Feroxyhyte crystallinity

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. 

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). 

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). 

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). 

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. 

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. 

Dominant constituent of the Fe oxide precipitate. PCL = poorly crystalline lepidocrocite L = lepidrocrocite A = Akaganeite F = feroxyhyte M = magnetite NC = non-crystaUine. The differential infrared data (not shown) are in accord with x-ray data. 

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