Magnetization of Sediments and the Environment - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 98

Magnetic mineralogy (Cioppa & Kodama 2003a). One point to keep in mind

when interpreting the S -ratio is that the spontaneous
magnetization of magnetite is about 200 times that of
hematite, so small deviations from an S - ratio of 1 could
mean respectable amounts of hematite in a sample.
The S -ratio can also be used with different backfi eld
strengths, designed to detect different magnetic miner-
alogies or magnetic grain sizes.
Even though heating is avoided in environmental
magnetic studies because of the potential for caus-
ing chemical changes to the magnetic minerals in a
sample, thermal behavior is a powerful tool for identify-
ing magnetic mineralogy. Magnetite loses its magneti-
zation at c. 580°C, hematite at c. 680°C and Fe sulfi des
lose their magnetization at c. 300°C. At these tem-
peratures, the sub-atomic magnetic order in a crystal
is overcome by the disordering infl uence of thermal
energy and due to the expansion of the crystal lattice.
The temperature at which the mineral loses its mag-
netization is referred to as the Curie temperature for
magnetite and the Neel temperature for hematite,
because hematite is an antiferromagnetic mineral.
Goethite, another antiferromagnetic mineral impor-
tant in environmental magnetic studies, has a Neel
temperature of c. 120 ° C.
The so-called Lowrie test (Lowrie 1990) is often used
to identify the magnetic mineralogy in a sediment.
Samples are given orthogonal IRMs in different fi elds,
usually a very high ( c. 1 T), an intermediate - strength
( c. 0.5 T) and a low - strength ( c. 0.1 T) fi eld. The sample
is then thermally demagnetized. The test exploits both
the thermomagnetic behavior and coercivities of dif-
ferent magnetic minerals to aid magnetic mineral
identifi cation. If the low fi eld IRM (0.1 T) disappears at
about 580°C this strongly suggests the presence of
magnetite. High coercivity IRMs (1 T) that are gone at
680°C indicate hematite; if the IRM decreases at 120°C
then goethite is suggested. Intermediate or low coer-
civities that are removed by 300°C temperatures indi-
cate the presence the iron sulfi des. Maghemite can be
mistaken for a sulfi de since it inverts to hematite at
about 350°C, greatly reducing the intensity of mag-
netization. Discriminating maghemite from Fe sulfi des
is aided by re-exposing the sample to the pre-heating
IRM fi elds. If much lower IRM intensities are acquired,
maghemite is implicated since it has changed to hema-
tite with its low spontaneous magnetization during
heating. An increase in IRM strength, greater than
before the heating, could implicate Fe sulfi des since
they usually oxidize to magnetite during the heating.

After magnetic mineral concentration and magnetic
mineral grain size, the third main property measured
in environmental magnetism studies is the magnetic
mineralogy of a sample. Different magnetic minerals
can be identifi ed, usually without heating, by their
different coercivities. The variation in the relative
amounts of different magnetic minerals can be the
result of changes in the depositional environment or
source area or environmental changes in the source
area. The relative amounts of these two magnetic min-
erals can be detected magnetically.
One of the best examples of using magnetic param-
eters to detect variations in the magnetic mineralogy
of a sediment is the S -ratio. This ratio detects the rela-
tive amounts of a low-coercivity ferromagnetic
mineral, i.e. magnetite, compared to the amounts of a
high-coercivity antiferromagnetic mineral, i.e. hema-
tite, in a sample. The sample is typically magnetically
saturated, i.e. the intensity of IRM no longer increases
as the sample is exposed to higher and higher DC mag-
netic fi elds in an IRM acquisition experiment, and then
the sample is exposed to a fi eld in the opposite direction
called the backfi eld. Typically, a 0.3 T fi eld is chosen for
the backfi eld since this strength is the theoretical
maximum coercivity of magnetite. The S - ratio is calcu-
lated as:

IRM
SIRM

− 0. T

S -ratio =−

where SIRM is the saturation isothermal remanent
magnetization. In the S - ratio experiment, the SIRM
activates all the magnetic minerals (both the magnetite
and the hematite) in a sample. The backfi eld magnet-
izes the magnetite in the reverse direction, leaving any
hematite in the sample magnetized in the original
direction. The ratio of these two IRMs indicates the
relative proportion of magnetite to hematite. The nega-
tive sign for the backfi eld IRM in the equation above
means that a sample containing only the ferrimagnetic
phase (magnetite) will have an S - ratio of +1. A sample
with only the antiferromagnetic mineral (hematite)
will have an S - ratio of − 1.
Variations in the S -ratio could be used to detect vari-
ations in the provenance of the sediments being
studied. For instance, more antiferromagnetic miner-
als in lake sediments could indicate that the subsoil in
a catchment is contributing more to erosion into a lake