Geoscience Reference
In-Depth Information
The hydrated sulphate of iron and potassium that forms is called
jarosite
. It has a pale yellow colour. All this amounts to saying that the
two preceding equations can be combined and written as:
FeS
2
+
15
4
O
2
+
1
3
K
+
+
5
2
H
2
O
Æ
1
3
KFe
3
(
SO
4
)
2
(
OH
)
6
+
4
___
__
__
__
__
3
SO
4
2 -
+ 3
H
+
(3)
But if the oxidation is limited (potential below 400 mV), sulphides
can be oxidized but not ferrous iron, which is rather stable in highly
acid medium. In these conditions, the reaction in equation (2) does not
occur and jarosite is not formed.
Actually, jarosite is not the only sulphate to form. As bases occur
in various proportions depending on the environment, the salts that
precipitate are variable, as shown in Table 12.10.
Table 12.10
Minerals found in the mud flats of Senegal (Le Brusq
et al
. 1987).
Mineral
Formula
Crystal system
KFe
3
(
SO
4
-
)
2
(
OH
)
6
Jarosite
Hexagonal
Natrojarosite
NaFe
3
(
SO
4
)
2
(
OH
)
6
Hexagonal
Alunite
(
K
,
Na
)
Al
3
(
SO
4
)
2
(
OH
)
6
Hexagonal
Tamarugite
NaAl
(
SO
4
)
2
, 6
H
2
O
Monoclinic
Pickeringite
MgAl
2
(
SO
4
)
4
, 22
H
2
O
Monoclinic
Halotrichite
FeAl
2
(
SO
4
)
4
, 22
H
2
O
Monoclinic
Blödite
Na
2
Mg
(
SO
4
)
2
, 4
H
2
O
Monoclinic
Hexahydrite
MgSO
4
, 6
H
2
O
Monoclinic
Rozenite
FeSO
4
, 4
H
2
O
Monoclinic
Coquimbite
Fe
2
(
SO
4
)
3
, 9
H
2
O
Hexagonal
If the material is calcareous, gypsum (CaSO
4
, 2H
2
O) can precipitate.
To this already long list of salts it is useful to now add
schwertmannite
[Fe
16
O
16
(SO
4
)
3
(OH)
10
, 10H
2
O] that plays a role in the WRB classification.
Certainly in future other mineral species with complex formulae will
be discovered in these swamps.
During oxidation, the very strong acidity of the medium results
in more or less intense breakdown of the crystal structure of the clay
minerals. These minerals release Al
3+
, Mg
2+
and silica that may or
may not precipitate. In principle, the remaining clay must exchange
