Ferrihydrite formula

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. 

The unit cell parameters given by Towe and Bradley (1967) for 6-line ferrihydrite are a = 0.508 nm and c = 0.94 nm. There are four formula units per unit cell. A con-... 

An exact formula for ferrihydrite has yet to be established because a precise separation of structural OH and H2O from adsorbed water has not been successful to date. Towe Bradley (1967) originally suggested the bulk formula Fes. H0g 4H20 which is certainly to be preferred over Fe(OH)3. Infrared measurements using D2O exchange have suggested that ferrihydrite contains OH and... 

Ferrihydrite A group of iron (oxy)(hydr)oxide compounds with highly variable chemical compositions. Rather than determining the potentially complex chemical formula of every ferrihydrite sample, researchers often classify the compounds by their number of broad powder X-ray diffraction (XRD) peaks, which include the poorly crystalline two-line and better crystalline six- or seven-line varieties. 

Two poorly ordered compounds both having the akaganeite structme but with sulfate or nitrate instead of chloride in the tunnel have been found recently, The sulfate form oecurs frequently in nature as an oxidation product of pyrite and has, therefore, been reeognized as a mineral with the name schwertmannite (Bigham et al. 1994). Chemically it can be considered a Fe oxyhydroxy sulfate with the ideal formula Feg0s(0Ff)6S04. The corresponding nitrate form can be synthesized by forced hydrolysis of an acidic Fe (N03)3 solution at 80°C and is a precursor of ferrihydrite (Schwertmann et al. 1996). 

Ferrihydrite (Fe50Hg.4H20) is common in young soil deposits but readily transforms to more stable iron hydroxides and oxides such as goethite and hematite (qq.v.). Ferrihydrite is also called amorphous iron oxide or hydrous ferric oxide, due to its disordered crystal structure the precise structure or formula of this mineral is not yet known (Schwertmann and Cornell, 2000). Ferrihydrite may be present in samples of naturally occurring raw ochres and sieimas (qq.v.) but identifications in works of art have not yet been made. 

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