Iron Oxides in Soil Environments
By
Sherwan Kafoor
Iron oxides are soil constitutes of great interest in soil chemistry and relevance to plant nutrition.
They are variable in structure, composition and degree of crystallinity. The forms in which they
exist depend substantially on soil physical conditions and the forms and transformations of soil
organic matter.
Due to the extremely low solubility of Fe oxides over the normal pH range of soils, Fe
3+ produced
during mineral weathering is precipitated as oxide or oxyhydroxide. Only a small part of the
oxidised iron will generally be incorporated into secondary layer silicate clay minerals and/or
complexed by organic matter.
Iron can exist in two valency states F
2+ and Fe3+. The reversible oxidation/reduction of iron plays
an important role in its translocation and transformation in soils.
Even at low concentrations iron oxides within soils have a high pigmenting power and determine
the colour of soils. Thus colour, and distribution of iron oxides within a profile, is helpful in
explaining soil genesis and is used widely in soil classification.
Iron oxides also affect soil structure and fabric, often being responsible for the formation of
aggregates and cementation of mineral particles.
The term Iron “oxides” covers a range of chemical and structural forms. These can be classified
as:
1. True Oxides
Magnetite Fe
3O4
Hematite
α – Fe2O3
Maghemite
γ – Fe2O3
2. Oxyhydroxides
Goethite
α – FeOOH
Lepidocrocite
γ – FeOOH
Akaganite
β – FeOOH
Feroxyhite
δ – FeOOH
3. Ferrihydrite Fe
2O3. 2FeOOH. 2.6H2O
4. Other minerals
Green rust (Fe
2+ Fe3+)3 (OH, O)8
and Fe3+ (O2, A)2
Feroxyhite
γ – Fe2O3
Akaganite
β-FeOOH
Hematite Fe
2O3
Hematite is the second most frequent occurring iron oxides in soils. Hematite colours the soil red
and has a great pigmenting effect. It is absent from soils recently formed under a humid
temperate climate such as northern and mid Europe and the northern part of American continent,
unless inherited in the parent material. In wormer climates hematitic soils are widespread and the
higher temperatures are accepted as an important factor in formation.
In the laboratory and probably in the field hematite forms through a dehydration – recrystallisation
process from Ferrihydrite. Aggregation of the small oxyhydroxide particles followed by nucleation
and crystallisation are essential steps in the process. Generally Hematite can be prepared in the
laboratory by heating either Ferrihydrite Lepidocrocite or Goethite at 600 C°.
The ideal composition of Hematite is Fe
2O3; its structure consists of layers of oxygen ions and
layers of Fe ions perpendicular to the triad axis. The oxygen ions are arranged approximately in
hexagonal closest packing. The Fe – O octahedral in hematite form an infinite, three dimensional
network “corundum structure” in which groups of three oxygen ions are linked to a pair of Fe ions.
Hematite can be recognised in the field by its bright red colour. Using X–ray diffraction analysis,
IR-spectra and Mossbauer spectroscopy, can identify hematite in the laboratory.
Goethite
α-FeOOH
The basic composition of goethite is
α-FeOOH. Goethite is orthorhombic with cell dimensions of
approximately a = 0.46 nm, b = 0.10 nm, and C = 0.30 nm. Goethite and Lepidocrocite are not
structurally hydroxylated. The surface Fe atoms in an aqueous complete the coordination shell of
their nearest neighbours through a reaction with water as follows:
Goethite is the most frequent occurring form of iron oxide in soils. Thermodynamically it has the
greatest stability under most soil conditions. Goethite occurs in almost every soil type in every
climatic region and is responsible for the yellowish colour “near 2Y” of many soils.
The formation of Goethite is governed by a number of factors such as temperature, acidity, rate of
hydrolysis, Iron concentration, nucleation and particle size. In the laboratory goethite can be
formed through nucleation from solution by slow hydrolysis of Fe
3+(OOH)6 at room temperature.
Goethite can be recognised by its strong X-ray diffraction lines and by Differential thermal
analysis DTA curves. Goethite can also be recognised from IR-spectra.
Lepidocrocite
γ-FeOOH
The basic composition of lepidocrocite is
γ–FeOOH. It contains double sheets of octahedral
with Fe ions at their centres, and the sheets themselves are composed of chains of octahedral.
In lepidocrocite the oxygen within the double octahedral layers are in cubic relationship whereas
in goethite the oxygens are in hexagonal close packed layer. Lepidocrocite is orthorhombic with
cell dimensions of approximately: a = 0.388 nm, b = 1.25 nm and c = 0.307 nm.
Lepidocrocite occurs in soils less frequently than goethite and hematite. It occurs typically under
oxidising conditions as a weathering product of Fe-bearing minerals. The occurrence of
lepidocrocite is indicative of hydromorphic conditions. It is commonly found in gleys and
pseudogleys particularly those high in clay, but it has not been reported in calcareous
hydromorphic soils where goethite forms instead.
Lepidocrocite occurs as bright orange mottles or bands. Under the electron microscope the
crystals identified as small lath shaped resemble those formed synthetically by oxidation of Fe(II)
salt solutions at pH 5-7.
The formation of Lepidocrocite is influenced by a number of factors such as the concentration of
Fe(II) in solution, pH, partial pressure of CO
2,
the rate of oxidation and the presence of various
solutes in the soil solution.
An orange colour in the field gives an indication that lepidocrocite may be present. It can be
recognised by X ray diffraction, IR-spectra and differential thermal analysis.
Ferrihydrite
Ferrihydrite was accepted in 1971 as anew mineral by the nomenclature Commission of
International Mineralogical Association. Previously it has been called incorrectly amorphous ferric
hydroxide.
Ferrihydrite appears as a rusty precipitate rich in adsorbed water and often rich in adsorbed
inorganic and organic matter. It was found in podzols and brown podzolic soils and in particular
environments associates with soils such as drainage ditches and slow running water-courses. It
occurs in very small spherical particles with a high surface area (200-350 m
2/g). Ferrihydrite
particles are generally highly aggregated and are 90-100% soluble in ).2M acidified ammonium
Oxalate.
Maghemite
γ–Fe2O3
The ideal composition of Maghemite is
γ – Fe2O3 which can be identified by X-ray diffraction and
its 600-800 °C DTA exothermic peek. Maghemite is c ommon in highly weathered soils of tropical
and sub-tropical climates, and also occurs in soil of temperate regions.
Green Rust
The greenish-blue crystalline compound has been defined from corrosion studies. It has a
structure consisting of alternating (Fe
2+ Fe3+)3 (OH, O)8 and Fe3+ (O2, A)2 layers
(A = monovalent
anion, divalent anion is also possible).
Akaganite
β-FeOOH
Akaganite is the
β-polymorph
of the oxyhydroxide FeOOH. It can be identified by its strong X ray
diffraction lines and by DTA. Akaganite can be prepared in the laboratory from Fe(II) chloride at
pH 1.3 - 2.3, and from FeCl
3 at pH 7.5.
Feroxyhite
δ-FeOOH
Feroxyhite has been found in the form of yellowish-brown deposits from the bottom of the Pacific
Ocean and the white Kara and Baltic seas. DTA curves under N2 atmosphere show strong
double endothermic peak at 150 and 200°C followed b y a weak curve at 400°C.
