Ch. 2] WEATHERING AND CLAY FORMATION 55 



ence of ground water usually contain carbonates regardless of the 

 nature of the parent material. 



Weathering and Clay Formation 



Many of the common soil-forming minerals, such as feldspars, micas, 

 pyroxenes, consist of chains and networks of tetrahedra and octahedra 

 whose corners are occupied by = and OH - ions. The small inter- 

 stices in the centers of the tetrahedra are occupied by Si ++++ or Al +++ 

 ions. Inside the octahedra are located Al +++ , Mg ++ , Fe +++ , and 

 Fe ++ ions. These negatively charged oxygen and hydroxyl polyhedra 

 share corners and edges, and they are balanced and held together by 

 positive cations, especially K + , Na + , Ca ++ , Mg ++ . 



Whereas the interior of any crystal is in electrical equilibrium (Paul- 

 ing's rules), the surfaces of many crystals are composed of ions whose 

 valences are not completely satisfied. For an orthoclase crystal, which 

 consists of joined Si- and Al-tetrahedra and K ions in intertetrahedral 

 cavities, the surface may be schematically depicted as in Fig. 10 (left 

 side) . 



H 2 H 2 HOH H H KOH 



i j \ H H \ 



-O-Si-O-AI-O-K-0 ^ -0-Si-0-AI-0-H-0- 



Crystal surface Crystal surface 



Fig. 10. Schematic presentation of orthoclase surface reacting with water. 

 Hydration of oxygen ions not shown. 



Upon the addition of water two reactions may occur. First, hydra- 

 tion, whereby water molecules (dipoles) are attracted to the unsatis- 

 fied valences of exposed Si and Al ions. The polarization of the at- 

 tracted water molecules may become so strong that some of the H 

 ions are expelled. They may become attached to exposed ions 

 which are thus converted to OH ions. The exposed polarizing Si and 

 Al ions become surrounded by OH ions (water molecules minus H). 



The second reaction, proceeding simultaneously and independently 

 of hydration, consists of an ion exchange (hydrolysis) between ex- 

 posed K ions of the lattice and H ions of water, as follows: 



Orthoclase K+ -f H+OH - ;=± lOrthoclasel H+ + K+OH - 



The liquid phase acquires alkalinity (pH 9-11), and the crystal sur- 

 face gains H ions, which tend to combine with 0= to form OH - . 



As a result of hydration and hydrolysis, the exposed oxygen tetra- 

 hedra become partial hydroxyl tetrahedra. Aluminum tends to attract 



