Geoscience Reference
In-Depth Information
6
−
−
7
Fe(OH)
3
precipitation
−
8
−
9
−
10
Fe(OH)
3
dissolution
−
11
5
6
7
8
9
10
pH
Figure 7.3
Ferric iron hydroxide saturation in pure water as a function of pH. The minimum in the solubility
curve results from the dominance of different hydroxide complexes at different pH values.
Fe
3
+
as:
which enables us to calculate the total ferric content
Fe
3
+
+
Fe
)
2
+
Fe
)
4
Fe
3
+
=
(
OH
(
OH
10
3
H
+
3
10
−
3
H
+
+
10
−
19
H
+
4.0
×
=
1.6
×
+
3.2
×
of variable complexation by the different hydroxides,
Fe
3
+
goes through a minimum for
pH
8.
In other reactions, CO
2
is consumed by silicate dissolution, such as:
≈
+
+
Mg
2
Si
2
O
6
4CO
2
2H
2
O
(pyroxene)
(atmosphere)
4HCO
3
2Mg
2
+
⇔
2SiO
2
+
+
(7.30)
(silica)
(solution)
(solution)
The CO
2
-consumption reactions are not fundamentally different from the proton-
exchange reactions, but show the overlapping relationship between erosion, the carbon
cycle - particularly marine carbonates and the CO
2
pressure of the atmosphere - and there-
fore, by way of the greenhouse effect, of climate. Proton-exchange reactions are very sensi-
tive to the pH of solutions, since, allowing for a unit concentration of solids, the mass action
rain, charged with sulfuric acid by oxidation of sulfur compounds at altitude, and ground-
water, charged with humic acids, will attack rock since their protons will be able to displace
the Na
+
,Ca
2
+
, or other ions of the minerals. Complexing anions (OH
−
,Cl
−
,CO
2
3
,SO
2
4
,
PO
3
4
, humic, and fulvic acids) also play a critical role in mineral solubility and transport.
These reactions are greatly dependent on temperature. In contrast to limestone dissolution
that recycles the fossil alkalinity produced since the origin of the Earth, the chemical ero-
sion of silicates
(Eqs. (7.22)
and
(7.30)
)
creates fresh alkalinity. Carbon dioxide outgassed
from the mantle will eventually be stored in newly formed sedimentary carbonates.
