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otherwise the iron is taken up by oxide species. Since pyr-
rhotite contains less sulphur than pyrite, the availability of
sulphur is a key control on which species is formed. In
reducing sedimentary environments, mostly saline ones, pyr-
rhotite may form by sulphurisation of iron, although pyrite is
produced if suf
cient time and sulphur allow. Primary pyr-
rhotite can occur in igneous rocks and may be created by de-
sulphurisation of pyrite during metamorphism. Higher
metamorphic grade tends to favour pyrrhotite formation.
For example, in their comparison of greenschist and
amphibolite facies orogenic gold deposits inWestern Austra-
deposits in lower grade hosts, but pyrrhotite when the grade
was higher. Geological descriptions rarely consider the habit
of the pyrrhotite. Whether the occurrences described above
are of monoclinic pyrrhotite will depend primarily on the
temperature/cooling history of the environment.
Monoclinic pyrrhotite often carries strong remanent
magnetisation. This can be stable for geologically long
periods at low temperatures, particularly in
ne-grained
materials. However, because of its relatively low Curie
temperature, remanent magnetism can be altered relatively
easily. Susceptibility is dependent on grain size, decreasing
with decreasing grain size. Monoclinic pyrrhotite has
strong intrinsic anisotropy within its basal plane which
results in signi
cant susceptibility anisotropy in the rocks
having a preferred orientation of the pyrrhotite grains.
Less common magnetic iron sulphide minerals include
greigite (Fe
3
S
4
) and smythite ((Fe, Ni)
9
S
11
). Greigite occurs
in young sediments; smythite, although rare, occurs in
some magmatic ores and some sedimentary rocks. In both
cases their magnetic properties probably resemble those of
monoclinic pyrrhotite.
to achieve than changing the magnetism of a domain itself.
These grains are magnetically less stable than single-
domain grains, i.e. the magnetism is
. An important
consequence is that the remanent magnetism of coarse
grains is much less stable than that of
fine grains and,
frequently, the remanent magnetism of coarse grains is
parallel to the present-day geomagnetic
field, this being a
viscous remanent magnetism (VRM; see
Section 3.2.3.4
).
The relatively large multidomain grains are often the dom-
inant cause of a rock
'
soft
'
s susceptibility.
Ultra-fine grains do not retain stable remanent
magnetism, but they do realign, or relax, easily with an
external field to produce very strong magnetism. This
effect is known as superparamagnetism and is particularly
important in electromagnetic surveying where strong local
magnetic
'
fields are produced by the transmitter (see Super-
paramagnetism in
Section 5.7.6
)
.
3.9.2
Magnetic properties of rocks
As would be expected, the susceptibility of rocks is depend-
ent upon the susceptibilities and proportions of their com-
ponent minerals. It also depends on the size and shape of
the magnetic mineral grains, so a range of susceptibilities is
possible for a given mineral content. For rocks with
strongly magnetic grains like magnetite occupying less
than about 10% of the volume, the relationship between
rock susceptibility and the volume fraction of the grains is
approximately linear on a log
-
log graph (
Fig. 3.41
). When
10
2
Approximate average
10
1
Magnetite
3.9.1.3
Grain size and rock magnetism
In general, very
fine grains of magnetic minerals have a
single magnetic domain structure, so the grain itself is
uniformly magnetised. The magnetism of these grains is
very stable or
10
0
10
-1
, because to alter it requires changing
the direction of the magnetism itself. They are the most
important carriers of intense remanence in many rocks
even though they may constitute a minor proportion of
the overall magnetic mineral assemblage.
For larger grains, the shape-dependent, internal self-
demagnetisation (see
Section 3.2.3.6
)
interacts with their
intrinsic magnetisation, producing multiple domains with
different magnetic orientations. The magnetism can be
changed by migration of domain boundaries (see Ferro-
magnetism in
Section 3.2.3.5
), a process that is much easier
'
hard
'
Monoclinic
pyrrhotite
10
-2
20%
10
-3
10
-1
10
0
10
1
10
2
Magnetic mineral content (vol %)
Figure 3.41
Magnetic susceptibility versus the volumetric content of
magnetite and pyrrhotite. Fine-grained:
<
20
μ
m, coarse-grained:
>
500
μ
m. Redrawn, with permission, from Clark (
1997
).
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