Carbonation

Carbonation is the chemical reaction that occurs when the mineral calcite, or calcium carbonate (CaCO3), comes in contact with an acid. In the natural environment, where rotting organic matter occurs in soils, the soil water reacts with the carbon dioxide from the organic matter to produce carbonic acid:

In turn, the weak carbonic acid that is produced then disassociates or comes apart into smaller atomic particles and compounds:

These materials can react in turn with the calcium carbonate. In this case, the hydrogen ion, H+, is a proton, which is an extremely small particle from the hydrogen nucleus that can work its way into many mineral particles and disrupt them. In the case of the disruption of the solid calcium carbonate mineral or the limestone bedrock, the disassociated ions of the carbonic acid disrupt the atomic matrix and recombine to form the new compound of calcium bicarbonate, which is then dissolved in water.

In this fashion, the carbonate rocks of the world (sedimentary limestone and dolostone, or the crystalline rocks of metamorphic marble and igneous carbonatite) are subject to the carbonation reaction, and caves or caverns result. As the rocks dissolve, the rock is carried away in the water in solution as ions of Ca+ and HCO3∼. Thus, the solid calcium carbonate rock goes into solution and is carried away in underground waters as Ca(HCO3)2, to eventually end up in the ocean as one of the many kinds of salts in the sea. Organisms in the sea such as coral, bivalves, and other sea life use this dissolved calcium carbonate to make their reefs and shells.

The double arrows in the above reaction equations (Equations 1, 2, and 3) indicate that these reactions are reversible, that is, they can go in both directions, with the result that the carbon dioxide (CO2) gas that is dissolved in liquids can come out of the solution as the effervescence or “fizz” of carbonated beverages. Thus, in situations in which surface waters containing a high proportion of dissolved calcium carbonate are agitated in rapids as they flow over an uneven stream-bed, they can lose some of their dissolved CO2 to the atmosphere and force the reactions to move CaCO3 back out of solution and into precipitation as solid carbonate rock. This is the same process inside caves wherein the carbonate-bearing water moves through the fissures and pore spaces until it arrives on the surface of the underground opening, loses some CO2, and thus produces stalactites that hang from the ceiling, stalagmites growing up from the floor, and various forms of flowstone, dripstone, and other cave speleothems that are produced in these carbonation reactions. Thus, first carbonation reactions produce caves, mostly below the water table in the limestone bedrock, and then, once the water table drops and exposes the new cave to the atmosphere, the process reverses, and the cave is then in-filled back again with new travertine calcium carbonate.