23
Iron [Fe, 26]
23.1 INTRODUCTION
Iron (Fe), a metal of the group 8 in the periodic table of elements, is a major constituent
of the lithosphere; its global abundance is calculated to be from around 4.5% to >5%,
and it is not considered as a trace element in rocks and soils. It plays a special role
in the behavior of several trace elements and is in the intermediate position between
macro and micronutrients in plants, animals, and humans. Iron reveals variable tendencies: siderophillic, chalcophillic, and lithophillic. It is the most important and
widely used metal.
Highest Fe contents are in mafic rocks, up to 8%, whereas in acidic igneous rocks,
its contents are up to 3%. Iron abundance in sedimentary rocks is about 4%, being
more concentrated in argillaceous ones. In coal, it may occur up to about 2%, and in
fly ash, up to 4%.
Iron occurs at several oxidation states, from +2 to +6, of which the most common
is +3. Geochemistry of Fe is very complex and is largely determined by the easy
change of the state of oxidation, and easy chemical reactions with other metals. Its
behavior is also closely linked to the cycling of O, S, and C. Iron-ore minerals are
mainly ferric oxides (hematite), hydrated ferric oxides (goethite), and various other
minerals, such as siderite, pyrite, and ilmenite. Bog iron ores are small deposits of
siderite, resulting from the precipitation of soluble Fe(HCO 3 ) 2 in lakes, swamps, and
shallow shelf regions.
The fate of Fe in weathering processes depends largely on the Eh–pH system,
and on the stage of the oxidation of Fe compounds. The general rules governing its
behavior are that oxidation and alkaline conditions promote the precipitation of Fe,
whereas reducing and acidic conditions promote the mobilization of Fe compounds
(Cornell and Schwertmann 2003).
Common iron minerals, including those that are present in soils, formed pedogenically and biologically are as follows:
r Hematite, α-Fe 2 O 3 , occurs in soils of arid, semiarid, and tropical regions,
and most often is inherited from parent rocks.
r Maghemite, γ-Fe 2 O 3 , is formed in highly weathered soils of tropical zones,
and often occurs as concentrations associated with other Fe minerals.
r Magnetite, Fe 2 O 3 , is mainly inherited from parent rocks, and is often
accompanied by maghemite.
r Goethite, α-FeOOH, is the most common mineral in soils, over broad climatic regions. Its crystallinity and composition may differ, depending upon
conditions in which it has formed.
r Lepidocrocite, γ-FeOOH, is common in poorly drained soils of humid temperate regions.
155
Iron [Fe, 26]
23.1 INTRODUCTION
Iron (Fe), a metal of the group 8 in the periodic table of elements, is a major constituent
of the lithosphere; its global abundance is calculated to be from around 4.5% to >5%,
and it is not considered as a trace element in rocks and soils. It plays a special role
in the behavior of several trace elements and is in the intermediate position between
macro and micronutrients in plants, animals, and humans. Iron reveals variable tendencies: siderophillic, chalcophillic, and lithophillic. It is the most important and
widely used metal.
Highest Fe contents are in mafic rocks, up to 8%, whereas in acidic igneous rocks,
its contents are up to 3%. Iron abundance in sedimentary rocks is about 4%, being
more concentrated in argillaceous ones. In coal, it may occur up to about 2%, and in
fly ash, up to 4%.
Iron occurs at several oxidation states, from +2 to +6, of which the most common
is +3. Geochemistry of Fe is very complex and is largely determined by the easy
change of the state of oxidation, and easy chemical reactions with other metals. Its
behavior is also closely linked to the cycling of O, S, and C. Iron-ore minerals are
mainly ferric oxides (hematite), hydrated ferric oxides (goethite), and various other
minerals, such as siderite, pyrite, and ilmenite. Bog iron ores are small deposits of
siderite, resulting from the precipitation of soluble Fe(HCO 3 ) 2 in lakes, swamps, and
shallow shelf regions.
The fate of Fe in weathering processes depends largely on the Eh–pH system,
and on the stage of the oxidation of Fe compounds. The general rules governing its
behavior are that oxidation and alkaline conditions promote the precipitation of Fe,
whereas reducing and acidic conditions promote the mobilization of Fe compounds
(Cornell and Schwertmann 2003).
Common iron minerals, including those that are present in soils, formed pedogenically and biologically are as follows:
r Hematite, α-Fe 2 O 3 , occurs in soils of arid, semiarid, and tropical regions,
and most often is inherited from parent rocks.
r Maghemite, γ-Fe 2 O 3 , is formed in highly weathered soils of tropical zones,
and often occurs as concentrations associated with other Fe minerals.
r Magnetite, Fe 2 O 3 , is mainly inherited from parent rocks, and is often
accompanied by maghemite.
r Goethite, α-FeOOH, is the most common mineral in soils, over broad climatic regions. Its crystallinity and composition may differ, depending upon
conditions in which it has formed.
r Lepidocrocite, γ-FeOOH, is common in poorly drained soils of humid temperate regions.
155