Geology Reference
In-Depth Information
(Table 2.2). Hematite has coercivities in the 100s to 1000 mT range, but a
much higher Neel temperature of 680°C. Therefore, to detect the goethite to
hematite ratio, a sample can be given a saturation magnetization (SIRM),
probably using field strengths in the 4-5 T range, then alternating field
demagnetized at ~100 mT (SIRM
100mT
), to remove the contribution of
any ferrimagnetic minerals (e.g., magnetite), and then heated to ~130°C
(SIRM
100mT 130°C
). Heating to this low temperature should remove the magne-
tization of any goethite that was activated by SIRM acquisition. The goethite/
hematite ratio is determined magnetically by:
G
H
SIRM
SIRM
SIRM
−
(2.5)
=
100
mT
100
mT
130
°
C
100
mT
2.5.2.4 S Ratio and Hard Isothermal Remanent Magnetization (HIRM)
Magnetic parameter ratios can also be used to detect the relative propor-
tions of different magnetic minerals based on their coercivities. The S ratio
measures the relative proportion of magnetite to hematite in a sample. It is
measured by saturating the remanence of a sample (SIRM), then applying a
backfield to the sample in a field strength that should magnetize all the
magnetite. Since the theoretical maximum microscopic coercivity for an
infinitely long magnetite needle is 300 mT, this value is often used for the
backfield. However, smaller field strengths can be used to detect different
grain sizes or shapes of the magnetite. Sometimes a range of values is used
to see how magnetic behavior changes as the strength of the backfield var-
ies. The S ratio can be calculated in a variety of ways, but the most common
formula is:
=
−
IRM
SIRM
Sratio
−03
.
T
(2.6)
The S ratio varies from +1 where all the magnetic grains in the sample are
magnetite (coercivities less than 0.3 T) so that the sample reaches saturation
both initially (SIRM) and in the backfield (IRM
-0.3T
). The negative sign
ensures that the sample containing mainly magnetite will have a positive
ratio. If the sample has only hematite particles, the 0.3 T backfield will not
be able to change its initial SIRM and the S ratio will have a value of -1.
Intermediate values indicate more or less contributions of magnetite and
hematite. The relative magnetizations of magnetite (92 Am
2
/kg) and hema-
tite (0.4 Am
2
/kg) need to be considered if the S ratio is used to quantitatively
estimate the relative proportions of magnetite and hematite. Liu et al. (2012)
point out that the variations in the S ratio are nonlinear and nonunique, so
it can only give a qualitative estimate of the proportion of low coercivity to
high coercivity magnetic minerals.
The results of saturation remanence (SIRM) and the backfield IRM
can be combined in a different way to determine the
hard isothermal
