Iron (Fe) – element-mineral links of Fe

Iron is an important element in chemostratigraphy interpretations. But what are the element-mineral links of Fe in a chemostratigraphy study?

Iron (symbol Fe; atomic number 26; relative atomic mass 55.845) [Link to webelements.com]

Fe has two main oxidation states, +2 and +3.

Common element-mineral links of Fe

Common minerals associated with Fe are pyrite (FeS₂), hematite (Fe₂O₃), goethite (FeO(OH)), magnetite (Fe₃O₄), and siderite (FeCO₃). Fe is, however, also common in other minerals, such as mica, chlorite, amphiboles, pyroxenes, garnet, and olivine.

The interpretation of iron in sedimentary rocks, however, is difficult because its abundance is controlled by a range of factors, including provenance, pH-Eh conditions, diagenetic alteration, and grain size.

In oxidizing environments, iron is present in the form of Fe³⁺ in oxides/hydroxides, while under strongly reducing (euxinic) conditions it is present as Fe2+ in form of sulfides, e.g., pyrite.

Fe³⁺ hydrous oxides often form haematitic or limonitic coatings around other minerals and can include other metals, such as Mn, Ti, V, Ni, Co, Cu, and Sc.

During secondary, chemical processes the prevailing pH-Eh conditions determine the format of Fe compounds. Generally, alkaline and oxidizing conditions stimulate precipitation of Fe, while acidic and reducing conditions promote the solution of Fe compounds. Consequently, Fe²⁺ is moderately mobile, whereas Fe³⁺ shows very low mobility.

Fe in siliciclastic rocks

In siliciclastic sedimentary rocks, Fe may be hosted by detrital minerals such as mica, amphiboles, pyroxenes, garnet, and olivine. Due to the sensitivity to pH-Eh conditions and diagenetic alterations of its host minerals (see above), Fe may also be present as grain-coatings and cement, such as hematite and chlorite. Under reducing conditions, Fe may be associated with sulfides, such as pyrite, and Fe-rich carbonates such as ankerite and ferroan dolomite (see below).

Fe in carbonate minerals

Several associations with carbonate minerals are possible:

• Siderite (FeCO₃) is an Fe-carbonate common as a diagenetic mineral in sandstones and shales. It sometimes occurs as concretions and forms at shallow burial depths. It is, however, also found as a hydrothermal mineral, where it often is associated with barite, galena, fluorite, etc.
• The ankerite formula (Ca(Fe,Mg,Mn)(CO₃)₂), is similar to that of dolomite, but Mg substitutes for Fe and Mn. In sedimentary rocks it may be present as an authigenic mineral formed under reducing conditions, or as a hydrothermal product.
• Fe-rich, or ferroan dolomite ((Ca,Mg,Fe)(CO₃)₂) may occur, similar to ankerite, as an authigenic mineral that formed under reducing conditions in sedimentary rocks.
• Fe-rich, or ferroan calcite ((Ca,Fe)CO₃) may be present as a cement which formed in oxidizing conditions.

Fe in igneous rocks

In igneous rocks, iron is mostly present as Fe²⁺ in ferromagnesian silicates, such as olivine, pyroxene, amphibole, and biotite, which alter to Fe³⁺-containing iron oxides and hydroxides during weathering processes.

Iron is generally enriched in mafic igneous rocks compared with felsic, intermediate, and ultramafic rocks.