Magnetization of Sediments and the Environment - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 96

sample, so the magnetic units typically employed to
report the results are A m 2 /kg.
Anhysteretic remanent magnetization (ARM) is
used to quantify magnetite concentration or, more
broadly, the concentration of low-coercivity magnetic
minerals. One reason is that most laboratories can only
apply ARMs with peak alternating fi elds up to 100-
200 mT. One other environmentally important mag-
netic mineral in this class would be the iron sulfi de
greigite (Fe 3 S 4 ) that is the sulfi de analogue of magnet-
ite. It has coercivities similar to that of magnetite
because its crystal structure is identical with sulfur
replacing oxygen and a spontaneous magnetization
about one-quarter that of magnetite. Peters & Dekkers
(2003) indicate that the iron sulfi de pyrrhotite and the
iron oxide maghemite also fall into the coercivity range
that could potentially be activated by a 100 mT ARM
but, as noted in Chapter 6, Horng & Roberts (2006)
suggest that pyrrhotite is less likely to result from envi-
ronmental processes such as reduction diagenesis.
Maghemite (γ - Fe 2 O 3 ) is important to environmental
magnetic studies because it is commonly found in soils.
Isothermal remanent magnetization (IRM) is also
used to measure magnetic mineral concentration in a
sample. Since most laboratories can apply fi elds of at
least 1 T and, in some cases, as high as 5 T, magnetic
minerals with much higher coercivities can be acti-
vated by application of an IRM. Magnetization by an
IRM is however a different process than that of ARM,
and so magnetic minerals respond differently. ARM is
considered a better analogue of thermal remanent
magnetization (TRM) processes than IRM, but without
the heating that could cause chemical changes of the
magnetic minerals in a sample. TRM is really the 'gold
standard' of activating the magnetization of particles
in a sample because it is the process by which nearly
all depositional magnetic grains were initially magnet-
ized. The alternating fi eld used to apply an ARM essen-
tially 'loosens up' the magnetization of a magnetic
grain so it can be reset by a weak magnetic fi eld, similar
in intensity to the Earth's magnetic fi eld. This is the
similarity to a TRM in which heating 'loosens up' the
magnetization of a grain while the Earth's fi eld (or a
weak laboratory fi eld) resets the grain's magnetization.
IRM is more of a blunt force approach in which a DC
magnetic fi eld 'whacks' the magnetization of a grain
into a new direction, depending on the orientation of
the applied fi eld with respect to the easy axis for the
grain's magnetization and the grain's coercivity. A
lightning strike is the natural process that remagnet-

izes a rock by an IRM. Nevertheless, IRM applied in the
laboratory can be used to measure the concentration
of magnetic minerals over a wide range of coercivities,
so minerals such as hematite and goethite (an iron
oxyhydroxide important in chemical weathering; α -
FeOOH) as well as magnetite and iron sulfi des can be
activated. Goethite has coercivities up in the tens of
Tesla (Peters & Dekkers 2003).
Finally, the last major magnetic measurement used
to determine the concentration of magnetic minerals
in a sample is susceptibility. Susceptibility measure-
ments are different from the two remanence measure-
ments mentioned above. Susceptibility is the induced
magnetization that arises during the application of a
low - strength magnetic fi eld to a sample:

J = χ H

ind

where J ind is the induced magnetization, χ is the suscep-
tibility and H is the applied fi eld.
When the applied fi eld is removed the sample loses
its induced magnetization. The fi eld strengths used for
a susceptibility measurement are not large enough to
give the sample a permanent magnetization like an
IRM or an ARM.
One of the problems with measuring magnetic min-
eral concentration using susceptibility is that many
different magnetic minerals have magnetic susceptibil-
ity, but due to different processes. Calcite and quartz
are examples of common diamagnetic minerals that
have weak negative susceptibilities, i.e. the induced
magnetization is opposite to the applied magnetic
fi eld. Iron-containing silicates (e.g. micas, amphiboles,
pyroxenes, clays) have paramagnetism with weak
positive susceptibilities. Ferromagnetic minerals have
strong positive susceptibilities. Usually the strong sus-
ceptibilities of the ferromagnetic minerals (magnetite
and hematite) swamp the diamagnetic and paramag-
netic contributions of silicates and calcite to the
induced magnetization in a sample, but for some sedi-
mentary rocks (e.g. limestones or 'clean' sandstones)
the concentration of ferromagnetic minerals is so low
that paramagnetic or diamagnetic minerals contribute
signifi cantly to the susceptibility.
In addition, the susceptibility of ferromagnetic min-
erals is a function of magnetic particle size, since the
process of acquiring an induced magnetization is dif-
ferent for single-domain and multi-domain grains.
Multi-domain grains readjust their domain structure
to minimize their energy with respect to the applied