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a)
c)
1.2
1.2
1.0
1.0
Fe 3+
H 2 O unstable
H 2 O unstable
0.8
0.8
0.6
0.6
0.4
0.4
Haematite
Haematite
Fe 2+
Pyrit +
0.2
0.2
Sulphur
0.0
0.0
Siderite
-0.2
-0.2
Pyrite
Pyrite
-0.4
-0.4
Siderite
Magnetite
-0.6
-0.6
Magnetite
H 2 O unstable
H 2 O unstable
Pyrrhotite
-0.8
-0.8
-1.0
-1.0
0
2
4
6
8
10
12
14
0
2
4
6
8
10
12
14
pH
pH
b)
d)
1.2
1.2
1.0
1.0
0.8
0.8
0.6
0.6
Rain
water
Rain
water
Stream
water
Stream
water
0.4
0.4
Bog
water
Bog
water
Ocean
water
Ocean
water
0.2
0.2
Ground
water
Ground
water
Saturated
soils
Saturated
soils
0.0
0.0
Euxenic
marine
Euxenic
marine
-0.2
-0.2
Organic-rich
saline
Organic-rich
saline
-0.4
-0.4
Field for natural
environment
Field for natural
environment
-0.6
-0.6
-0.8
-0.8
-1.0
-1.0
0
2
4
6
8
10
12
14
0
2
4
6
8
10
12
14
pH
pH
Figure 3.53 Eh
fields of various iron oxides, sulphides and carbonates in relation to conditions in near-
surface environments in the presence of water. Pressure
-
pH diagrams showing the stability
¼
0.1 MPa, temperature
¼
25 °C. (a) Haematite, pyrite, magnetite and pyrrhotite for
10 - 1.5 ,
10 - 2.5 M (as in sea water) (b) Comparison of
Σ
S
¼
Σ
CO 2 ¼
fields in (a) with those in the natural environment. (c) Stability
fields after an
10 - 6 ,
increase in the availability of CO 2 and decrease in the availability of S;
fields in (c) with those in the
natural environment. (a) Redrawn, with permission, from Mueller ( 1978 ); (c) redrawn, with permission, from Garrels and Christ ( 1965 ).
Σ
S
¼
Σ
CO 2 ¼
1M. (d) Comparison of
The metal oxide and sulphide minerals are mostly
weakly magnetic, although magnetite mineralisation is an
obvious exception. Deposits containing massive magnetite
are extremely magnetic, for example magnetite-bearing
skarns, although self-demagnetisation (see Section 3.2.3.6 )
may reduce this. Magnetite-bearing heavy mineral sands
may have high susceptibilities, and available data suggest
that remanent magnetism of the deposits is very weak
(Lawton and Hochstein, 1993 ) , probably owing to random
orientation of the magnetic grains. Other heavy minerals
are weakly magnetic (ilmenite) or non-magnetic. Mineral-
isation comprising nickel and base metal sulphide and
oxide species can be strongly magnetic (see Fig. 3.42 ) . This
is due to magnetite and pyrrhotite in the mineralisation,
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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