In hydration certain rocks, chiefly those containing salt minerals, are capable of absorbing water into their structure, causing them to swell and become vulnerable to future breakdown. An example of hydration is the reversible change between anhydrite and gypsum: .
CaSO4 + 2H2O .
CaSO4 2H2 O.
Anhydrite Gypsum.
Since hydration is accompanied by volume increase, the growth of hydrated minerals in rocks can exert pressure which may cause the rocks to break up. According to Waugh this process appears to be most active following successive periods of wet and dry weather and is important in forming clay particles. Hydration is in fact a physio-chemical process as the rocks may swell and exert pressure as well as changing their chemical structure. According to McLeish, the most important weathering reaction is hydrolysis in which minerals break down by reacting with water. Under acidic conditions clay minerals may be produced by the hydrolysis of feldspar. Hydrogen in water reacts with minerals in the rock or there is a combination of the H+ and OH- ions in the water and the ions of the mineral. The rate of hydrolysis depends on the amount of H+ ions, which in turn depends on the composition of air and water in the soil, the activity of organisms, the presence of organic acids and the cation exchange. .
Oxidation occurs when rocks are exposed to oxygen in the air or water. According to Haldar the oxidation occurs mainly above the basic level of groundwater table in areas with steep relief and warm climate. The most recognised example of oxidation is when iron in a ferrous state is changed by the addition of oxygen into a ferric state. The rock or soil, which may have been blue or grey in colour (characteristic of a deficiency of oxygen), is discoloured into a reddish-brown- a process better known as rusting. However, in waterlogged areas, oxidation may operate in reverse and is known as reduction.
