ferrigmagnet of considerable technical interest, several other iron oxides with closely related hexagonal structures have been synthesized and studied. Of particular interest for this review are those containing mixed valency. 

Better than other iron oxides because it gives higher burn rate, lower pressure exponent, greater effective impulse (hence greater range, velocity, payload) and improved mission versatility. 

Rusticyanin is found in Thiobacillus ferrooxidans, an acidophilic, chemolithotrophic sulfur bacterium utilizing Fe + and reduced sulfur compounds as its sole energy source. T. ferrooxidans does not produce rusticyanin when grown on reduced sulfur. Similar to other substrate-inducible cupredoxms, the msticyanin gene is activated when soluble iron is present in the media. Little is known about its redox partners and it should be noted that rusticyanin itself does not carry out Fe + oxidation. Other iron-oxidizing bacteria, for example, Leptospirillum ferrooxidans, prodnce a cytochrome which substitutes rusticyanin functionally. To date T. ferrooxidans remains the only source for rusticyanin. 

HFO and other iron oxides may also play a significant role in the oxidation of As(Ill) in natural waters since the oxidation of As(lll) adsorbed by HFO is catalyzed by H2O2 (Voegelin and Hug, 2003). This reaction may be significant in natural environments with high H2O2 concentrations (1-10 p,M) and alkaline pH values, and in water treatment systems where H2O2 is used. 

Recent studies at the Iron Mountain acid mine-drainage site in California suggest that the main role of A. ferrooxidans is to oxidize iron downstream from the principal acid-generating site, and that the primary effect is to enhance the precipitation of iron oxyhydroxides (Banfield and Welch, 2000). Other iron-oxidizing bacteria (e.g., L. ferrooxidans) and archeal (e.g., Thermoplasmales) species have been observed proximal to the sulfide ore (Edwards et al., 2000). The utilization of energy derived from iron or sulfur oxidation in other prokaryotes remains unclear, but it can be surmised that the mechanism involved could be broadly similar to that determined for A. ferrooxidans (Banfield and Welch, 2000). 

Not all iron oxides are available for reduction. Some iron minerals are solid crystals or even entire iron grains, which makes them resistant to microbial reduction (Lovley, 1991 Postma, 1993 Heron et al., 1994b). Other iron oxides or hydroxides are amorphous and readily reducible. Over time, even some crystalline minerals such as goethite and hematite may be reduced in the complex environment in leachate (Heron and Christensen, 1995). This indicates that the importance of iron as a redox buffer controlling the size of plumes is not given just by the amount of iron oxides present. The composition and microbial availability of iron for reduction are key parameters. Methods for the actual quantification of the microbial iron reduction capacity have, however, not been developed. 

Fe(OH)2 is prepared from Fe° solutions by precipitation with alkali. When freshly precipitated under an inert atmosphere (in a Schlenck apparatus for example) Fe(OH)2 is white (Bernal et al., 1959). It is, however, readily oxidized by air or even water upon which it darkens. Fe(OH)2 has the CdL type structure with hep anions and half of the octahedral interstices being filled with Fe ions. The crystals form hexagonal platelets. In solution Fe(OH)2 transforms by a combination of oxida-tion/de-hydration/hydrolysis reactions to other iron oxides and hydroxides. The end product depends both upon the order in which these processes occur and upon their rates. 

P-Fe20s has been obtained by dehydration of P-FeOOH in high vacumn at 170 °C (Braun and Gallagher, 1972). s-FeaOs ean be produced by the reae-tion of alkaline potassium ferrieyanide solution with sodium hypochlorite. It is also obtained (together with a mixture of other iron oxides) in an electric arc under an oxidizing atmosphere (Buttner, 1961). Its magnetic and thermal properties have been investigated by Dezsi and Coey (1973). 

Murad, E. and Schwertmann, U. (1980) The MOssbauer spectrum of ferrihydrite and its relations to those of other iron oxides. Amer. Miner. 65 1044-1049. 

Good heat stability, inert, light fast, good weatherability, more expensive than other iron oxides... 

The Haber-Bosch process has been known and used for over a century, and considerably little has changed over such a long period of time. In early work, Mittasch developed a highly active heterogeneous iron catalyst prepared from magnetite (Fe304) very similar catalyst formulations are still used in modern ammonia synthesis. It was also demonstrated that catalysts prepared from magnetite had superior catalytic activity in comparison to catalysts prepared from other iron oxides. 

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