Magnetization of Sediments and the Environment - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 95

netizing) different subpopulations of magnetic grains
of different coercivities so that a measurement of the
magnetized sample essentially 'counts' the number of
magnetic grains, a measure of the amount of magnetic
material. For the complete set of equations and basic
rock magnetic principles the reader is referred to either
Butler ' s (1992) or Tauxe ' s (2010) excellent paleomag-
netism textbooks. We will provide only the most essen-
tial equations here and only a simple explanation of
the rock magnetic principles; the reader is urged to
read more deeply in Butler or Tauxe.
The two most common magnetic minerals in sedi-
ments are magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ).
These two Fe oxides differ markedly in their crystal
structure with magnetite being a cubic, inverse spinel
and hematite being rhombohedral with a corundum
structure and a hexagonal unit cell. These differ-
ences affect and control their ferromagnetism. The
sub-lattices of the magnetite inverse spinel crystal have
unequal numbers of antiparallel magnetic moments,
thus making magnetite a ferrimagnetic mineral (as
opposed to ferromagnetic which is the general term for
minerals that have a permanent magnetism or rema-
nence). The sub-atomic level magnetic moments that
cause the spontaneous magnetization, or ferromagnet-
ism, of the iron oxide minerals arises from interactions
between the uncoupled electron spin moments in the
d-shells of the iron atoms. The quantum mechanical
interactions between the uncoupled spin moments of
adjacent Fe atoms leads to a magnetic 'order' through-
out the crystal. This magnetic structure leads to
a strong spontaneous magnetization for magnetite
(480 kA/m). Hematite, on the other hand, has a much
weaker spontaneous magnetization (2 kA/m) because
its crystal structure results in a canted antiferromag-
netism. Essentially, equal numbers of antiparallel spin
moments in the crystal are misaligned by only a very
small angle (0.065°; Morrish 1994).
Because magnetite has a much stronger spontane-
ous magnetization than hematite, its particle-scale
anisotropy (which controls the direction of magnetism
in a particle) is dictated by the shape of the particle; in
contrast, the particle-scale anisotropy of hematite is
dictated mainly by its crystal structure. Magnetite par-
ticles tend to be magnetized along their longest shape
axis, while hematite particles tend to be magnetized in
the basal plane of the hexagonal crystal lattice. The
different causes of particle anisotropy for these two
magnetic minerals lead, in turn, to magnetite having
much lower coercivities than hematite. Magnetite has

typical coercivities less than 100 mT while hematite's
coercivities are several hundred to one thousand mT
(Peters & Dekkers 2003 ).
Furthermore, the differences in spontaneous mag-
netizations between the two minerals cause different
coercivity behavior for the particle sizes typically seen
in natural sediments and sedimentary rocks (micron to
submicron). Magnetite will be single domain (SD) (a
particle is magnetized in the same direction through-
out its volume) in the submicron range ( c. 0.02 - 0.2
microns; Tauxe 2010) and multi-domain (MD) (a par-
ticle divides itself into volumes with different magnetic
directions) at larger grain sizes. The subdivision into
multi-domain grains occurs at a magnetic mineral's
critical diameter, when it saves enough magnetostatic
energy to build the walls between the different regions
of different direction magnetization. By changing to a
multi-domain structure, a magnetic particle minimizes
its overall magnetostratic energy by arranging the
north magnetic poles of one domain close to the south
magnetic poles of an adjacent domain. Multi-domain
particles can more easily adjust to externally applied
magnetic fi elds by moving their domain walls to change
the volumes of different domains in the grain. Domains
parallel to the applied fi eld become larger and those
antiparallel become smaller. Magnetite's coercivity is
then observed to increase as its grain size decreases
into the SD grain-size range. Hematite, in contrast, will
be single domain through much of its natural grain-
size range. It switches to multi-domain confi gurations
at particle sizes close to 10-20 microns (Dunlop &
Ozdemir 1997 ).

Magnetic mineral concentration

A magnetic concentration measurement results from
application of a laboratory remanence to a sample and
then measurement of the strength of the sample's
magnetization. The direction of the sample's magnet-
ism is not important to this measurement; in fact, any
natural remanent magnetization (NRM) of the sample
is destroyed by the application of the laboratory fi eld
and the sample is no longer suitable for paleomagnetic
study. As mentioned earlier, the laboratory-applied
fi eld 'activates' or magnetizes a subpopulation of the
magnetic grains in a sample. Measurement by rock
magnetometer essentially 'counts' the number of mag-
netic grains activated in the sample. The magnetic
measurement is usually normalized by the mass of the