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Title: Ferrihydrite  
Author: World Heritage Encyclopedia
Language: English
Subject: Hydrous ferric oxides, Iron oxide pigments, Superferromagnetism, FH, Fougèrite
Publisher: World Heritage Encyclopedia


Mine drainage from Ohio. The orange coating on the logs is ferrihydrite.
Category Oxide minerals
(repeating unit)
Strunz classification 04.FE.35
Dana classification
Crystal symmetry Trigonal hexagonal scalenohedral
H-M symbol: (32/m)
Space group: P31c
Unit cell a = 2.96 Å, c = 9.4 Å; Z = 1
Formula mass 168.70 g
Color Dark brown, yellow-brown
Crystal habit Aggregates, microscopic crystals
Crystal system Trigonal
Streak Yellow-brown
Diaphaneity Opaque
Density 3.8 g/cm3
References [1][2][3]
X-ray diffraction patterns for six-line and two-line ferrihydrite.
X-ray diffraction patterns for six-line (top) and two-line (bottom) ferrihydrite. Cu Kα radiation.

Ferrihydrite (Fh) is a widespread hydrous ferric oxyhydroxide

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Better crystallized and less hydrated iron oxy-hydroxides are amongst others:

See also

Under reducing conditions as those found in gley soils, or in deep environments depleted in oxygen, and often with the assistance of microbial activity, ferrihydrite can be transformed in green rust, a layered double hydroxide (LDH), also known as the mineral fougerite. However, a short exposure of green rust to atmospheric oxygen is sufficient to oxidize it back to ferrihydrite, making it a very elusive compound.

Ferrihydrite is a metastable mineral. It is known to be a precursor of more crystalline minerals like hematite and goethite[31][32][33][34] by aggregation-based crystal growth.[35][36] However, its transformation in natural systems generally is blocked by chemical impurities adsorbed at its surface, for example silica as most of natural ferrihydrites are siliceous.[37]


Because of the small size of individual humic and fulvic acids).[20][21][22][23] Its strong and extensive interaction with trace metals and metalloids is used in industry, at large-scale in water purification plants, as in North Germany and to produce the city water at Hiroshima, and at small scale to clean wastewaters and groundwaters, for example to remove arsenic from industrial effluents and drinking water.[24][25][26][27][28][29] Its nanoporosity and high affinity for gold can be used to elaborate Fh-supported nanosized Au particles for the catalytic oxidation of CO at temperatures below 0 °C.[30]

Porosity and environmental absorbent potential

Due to the nanoparticulate nature of ferrihydrite, the structure has remained elusive for many years and is still a matter of controversy.[14][15] Drits et al., using X-ray diffraction data,[10] proposed a multiphase material with three components: defect-free crystallites (f-phase) with double-hexagonal stacking of oxygen and hydroxyl layers (ABAC sequence) and disordered octahedral Fe occupancies, defective crystallites (d-phase) with a short-range feroxyhite-like (δ-FeOOH) structure, and subordinate ultradisperse hematite (α-Fe2O3). Recently, a new single phase model for both ferrihydrite and hydromaghemite[16] has been proposed by Michel et al.,[17][18] based on pair distribution function (PDF) analysis of x-ray total scattering data. The structural model, isostructural with the mineral akdalaite (Al10O14(OH)2), contains 20% tetrahedrally and 80% octahedrally coordinated iron.

Ferrihydrite only exists as a fine grained and highly defective nanomaterial. The powder X-ray diffraction pattern of Fh contains two scattering bands in its most disordered state, and a maximum of six strong lines in its most crystalline state. The principal difference between these two diffraction end-members, commonly named two-line and six-line ferrihydrites, is the size of the constitutive crystallites.[10][11] The six-line form has been classified as a mineral by the IMA in 1973[12][13] with the nominal chemical formula 5Fe2O3•9H2O. However, its formula is fundamentally indeterminate as its water content is variable. The two-line form is also called hydrous ferric oxides (HFO).



  • Structure 1
  • Porosity and environmental absorbent potential 2
  • Metastability 3
  • See also 4
  • References 5


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