An Introduction to Environmental Chemistry

tetrahedral framework is particularly weak where aluminium has substituted for

silicon, since the aluminium–oxygen bond has more ionic character. The product

released to solution is H 4 SiO 4 (Fig. 4.5). Equation 4.14 expresses quantitatively

the reaction for the sodium (Na)-rich feldspar, albite.

eqn. 4.14

We might conclude that the dominant weathering mechanism of the upper

crust is acid hydrolysis, resulting in a partially degraded and hydrated residue,

with silicic acid and metal bicarbonate dissolved in water. We should, however,

remember that much of the Earth’s continental area is mantled by younger sed-

imentary rocks, including relatively soluble ones, for example limestone. Lime-

stones are common in young mountain belts, such as the European Alps and the

Himalayas, where rates of physical weathering are high. It is therefore probable

that average weathering reactions are really biased toward the weathering of the

sediment cover, rather than average continental crust. Studies of Alpine weath-

ering seem to confirm this. Many Alpine streamwaters are low in dissolved

sodium and H 4 SiO 4 but high in calcium and HCO 3 -. These results suggest that

the dissolution of limestone—not feldspar—is the locally important weathering

reaction.

292

22 4

38 2 2 3

225 4 344

NaAlSi O H O H CO

Al Si O OH Na HCO H SiO

sl aq

s aq aq aq

() () ()

() ()+ -() ()

++ Æ

()++ +

86 Chapter Four

Fig. 4.6Scanning electron micrograph showing square-shaped etch pits developed on

dislocations in a feldspar from a southwestern England granite. Note that in places the pits

are coalescing, causing complete dissolution of the feldspar. Scale bar= 10 mm. Photograph

courtesy of ECC International, St Austell, UK.