Geology Reference
In-Depth Information
Table 2.1 Conversion between SI and cgs magnetic units
Magnetic Parameter
SI unit
cgs unit
Conversion
Amp-m 2 (Am 2 )
emu
1 Am 2 = 10 3 emu
Magnetic moment
Magnetization/mass
Am 2 /kg
emu/g
1 Am 2 /kg = 1 emu/g
Magnetic field (H)
A/m
Oersted (Oe)
1 A/m = 4 π × 10 -3 Oe
Magnetic induction (B)
Tesla (T)
Gauss (G)
1 T = 10 4 G
Susceptibility/mass
m 3 /kg
emu/g/Oe
1 m 3 /kg = (10 3 /4 π ) cgs
Source: B = μ (H + M) where M is the magnetization of a material and μ is the magnetic permeability.
the applied magnetic field. All materials are diamagnetic, but in some mate-
rials stronger magnetizations (e.g., paramagnetism and ferromagnetism)
swamp the diamagnetism. Diamagnetism is due to the interaction of the
magnetic moment of orbiting electrons with the applied magnetic field.
Orbiting electrons are like current loops and generate dipole moments.
Calcite and quartz are important rock-forming minerals that are diamag-
netic. Water and organic material are also diamagnetic. Typical magnitudes
of diamagnetic susceptibilities are as follows: quartz = -0.62 × 10 -8 m 3 /kg,
calcite = -0.48 × 10 -8 m 3 /kg, and water = -0.9 × 10 -8 m 3 /kg.
Paramagnetism is an induced magnetization in which a material acquires
a magnetization with a magnetic moment parallel to the direction of the
applied magnetic field. Paramagnetism is due to materials containing
atoms with net magnetic moments because of unpaired electrons. Electrons
have spin and the spin of an electric charge generates a magnetic dipole
moment. Paired electrons means that two electrons occupy the same
atomic orbital but their spins are opposite to each other. Therefore, their
spin magnetic moments cancel out. The unpaired electron spin moments,
resulting when only one electron occupies an orbital, will align with an
applied magnetic field. Iron has unpaired electrons in its 3D electron shell,
so iron-containing silicates (e.g., amphiboles, pyroxenes, and iron-rich
clays) are paramagnetic. Paramagnetism is about 100 times stronger than
diamagnetism. Values for common Fe-rich Earth materials are as follows:
biotite = 79 × 10 -8 m 3 /kg and nontronite (Fe-rich clay) = 65 × 10 -8 m 3 /kg.
Paramagnetism is temperature dependent and this dependence is described
by the Curie law of paramagnetism (see Butler (1992) for complete cov-
erage) which indicates that the strength of the induced paramagnetism is
dependent on the following:
MH
kT
J
ind
(2.3)
where J ind is the induced paramagnetism, M is the moment resulting
from the unpaired electrons, and H is the applied field that rotates the
moment into alignment. T is the temperature multiplied by Boltzmann's
 
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