Diagenetic imprints on magnetic mineral assemblages in marine ...

Chapter 1
6
Magnetic intergrowths of ulvöspinel composition are relatively rare in nature, there is
usually enough oxygen present to oxidise the titanomagnetite completely. Low-
temperature oxidation converts a single-phase spinel into another single-phase spinel with
a different lattice parameter, whereas high-temperature oxidation (deuteric oxidation),
during the initial cooling, results in intergrowths of spinel (near magnetite) and
rhombohedral (near ilmenite) phases. This process is also known as oxyexsolution.
In chapter 2.4, low-temperature magnetic properties of exsolved titanomagnetite and
titanohematite, which have been found in the sediments near the Rio de la Plata estuary,
are discussed in more detail.
2.1.2 Iron oxyhydroxides
The only magnetically significant oxyhydroxide is goethite. It has a slightly lower
spontaneous magnetic moment than hematite and its Néel point lies at about 120 o C
(Table 1, e.g., Dunlop and Özdemir, 1997). Recent studies have showed the wide spread
occurrence of goethite in soils and sediments (France and Oldfield, 2000).
Two other oxyhydroxides, i.e. ferrihydrite and lepidocrocite, are worth mentioning, as
they may undergo chemical changes and produce hematite and magnetite in soils
(Schwertmann, 1988) and sediments.
2.1.3 Sulphides
The ferromagnetic magnetic iron sulphide, greigite was formerly thought to be rare in
nature, but commonly occurs in sediments formed under anoxic, i.e. sulphate reducing,
conditions (Roberts, 1995) and may also be bio-mineralised by magnetotactic bacteria
(Mann et al., 1990). Greigite is the sulphide equivalent of magnetite, and has a saturation
remanent magnetisation, that is approximately a quarter of that of magnetite (Table 1).
Thermomagnetically it may be identified by its Curie temperature, which lies at
approximately 330 o C (e.g., Dunlop and Özdemir, 1997).
Pyrrhotite is found in igneous, metamorphic and sedimentary rocks, as well as in
sulphide ores, but it seldom dominates the remanent magnetic signal. It has a Curie point
close to that of greigite, ~320 o C (e.g., Dunlop and Özdemir, 1997). Differently from
greigite pyrrhotite displays a low-temperature transition at ~34 K (Dekkers, 1989; Rochette
et al., 1990), which can be used to distinguish between the two sulphidic phases. Grain-
size dependent parameters of pyrrhotite have been studied by Clark (1984) and Dekkers
(1988; 1989).