Silicate weathering

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. ${\displaystyle H^{+}}$ ${\displaystyle OH^{-}}$

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 : ${\displaystyle K^{+}}$ ${\displaystyle K^{+}}$

${\displaystyle KAlSi_{3}O_{8}+H^{+}+OH^{-}\rightarrow HAlSi_{3}O_{8}+K^{+}+OH^{-}}$

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. ${\displaystyle OH^{-}}$ ${\displaystyle K^{+}}$ ${\displaystyle OH^{-}}$ ${\displaystyle Si-O}$ ${\displaystyle Al-O}$

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

${\displaystyle HAlSi_{3}O_{8}+7H_{2}O\rightarrow Al(OH)_{3}+3H_{4}SiO_{4}}$

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

• Carbonate-silicate cycle

literature

• 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 .