Geoscience Reference
In-Depth Information
Rutile
(TiO
2
)
a)
Hexagonal pyrrhotite + pyrite
300
Hexa' + mono'
pyrrh'
200
Hexagonal
pyrrhotite
Monoclinic pyrrhotite + pyrite
Ilmenite
(FeTiO
3
)
Mono'
pyrrh'
100
Titanohaematites
(Fe
2-y
Ti
y
O
4
)
Ulvospinel
(Fe
2
TiO
4
)
Hexa'
pyrrh'
+
troilite
Smythite
+
mono'
pyrrh'
Smythite + pyrite
Titanomaghaemites
Oxidation
0
50
48
46
44
42
40
38
36
34
Titanomagnetites
(Fe
3-x
Ti
x
O
4
)
Atomic Fe (%)
Reduction
Wüstite
(FeO)
Magnetite
(Fe
3
O
4
)
Haematite
( Fe
2
O
3
)
Maghaemite
( Fe
2
O
3
)
Figure 3.40
Relationship between condensed phases in part of the
Fe
-
S systemfor temperatures below 350 °C. Redrawn, with
Fe
2.2
Ti
0.8
O
4
b)
Titanohaematites
The titanohaematite series (Fe
2
-
x
Ti
x
O
3
0
4.0
120
m
m
<
x
<
1) has
30
m
m
2.0
6
m
m
ilmenite
(FeTiO
3
) and haematite
(
α
Fe
2
O
3
) as
end-
1
m
m
members. Titanohaematites with 50
80% ilmenite are
strongly magnetic and carry remanence. Pure ilmenite is
paramagnetic and therefore carries no remanence and has
low susceptibility. Some ilmenite compositions exhibit the
rare phenomenon (in rocks) of acquiring a self-reversal
remanent magnetisation, where the remanence is opposite
to the applied magnetic field. Haematite exhibits weak
susceptibility and its Curie temperature is 680 °C.
Maghaemite (
-
0.0
2.0
1
m
m
1.0
10
m
m
30
m
m
0.0
Ulvospinel
(Fe
2
TiO
4
)
Magnetite
(Fe
3
O
4
)
Increasing titanium content
Fe
2
O
3
) is ferrimagnetic and strongly
magnetic, although its magnetic characteristics are com-
plex and poorly understood.
γ
Figure 3.39
(a) Ternary diagram showing the chemistry of Fe-Ti
-
Ti
oxides and their change with increasing oxidation. (b)
Compositional control of magnetic properties in the titanomagnetite
series. The individual curves are for the grain sizes shown. Redrawn,
with permission, from Clark (
1997
).
3.9.1.2
Iron sulphide minerals
The iron sulphide mineral pyrrhotite can be magnetic. The
general formula for the pyrrhotites is Fe
1
-
x
S(0
0.13)
with values of x appearing to correspond with particular
ratios of Fe:S. Its crystal structure is temperature-
rhotites occur with various crystal structures, but a key
generalisation, from a geophysical perspective, is that
monoclinic (4C) pyrrhotite (Fe
7
S
8
) is the only common
pyrrhotite that is ferrimagnetic with high susceptibility.
It has a Curie temperature of 320 °C. Note that monoclinic
pyrrhotite is primarily a lower-temperature phase. It is
unstable above about 250 °C, and higher-temperature
forms have hexagonal structure and are antiferromagnetic.
Pyrrhotite often occurs in association with pyrite (FeS
2
),
which is paramagnetic, especially in ore environments. Both
minerals are favoured by strongly reducing conditions,
<
x
<
range. Grains with compositions in this range undergo sub-
solidus exsolution into magnetite- and ulvospinel-rich, usu-
ally lamellar, intergrowths. Photomicrographs illustrating
this phenomenon in an ore environment are provided by
ulvospinel then tends to be oxidised to ilmenite and magnet-
ite, if dissociated water is present, i.e. deuteric oxidation. Both
phenomena tend to increase the magnetism of the grain. In
more rapidly cooled extrusives, the titanium and iron oxides
tend to remain in metastable equilibrium. In addition to
producing iron-rich magnetic grains, exsolution may also
effectively partition larger grains into smaller ones, whose
'
grain size affects their magnetic properties (see
effective
'
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