Other Fe oxides

Fe(OH)2 is difficult to investigate because it is so readily oxidized. At room temperature it is paramagnetic and is antiferromagnetic below 33 K (Miyamoto, 1976). Fe(OH)2 has a layer structure. In the antiferromagnetic state all spin moments within a layer are parallel and also parallel to the c-axis spins between adjacent 

Colour (Munsell) Oxide R ractive index Other 

5 Most minerals polarise light that passes through them, i. e. show anisotropic behaviour Birefringence-the maximum difference between the refractive indices of a mineral 

6 Pleochroism-exhibits two different colours upon rotation of the stage of the polarising microscope 

As a pigment, each iron oxide has an optimum particle size which is that with the maximum scattering cross section. This optimum particle size is lower, the higher the refractive index of the mineral. For hematite, the size corresponding to the maximum in scattering/absorption cross section is ca. 1 pm. As the particle size decreases, the relative scattering cross section drops to zero and the relative absorption cross section levels out. As a result, very small particles of hematite are transparent. 

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The very low Neel temperature (77 K) of lepidocrocite ensures that this compound is paramagnetic at RT. The Mossbauer spectrum at RT consists of a doublet with a AEq of 0.53 mm s . Magnetically ordered lepidocrocite shows a sextet with a Bhf of 45-46 T at 4K that is lower than that of any other Fe oxide (Johnson, 1969 Murad Schwertmann, 1984). 

In general, foreign species in the system can have two different effects on the transformation of ferrihydrite to other Fe oxides they can either modify the rate of the transformation, usually by slowing the process, or change the composition (mainly the hematite/goethite ratio) and properties of the end product. Two principal mechanisms of interaction operate ... 

The laboratory derived model of hematite formation in soils via ferrihydrite has received general acceptance. So far, it is the only way to produce hematite at ambient temperatures and in the pH range of soils. Support from soil analysis, however, is meagre. Hematite is usually associated with other Fe oxides, mainly with goethite but not with ferrihydrite. There seems to be only one report of a ferrihydrite-hema-tite association (based on XRD and Mossbauer spectra) viz. in several andisols formed from basalt in the warm and moist climate of Hawaii (Parfitt et al., 1988). In this case, in addition to the low age of the soils, high release of Si may retard the transformation of ferrihydrite to hematite, whereas normally, the rate of transformation of ferrihydrite seems to be higher than that of ferrihydrite formation, so that this mineral does not persist. 

Precipitation, dissolution and reprecipitation of the various Fe oxides in the environment depend predominately on factors such as pH, Eh, temperature and water activity. For this reason, the different Fe oxides may serve as indicators of the type of environment in which they formed. Goethite and hematite are thermodynamically the most stable Fe oxides under aerobic surface conditions and they are, therefore, the most widespread Fe oxides in soils and sediments. Other Fe oxides are, however, also found in the enviroirment because, although they are thermodynamically less stable, their formation is kinetically favored and their transformation to more stable forms proceeds sluggishly. 

Ferrihydrite is generally the initial precipitate that results from rapid hydrolysis of Fe solutions. Its crystallinity, i.e. crystal size and order, is usually lower than that of any of the other Fe oxides described except feroxyhyte and schwertmannite. It is usually named according to the number of its XRD peaks, with 6-8 broad peaks for well crystalline (6-line-) ferrihydrite and only two very broad ones for the most poorly crystalline form (2-line-ferrihydrite). The 2-line ferrihydrite is commonly but incorrectly called hydrous ferric oxide (HFO) or, amorphous iron oxide . In natural environments all forms of ferrihydrite are widespread usually as yoimg Fe oxides and they play an important role as an active sorbent due to their very high surface area. 

Maghemite can be produced by heating either lepidocrocite or synthetic magnetite, by oxidation of a mixed Fe -Fe ° solution at RT or by heating ferrihydrite (or other Fe oxides) in the presence of an organic substance. 

Lepidocrocite is much less common than hematite and goethite, but it is not rare. Iron coatings around rice roots, formed of goethite and lepidocrocite, have been identified. Maghemite, magnetite, schwertmannite, and akaganeite are other Fe-oxides present in soil environments, which form under specific conditions (Cornell and Schwertmann, 1996). 

Fe(H20)6] (and, indeed, all other Fe" species in Table A) unstable wrt atmospheric oxidation. In practice the oxidation in acidic solutions is slow and, if the pH is increased, the potential for the Fe "/Fe" couple remains fairly constant until the solution becomes alkaline and hydrous Fe203 (considered here for convenience to be Fe(OH)3) is precipitated. But here the change is dramatic, as explained below. 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

Besides this iron meteorite, there have been four other rocks identified to be probably of meteoritic origin. These centimeter-sized pebbles, named Barberton, Santa Catarina, Santorini and Kasos, show troilite and/or kamacite signatures in the corresponding Mossbauer spectra [359]. The range of Fe oxidation states suggests the presence of a fusion cmst. The four cobbles have a very similar chemical composition determined by the APXS, and therefore they may be fragments of the same impactor that created Victoria Crater [361]. 

Grossman and Millet (1961) found that the free Fe-oxide concentration in noncalcareous soils was unchanged after contact with this buffer for nine weeks. Other researchers have shown that acetic acid at a concentration of 2.5% and pH 2.5 led to a partial attack of Fe and Mn oxides (Nissenbaum, 1972 Mclaren and Crawford, 1973 Tessier et al., 1979). Tessier et al. (1979) also indicated that this buffer solution at pH 5.0 was minimal in the attack of silicate minerals and sulfide. 

Metal sorption on Fe/Al oxides is an inner sphere complexion. The formation of a surface-metal bond releases protons for every metal ion adsorbed. Heavy metal sorbed on Fe oxides can be exchanged only by other metal cations having a similar affinity or by H (McBride, 1989). Metal adsorption on Fe oxides is an initial rapid adsorption reaction, followed by slow diffusion (Barrow et al., 1989). Metal ions (Ni2+, Zn2+ and Cd2+) slowly... 

Cobalt is capable of substitution for Fe2+ and other transition metals in the phyllosilicates due to the similarity of ionic radii. On the other hand, cobalt (Co2+) is specifically adsorbed by Mn and Fe oxides, and concentrations of Co sorbed by Mn oxides are much greater than those by Fe oxides (Backes, et al., 1995). Traina and Donor (1985) suggested that the Mn release during Co2+ ion sorption resulted not only from the oxidation of... 

It may be concluded that the hardness of A1203 is determined by the strength of its chemical bonds. This is probably also the case for other A2B3 oxides, such as those with A = Fe, Cr, Ti, Nb, Y, etc. 

The zinc oxide ores of any economic value are represented by smithsonite and calamine. Willemite, franklinitc and other zinc oxide minerals are quite rare. The gangue minerals are usually represented by calcite ferooxides, dolomite and hemimorphite. The composition of gangue minerals, however, varies considerably and may also contain clay, talk, Fe-hydro-xide and other minerals. 

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