CaAO 2H2CO3aq H2Ol

The formula of the solid product (Al2Si2O5(OH)4) is that for kaolinite, an important 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 situ reprecipitation 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 H2CO3 weathering solution. Natural crystal surfaces have areas of excess electrical charge at their corners and edges, usually because of imperfections (rows of atoms slightly out

Table 4.4 Percentage mineral composition of the upper continental crust. After Nesbit and Young (1984).

Average upper continental crust

Average exposed 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

(a) Anorthite Solution crystal \

(b) Anorthite Solution crystal

(c) Anorthite crystal

Solution

Ca2"

HCO,

HCO,

(d) Anorthite crystal

Solution

0 0

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