Diagenetic imprints on magnetic mineral assemblages in marine ...

Chapter 1
2.1.1 Iron (titanium) oxides
4
Magnetically, three naturally occurring iron oxide minerals are important; magnetite,
hematite and maghemite. Some properties of these minerals and several others are given
in table 1. Magnetite is found in rocks and sediments and can also be produced by certain
types of bacteria, known as magnetotactic bacteria, which use it for navigational
purposes. Magnetite is characterised a high spontaneous magnetic moment and by two
important temperatures, i.e., the Curie point at 580 o C and the Verwey transition at about
-150 o C (e.g., Dunlop and Özdemir, 1997). These temperatures and the respective
changes in observed magnetisation are frequently used in diagnostic tests.
Table 1. Properties of some common magnetic minerals
Mineral Formula Ms (kA/m) TC ( o C) magnetic structure
Magnetite Fe3O4 480 580 ferrimagnetic
Hematite α-Fe2O3 ~2.5 675 canted antiferromagnetic
Maghemite γ-Fe2O3 380 590-675 ferrimagnetic
Goethite α-FeOOH ~2 120 antiferromagnetic
Greigite Fe3S4 ~125 ~330 ferrimagnetic
Pyrrhotite Fe7S8 ~80 320 ferrimagnetic
A more detailed description is given by Hunt et al. (1995), Dekkers (1997) and Dunlop and Özdemir (1997).
Hematite is common in soils and sediments, and is the main carrier of the
magnetisation of ‘red beds’, a major source of information in classic paleomagentism
(Evans and Heller, 2003). The mineral is like magnetite characterized by two
temperatures (e.g., Dunlop and Özdemir, 1997), the Néel temperature at 675 o C and the
Morin transition at approximately -15 o C (Morin, 1950).
Maghemite, the fully oxidised form of magnetite, is also frequently encountered in
soils. It has a Curie temperature at around 645 o C, that is unfortunately difficult to
measure, there the mineral is metastable, i.e. it transforms to hematite with a loss of
magnetisation (e.g., Dunlop and Özdemir, 1997). This so-called inversion temperature can
be used in identification, however it is very variable and many different values have been
reported.
In nature varying compositions for the above-mentioned minerals are observed, in
most cases some iron has been replaced by titanium. In Fig. 1 a ternary diagram
illustrates the solid solution series in which iron titanium minerals can occur. Two main