Geology Reference
In-Depth Information
as oxidizing potential and negative values as reducing
potential (Box 3.2).
dioxide diffuses from the air to the water, which enables
further solution of limestone through the chain of reac-
tions. Diffusion of carbon dioxide through water is a slow
process compared with the earlier reactions and sets the
limit for limestone solution rates. Interestingly, the rate
of reaction between carbonic acid and calcite increases
with temperature, but the equilibrium solubility of car-
bon dioxide decreases with temperature. For this reason,
high concentrations of carbonic acid may occur in cold
regions, even though carbon dioxide is produced at a
slow rate by organisms in such environments.
Carbonation is a step in the complex weathering
of many other minerals, such as in the hydrolysis of
feldspar.
Carbonation
Carbonation
is the formation of carbonates, which
are the salts of carbonic acid (H
2
CO
3
). Carbon diox-
ide dissolves in natural waters to form carbonic acid.
The reversible reaction combines water with carbon
dioxide to form carbonic acid, which then dissociates
into a hydrogen ion and a bicarbonate ion. Carbonic
acid attacks minerals, forming carbonates. Carbonation
dominates the weathering of calcareous rocks (limestones
and dolomites) where the main mineral is calcite or cal-
cium carbonate (CaCO
3
). Calcite reacts with carbonic
acid to form calcium hydrogen carbonate (Ca(HCO
3
)
2
)
that, unlike calcite, is readily dissolved in water. This is
why some limestones are so prone to solution (p. 188).
The reversible reactions between carbon dioxide, water,
and calcium carbonate are complex. In essence, the
process may be written:
Hydrolysis
Generally,
hydrolysis
is the main process of chemical
weathering and can completely decompose or dras-
tically modify susceptible primary minerals in rocks.
In hydrolysis, water splits into
hydrogen cations
(
H
+
)
and
hydroxyl anions
(
OH
−
) and reacts directly with
silicate minerals in rocks and soils. The hydrogen
ion is exchanged with a metal cation of the silicate
minerals, commonly potassium (K
+
), sodium (Na
+
),
calcium (Ca
2
+
), or magnesium (Mg
2
+
). The released
cation then combines with the hydroxyl anion. The
reaction for the hydrolysis of orthoclase, which has the
chemical formula KAlSi
3
O
8
, is as follows:
Ca
2
+
+
2HCO
3
CaCO
3
+
H
2
O
+
CO
2
⇔
This formula summarizes a sequence of events starting
with dissolved carbon dioxide (from the air) reacting
speedily with water to produce carbonic acid, which is
always in an ionic state:
H
+
+
+
⇔
CO
2
H
2
O
HCO
3
2H
+
+
2OH
−
→
+
+
2KAlSi
3
O
8
2HAlSi
3
O
8
2KOH
Carbonate ions from the dissolved limestone react
at once with the hydrogen ions to produce bicarbonate
ions:
So the orthoclase is converted to aluminosilicic acid,
HAlSi
3
O
8
, and potassium hydroxide, KOH. The alumi-
nosilicic acid and potassium hydroxide are unstable and
react further. The potassium hydroxide is carbonated to
potassium carbonate, K
2
CO
3
, and water, H
2
O:
CO
3
2
−
+
H
+
⇔
HCO
3
2
−
This reaction upsets the chemical equilibrium in the
system, more limestone goes into solution to compen-
sate, and more dissolved carbon dioxide reacts with the
water to make more carbonic acid. The process raises
the concentration by about 8 mg/l, but it also brings the
carbon dioxide partial pressure of the air (a measure of
the amount of carbon dioxide in a unit volume of air)
and in the water into disequilibrium. In response, carbon
+
→
+
2KOH
H
2
CO
3
K
2
CO
3
2H
2
O
The potassium carbonate so formed is soluble in
and removed by water. The aluminosilicic acid reacts
with
water
to
produce
kaolinite,
Al
2
Si
2
O
5
(OH)
4
