Environmental Engineering Reference
In-Depth Information
Figure 13.8
Mineral susceptibility to chemical weathering,
closely linked with magma geochemistry and melt
temperatures.
other minerals, creating new species which are readily removed (Figure 13.9). It is
therefore both
reagent
and
solvent
. Hydrogen ion concentration is expressed as the
pH
of water, which is neutral when pH = 7·0,
acid
at less than 7·0 and
alkaline
at over 7·0.
Chemical weathering potential increases inversely with pH and proportionally with
increasing equilibrium solubility of minerals and temperature, until saturation is reached.
Most minerals dissolve slowly in water, but those dominated by ionic bonds, e.g.
mafic minerals Mg
++
, Fe
++
, CaAl
2
Si
2
O
8
(calc-plagioclase), K
+
(potassium), Na
+
(sodium)
and Ca
+
(calcium), are more susceptible to
solution
than felsic minerals dominated by
covalent bonds, e.g. KAlSiO
3
O
8
(orthoclase), KAl
2
(OH)
2
Si
3
AlO
10
(muscovite) and (SiO
2
)
quartz. Al
2
SO
3
(alumina) is soluble in very acid (pH < 4·0) or alkaline (pH > 9·0) water.
Solution potential is enhanced through carbonation - the incorporation of dissolved
atmospheric CO
2
in water during or after precipitation - forming dilute carbonic acid,
H
2
CO
3
. This stage in the process provides a rare instance of a chemical reaction rate
inversely proportional to temperature. Other dilute acids may form in association with
atmospheric constituents, including sulphuric acid (H
2
SO
4
) as
acid rain
. Carbonation is
enhanced by passing through soil rich in biogenic carbon dioxide and is a common form
of chemical weathering, particularly of limestone. When hydration equilibrium between
H
2
O and CO
2
has been reached, the carbonic acid is dissociated to HCO
3
−
and H
+
ions.
CaCO
3
(calcium carbonate), in contact with the solution, is dissociated into Ca
++
and
CO
3
-
ions, combining with H
+
to form more HCO
3
−
