Geology Reference
In-Depth Information
Table 2.2
Important rock magnetic properties.
Spontaneous
magnetization
(Am
2
/kg)
Magnetic
mineral
Range of
coercivity
Curie
temperature (°C)
Magnetite
92
10-100 mT
580
TM60
24
~8 mT
150
Hematite
0.4
100s of mT
to several T
680
Greigite
25
60 > 100 mT
~320
Goethite
Varies
≲
1
5-10 T
~125
Maghemite
74
10-100 mT
Inverts to hematite
at about 300-350
Source: Table based on the information in Tauxe (2010), O'Reilly (1984), Dunlop and Ozdemir (1997).
weak, ferromagnetism results (spontaneous magnetization = 0.4 Am
2
/kg).
Hematite can be a primary magnetic mineral, but it is more likely to be a
secondary magnetic mineral. Many times it can be formed early in the postde-
positional history of a sediment by secondary chemical growth, in which case
it is magnetized by its growth in the Earth's magnetic field and acquires a
chemical or crystallization remanent magnetization (CRM)
. Secondary
hematite is formed by the oxidation of Fe-rich silicates probably in arid envi-
ronments or perhaps during the dry season of monsoonal climates (Kodama
2012). Many red bed sedimentary sequences, whose red color is due to very
fine-grained (submicron), pigmentary hematite and whose paleomagnetic
signal is carried by either the pigmentary hematite or large grain size (micron
size) specular hematite particles, can record a magnetostratigraphy and show
evidence of inclination shallowing. Inclination shallowing, in which the angle
that the paleomagnetic vector makes with the paleohorizontal is smaller than
the geomagnetic field in which the rocks were deposited, suggests that the
hematite is either depositional or formed very soon after deposition, so that
burial compaction will affect its inclination (Kodama 2012). A magnetostratig-
raphy carried by hematite is also evidence of either a primary depositional
remanence or a CRM acquired soon after deposition. The pigmentary hema-
tite in red bed sequences is usually secondary and probably formed on the
order of 10
5
-10
6
years after deposition (Kodama 2012).
Iron sulfides are formed during the reductive diagenesis that occurs in
organic-rich sediments, both in marine and lacustrine settings. Iron sul-
fides are clearly secondary but can be formed soon after deposition (10
3
-
10
5
years) in the top meter of the sediment column (see Table 6.1 in
Kodama 2012). During reductive diagenesis, the primary, depositional
iron oxide (typically magnetite) is dissolved and then a sequence of iron
sulfides are formed, including greigite (Fe
3
S
4
), as intermediate products,
