An Introduction to Environmental Chemistry

of place) or damage (broken bonds). Areas of excess negative charge are prefer-
entially attacked by soil acids, resulting in the formation of etch pits on the
mineral surface (Fig. 4.6). Hydrogen ions dissociated from H 2 CO 3 hydrate the
silicate surface. The ionic bonds between Ca^2 +and SiO 4 tetrahedra are easily
severed, releasing Ca^2 +into solution. The result is a metal-deficient hydrated sil-
icate and a calcium bicarbonate (Ca^2 ++2HCO 3 - ) solution. Continued reaction
may break the more covalent bonds within the tetrahedral framework. The

The Chemistry of Continental Solids 85

H 2 CO 3 H+ + HCO 3

H+

H+

H+

• 
  

H+

• 
  
  
  
  • H+
  
  
  
  


• H+

H+

Ca2+

Ca2+

Anorthite

crystal

(a)

Ca2+

Ca2+

Anorthite

crystal

(b)

• 
  

Solution

H+

H+

• 
  
  
  
  • 
    
  
  
  
  


• –


• H+


• H+


• H+

HCO 3 –

Solution

Ca2+

Ca2+

Anorthite

crystal

(c)

Ca2+

Anorthite

crystal

Solution (d)

• 
  

  –

  
  


• H+

H+

• 
  

Solution

H+

H+

HCO 3 – HCO 3 –

H+

• 
  

H+–

Oxygen

Silicon

Aluminium

Bond

Crystal surface

H+

Fig. 4.5Weathering reactions at the surface of a feldspar (after Raiswell et al. 1980). (a)
Broken bonds become protonated by H+dissociated from carbonic acid and ionic-bonded
Ca^2 +is released to solution. (b) Protonated lattice. (c) Further severing of ionic bonds causes
complete protonation of the edge tetrahedron. (d) Edge tetrahedron is completely removed
to solution as H 4 SiO 4.