Earth

Carbon dioxide: allied with karst

Among the substances that can increase the corrosive power of  natural water, carbon dioxide (CO₂) has the most important role. It is already present in meteoric waters, as it is one of the gases of the atmosphere, however its percentages  are very low (0.03 atm). Its concentration increases greatly in waters that cross through thick layers of soil covered with dense vegetation. Enrichment with  CO₂ and other organic acids produced by vegetation and biological activity  of water coming into contact with rock can lead to a remarkable increase in the solubility of minerals like calcite (calcium carbonate, CaCO₃) and dolomite (calcium carbonate and magnesium, CaMg(CO₃)₂), which increases from 10-12 mg/l to 200-400 mg/l.  Other minerals, such as quartz or rock salt are on the contrary insensitive to the presence of CO₂ in the water they come into contact with, and  their solubility therefore remains unchanged.
The dissolution reaction of calcium carbonate responsible for the formation of most caves is: CaCO₃ + CO₂ + H₂O = Ca₂+ + 2HCO^3- In the case of dolomite, the reaction is very similar : CaMg(CO₃)₂ + 2CO2 + 2H₂O = Ca₂+ + Mg₂+ + 4HCO^3-