Page 73 - Fundamentals of Geomorphology

56 INTRODUCING LANDFORMS AND LANDSCAPES

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