An Introduction to Environmental Chemistry

The combined effects of dissolving CO 2 into soilwater (eqn. 4.7), the subse-

quent dissociation of H 2 CO 3 (eqn. 4.8) and the production of HCO 3 - by acid

hydrolysis weathering reactions (eqns. 4.11 & 4.12) mean that surface waters have

near-neutral pH, with HCO 3 - as the major anion.

4.4.4 Weathering of complex silicate minerals

So far we have considered the weathering of monomer silicates (e.g. olivine),

which dissolve completely (congruent solution). This has simplified the chemi-

cal reactions. The presence of altered mineral residues during weathering,

however, suggests that incomplete dissolution is more usual. Upper crustal rocks

have an average composition similar to the rock granodiorite (Table 4.4). This

rock is composed of the framework silicates, plagioclase feldspar, potassium

feldspar and quartz (Table 4.4), plagioclase feldspar being most abundant. Thus,

a simplified weathering reaction for plagioclase feldspar might best represent

average chemical weathering. We can illustrate this using the calcium (Ca)-rich

plagioclase feldspar, anorthite.

eqn. 4.13

The formula of the solid product (Al 2 Si 2 O 5 (OH) 4 ) is that for kaolinite, an impor-

tant member of the serpentine–kaolin group of clay minerals (Section 4.5.1). This

reaction demonstrates incongruent dissolution of feldspar, i.e. dissolution with in

situreprecipitation of some compounds from the weathered mineral.

Schematic representation of the anorthite chemical weathering reaction (Fig.

4.5) shows the edge of the anorthite crystal in contact with an H 2 CO 3 weather-

ing solution. Natural crystal surfaces have areas of excess electrical charge at their

corners and edges, usually because of imperfections (rows of atoms slightly out

CaAl Si O H CO H O

Ca HCO Al Si O OH

saql

aq aq s

228 2 3 2

(^232254)

2

2

() () ()

()

+ () ()

++Æ

++()

84 Chapter Four

Table 4.4Percentage mineral composition of the upper continental crust. After Nesbit and

Young (1984).

Average upper Average exposed

continental crust continental crust surface

Plagioclase feldspar 39.9 34.9

Potassium feldspar 12.9 11.3

Quartz 23.2 20.3

Volcanic glass — 12.5

Amphibole 2.1 1.8

Biotite mica 8.7 7.6

Muscovite mica 5.0 4.4

Chlorite 2.2 1.9

Pyroxene 1.4 1.2

Olivine 0.2 0.2

Oxides 1.6 1.4

Others 3.0 2.6