CHEMICAL WEATHERING (CONTINUED)

In oxidation reactions we saw a mineral's anion replaced by a different anion and removed.  The third main type of chemical weathering is analogous, but involves the replacement of a cation in a mineral by hydrogen ions, accompanied by the solution and removal of the original cation.  This is probably the most important chemical weathering process simply because so many common silicates are subject to it.  It is called HYDROLYSIS.

The hydrogen ions are supplied, as you might suspect, by acids.  Usually the acid is carbonic acid, though any source of the ions will do.  Acid rain and acid mine drainage, both man-made problems, are particularly effective at this process.

This formula describes what happens to a sodium feldspar during hydrolysis:

2NaAlSi3O8 + 2H2CO3 + 2H2O  ---> Al2Si2O5(OH)4 + 2Na+ + 2HCO3- + 4SiO4-4 + H2O

Feldspar in an acid, in other words, is altered to clay (kaolinite in this case) plus dissolved silica and sodium.  Note that some of the water is added to the clay structure as hydroxide.  The sodium and some of the silica (but not all) are dissolved and carried away by the remaining water.  A potassium or calcium feldspar would weather in the same way, but obviously would produce different dissolved cations.

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