Chemical weathering processes

Chemical weathering alters or dissolves minerals in place. Water chemistry and mineral composition determine the reaction.

Weathering changes rock in place. Chemical weathering changes its minerals; erosion removes material. Several reactions can operate together, so identify the reactant, mineral and product rather than treating every chemical process as the same kind of dissolving.

Carbonation begins when carbon dioxide dissolves in water to form weak carbonic acid. This reacts with calcium carbonate in limestone, producing dissolved calcium and bicarbonate ions. The products can be carried away in solution. Soil respiration often supplies more carbon dioxide than the atmosphere alone.

Hydrolysis occurs when water and hydrogen ions react with silicate minerals. Feldspar can alter to clay minerals, releasing dissolved substances. Granite may therefore become weaker as feldspar changes while more resistant quartz grains remain.

Oxidation involves minerals losing electrons, commonly through reactions involving oxygen. Iron-bearing minerals can form iron oxides or hydroxides, giving weathered material a red or brown colour. Reduction is the gain of electrons. In waterlogged, oxygen-poor conditions, microbes can help convert ferric iron to more soluble ferrous forms, allowing iron to be removed and leaving grey zones.

Solution is the dissolving of soluble minerals into water, such as halite dissolving into sodium and chloride ions. Carbonation also leads to dissolution, but specifies the carbonic-acid reaction. Distinguish it from simple solution, and distinguish hydrolysis from physical break-up: hydrolysis creates different minerals.