Silicate weathering

from Wikipedia, the free encyclopedia

The silicate weathering refers to the hydrolytic decomposition of silicates , so the chemical reaction of mineral components with - and ions of the dissociated water. Compounds that consist of a weak acid and / or a weak base are particularly exposed to hydrolytic decomposition . These are mainly carbonates (which dissociate to form calcium hydrogen carbonate ) and silicates - thus a large part of the rock-forming minerals is subject to hydrolysis.

For the weathering of silicates, the process of hydrolysis will be explained using the example of potash feldspar , as it is one of the most common minerals in the earth's crust with a volume fraction of around 20% .

If the mineral surface comes into contact with water (as a solvent), the ions lying on the edge in the crystal lattice are hydrated (especially the -ions), which leads to a loosening of the bond and subsequently the -ions are split off by the protons of the water :

However, due to the naturally occurring soil pore solutions, the ions are neutralized by the protons of the acids they contain. The ions are washed out , adsorbed or absorbed by the plants as an important soil nutrient . If the accumulation of protons and ions on the crystal lattice of the feldspar progresses, the and bonds within the tetrahedral compounds typical of silicate are broken.

Ultimately, the mineral structure is completely destroyed with the formation of aluminum hydroxide and orthosilicic acid , which are both end products of silicate weathering:

This step of silicate weathering is called desilification , as a large part of the silicate is removed from the solution.

Other silicates, such as mica , hornblende or olivine , are in principle just as affected by these weathering processes. Secondary (clay) minerals ( e.g. kaolinite or illite ) can recrystallize as solid products from the ionic and molecular decomposition products of hydrolytic cleavage . The newly formed minerals can fill the volume of the dissolved mineral and thereby adopt its former shape ( pseudomorphism ) or deposit on the rock surface.

See also


  • Paul Schachtschabel, Fritz Scheffer : Textbook of soil science (=  spectrum textbook ). 15th, revised and expanded edition. Spectrum - Akademischer Verlag, Heidelberg et al. 2002, ISBN 3-8274-1324-9 .
  • Herbert Kuntze , Günter Roeschmann, Georg Schwerdtfeger: Soil Science (=  UTB for science - large series. 8076 ). 5th, revised and expanded edition. Ulmer, Stuttgart 1994, ISBN 3-8252-8076-4 .

Individual evidence

  1. Frank Ahnert: Introduction to Geomorphology . 1st edition. Verlag Eugen Ulmer, Stuttgart 1996, ISBN 3-8252-8103-5 , p. 108, 116 .