Feroxyhyte

Feroxyhyte is a poorly ordered form of 5-FeOOH (and therefore called 5 -FeOOFI) discovered and introduced as a mineral by Chukhrov et al. in 1977. It is structurally related to the ferrimagnetic 5-FeOOH described by Glemser and Gwinner in 1939. 

This method yields about 2.5 g of feroxyhyte with broadened X-ray lines (Fig. 7-1) whose widths follow the order 100, 110 101 102 indicating 

An initial pH of 7-8 before addition of H2O2 is probably the optimum value for the production of reasonably well crystalline material. Lowering the pH yields less crystalline feroxyhyte. 

Very rapid oxidation is essential for formation of feroxyhyte. As the oxidation rate is lowered, lepidocrocite and/or magnetite may form. 

Koch et al. (1985) produced feroxyhyte in the same way as described above, except that the 0.1 M FeCb solution was produced by dissolving Ferrum reductum (a mixture of iron metal and Fe304) in HCl. They obtained platy crystals 15-90 nm across and 2-3 nm thick with a surface area of 110 m /g and a magnetic hyperfine field at 5 K of 52.5 T. The magnetic properties of this material are described in Bender-Koch et al. (1995). 

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Mineral name Goethite Lepidocrocite Akaganeite Schwertmannite Feroxyhyte... 

FeOOH (synthetic) and its poorly crystalline mineral form, feroxyhyte (5 -FeOOH), are reddish-brown, ferrimagnetic compounds. Their structures are based on hep anion arrays and differ in the ordering of the cations. Feroxyhyte was first described by Chukhrov et al. in 1976 and occurs (rarely) in various surface environments. 

FeOOH is isostructural with Fe(OH)2. It has a hexagonal unit cell with a = 0.294 nm and c = 0.449 nm. A related compound exists in nature under the mineral name feroxyhyte (named by F.V. Chukhrov see Chukhrov et al., 1976), also termed 5 -FeOOH. Feroxyhyte has a hexagonal unit cell with a = 0.294nm and c = 0.456 nm. It contains two formula units per unit cell, whereas the unit cell of the synthetic material contains only one formula unit. 

Fig. 2.7 Structure of feroxyhyte. a) Hexagonal close packed anion layer with cations distributed over the interstices, b) Layers of edge-sharing octahedra. (a, b Stanjek, unpubl.)...

The natural and synthetic compounds differ in the arrangement of cations in both compounds the anion packing is disordered. Feroxyhyte is a disordered form of the synthetic material with the cations being distributed almost entirely randomly over the interstices. 

Fig. 4.27 Upper Platy crystals of synthetic feroxyhyte formed by rapid oxidation of FeCl2 solution at pH 8 (Carlson Schwert-mann, 1980 with permission). Lower Vermiform natural feroxyhyte aggregates from the Clara Mine, Black Forest (Courtesy K.Walenta).

FeOOH precipitates as platy crystals. When formed by fast oxidation of Fe(OH)2 at pH 12 well formed hexagonal plates result, whereas simultaneous precipitation/oxi-dation gives thin plates which are often rolled up (Feitknecht, 1959). Feroxyhyte (5 -FeOOH) produced by rapid oxidation (e.g. with H2O2) of FeQ2 solution at pH 8 also forms thin platy crystals around 100 nm in size (Fig. 4.27, upper). As the pH is lowered, the crystals become smaller and develop a grassy appearance (Carlson Schwertmann, 1980). Natural feroxyhyte from the Clara Mine in the Black Forest occurred as vermiform aggregates (Fig. 4.27, lower) (Walenta, 1997). 

Synthetic 5-FeOOH has a surface area which ranges from 20-300 m g depending on the thickness of the crystals. In a series of seven synthetic feroxyhytes the surface area increased from 140 to 240 m g (EGME method) as the crystallinity decreased (Garlson and Schwertmann, 1980). 5-EeOOH displays interpartide porosity, i.e. slitshaped micro- or mesopores between the plate like crystals (Jimenez-Mateos et al., 1988 Ishikawa et al., 1992). Both TEM observations and t-plot analysis showed that 0.8 nm micropores formed upon dehydroxylation at 150 °G in vacuo. The surface area rose steeply as the temperature exceeded 100 °G and reached a value close to 150 m g at 200 °C at which temperature, the sample was completely converted to hematite. 

Owing to its small particle size and poor crystallinity, the mineral feroxyhyte (5 -FeOOH) is superparamagnetic at room temperature. Superparamagnetic relaxation is also observed for small particles of the synthetic compound, which had a Bhf at 4.2 K of 51.3 T (Carlson Schwertmann, 1980). 

To the naked eye, goethite and akaganeite appear yellow-brown, lepidocrocite orange and hematite usually red (Plate 6.1). Feroxyhyte and ferrihydrite are dark reddish brown, maghemite brown to brownish red and magnetite and wiistite are black. 

The IR spectrum for d-FeOOH shows an OH stretch at 3130 cm", OH bending bands at 1124, 890 and 810 cm" and Fe-O stretch bands at 580 and 480 cm" (Oka-moto, 1968). In its poorly crystalline form as feroxyhyte, 5 -FeOOH, this material shows similar, but less well expressed features. High pressure FeOOH shows a broad band at 2800 cm due to the bulk OH stretch, a doublet at 1150 and bands at 970 and 685 cm corresponding to OH bending vibrations and an Fe-O stretch, respectively (Pemet et ak, 1973). 

Based on these results, and in spite of considerable variation in the band positions of samples of the same Fe oxide, at least 80 % of the pure akaganeite, feroxyhyte, ferrihydrite, hematite and lepidocrocite samples could be correctly classified by Scheinost et al. (1998) by discriminant functions based on the above four bands (Fig.7.3 right) Magnetite could be identified by its band at 1500 nm, but for goethite. 

Goethite 29-713 Lepidocrocite 44-1415 Akaganeite 13-157 Schwertmannite 6-FeOOH > Feroxyhyte Ferrihydrite 29-712 HP FeOOH > ... 

With lepidocrocite the dehydroxylation endotherm due to transformation to maghemite is followed by an exotherm indicating transformation of maghemite to hematite. The temperature of the dehydroxylation endotherm was found to increase from 270 to 300 °C as A1 substitution rose from Al/(Fe-tAl) of 0 to 0.12 (Schwertmann Wolska, 1990) and that of the exotherm rose from 500 to 650 °C (Wolska et al., 1992). Synthetic feroxyhyte shows a weak dehydroxylation endotherm at ca. 260 °C (Carlson Schwertmann, 1980). 

Feroxyhyte Goethite Dissolution/reprecipitation Alkaline solution... 

The thermal transformation of feroxyhyte (5 -FeOOH) was studied by Carlson and Schwertmann (1980). Synthetic feroxyhyte transformed to hematite with non-uni-formly broadened XRD lines at 240 °C (DTA). As the temperature increased further, an exothermic peak appeared and the crystallinity of the hematite improved. In an atmosphere of N2 the transformation of natural feroxyhyte was impeded. As the temperature rose, the crystallinity of this feroxyhyte improved and at 460 °C, the a unit cell edge length dropped from 0.5062 to 0.5027 nm. As this sample contained organic impurities, the final transformation product in this case, even at 800 °C, was maghemite (see p. 368). 

As lepidocrocite is metastable relative to goethite, it can be expected that lepidocrocite may transform into goethite. As demonstrated in the laboratory, this transformation proceeds via solution (see Chap. 14). Electron micrographs from a redoxi-morphic soil in Australia indicate that the same process seems to occur in soils (Fig. 16.5). The lepidocrocite crystals show dissolution features and there are small, acicu-lar, goethite crystals in their neighbourhood. Feroxyhyte was reported in two allopha-... 

Magnetic properties of feroxyhyte (8-FeOOH) Phys. Chem. Min. 22 333-341 Bender-Koch, C. Morup, S Madsen, M.B. ... 

Chukhrov, F.V. Zvyagin, B.B. Gorshkov, A.I. Ermilova, L.P. Korovushkin,V.V. Rudnits-kaya, E.S. Yakubovskaya (1976) Feroxyhyte, a new modification of FeOOH. Izvest. Akad. Nauk SSSR, Ser. Geol. 5 5-24... 

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