Carbonation in Chemical Weathering
Chemical weathering is caused by rain water reacting with the mineral grains in rocks to
form new minerals (clays) and soluble salts. These reactions occur particularly when the
water is slightly acidic. These chemical processes need water, and occur more rapidly at
higher temperature, so warm, damp climates are best. Chemical weathering is the first
stage in the production of soils.
Carbonation, sometimes known as solution, is the process of acid rain dissolving the rock
face, causing the rock to decay and in some cases forming caves. The process starts
when carbon dioxide in the air dissolves in rainwater and becomes weakly acidic. Carbonic
acid once in contact with calcium carbonate produces calcium hydrogen carbonate (also
known as calcium bicarbonate). The calcium bicarbonate which already is dissolved in water
is removed rapidly leaving only impurities of the rock behind. As the acid rain seeps into
cracks and cavities, over many years, carbonation of the rock can form spectacular cave
systems. Here is the chemical equation that occurs when carbonation takes place...
CO2 + H2O -> H2CO3
carbon dioxide + water -> carbonic acid
H2CO3 + CaCO3 -> Ca(HCO3)2
carbonic acid + calcium carbonate -> calcium bicarbonate
Also, stalagmites (on the ground) and stalactites (on the roof) form in caves as water drips
from the roof, depositing some of its dissolved calcium carbonate, as carbon dioxide is
released into the air.
The picture below is an example of a cave formed by carbonation, with both stalagmites and
stalactites present...
By Zak Flesch 10N