What Is Chemical Weathering?
Rocks, soils, minerals, wood, and even artificial materials exposed to the elements of nature like air and water will undergo significant changes over a period of time both in morphology and in chemical composition and ultimately break down into smaller pieces by the processes of weathering.
When weathering occurs through chemical reactions that change the chemical composition of things like rocks, soils, and minerals, the process is then known as chemical weathering. The process takes place gradually and results in the formation of secondary or new minerals.
The most common types of chemical reactions that induce chemical weathering are oxidation, hydrolysis, hydration, carbonation, and reduction. Below, we discuss these processes and inspect the way they affect rocks.
What are the Processes Of Chemical Weathering?
Now that we have a basic idea of what chemical weathering is, let us take a look at the different processes by which chemical weathering of rocks takes place.
In the case of chemical weathering of rocks, oxidation refers to the addition and combination of oxygen to rock minerals. Rocks undergo oxidation when some minerals in the rock react with the oxygen present that is present in soil water or in the atmosphere. In the presence of moisture, the process of oxidation speeds up. As an end result, hydrated oxides are produced.
Most of us are familiar with the oxidation reaction of rusting. It is the result of a reaction whereby iron (in the form of Fe2+) reacts with oxygen to form Fe3+ oxides and hydroxides. Rusting of metallic objects we use in our everyday life often leads to the degradation of the object as the rusted parts become fragile and easily break-off. Similar chemical reactions also occur in rocks with iron content.
Example of an oxidation reaction that takes place in a rock is as follows:
4FeO (Ferrous oxide) + O2 → 2Fe2O3 (Ferric oxide)
4Fe3O4 (Magnetite) + O2 → 6Fe2O3 (Hematite)
Rocks with iron content undergoing oxidation often develop a reddish-brown color which indicates that the rock is undergoing oxidation.
In the presence of moisture, the reaction proceeds further to yield:
2Fe2O3 (Hematite) + 3H2O -> 2Fe2O3 .3H2O (Limonite)
Hydrolysis can be regarded as another important process of chemical weathering. The term hydrolysis is derived from "hydro", meaning water, and "lysis" meaning break-down. The process is driven by the dissociation of water into hydrogen (H+) and hydroxide (OH-) ions. These ions react with minerals present in rocks to induce changes in the composition of the rocks. Silicate and carbonate minerals are most commonly hydrolyzed.
Under ideal conditions, pure water ionizes slightly to yield H+ and OH- ions which then react with minerals to hydrolyze them. Theoretically, if enough water is available, the original mineral will be completely dissolved. For example:
Mg2SiO4 + 4 H+ + 4 OH− ⇌ 2 Mg2+ + 4 OH− + H4SiO4
However, in reality, water rarely donates H+ ions. The situation changes, however, if carbon dioxide is present. The gas readily dissolves in water to form a weak carbonic acid which then acts as a H+ donor.
The reaction is as follows:
Mg2SiO4 + 4 CO2 + 4 H2O ⇌ 2 Mg2+ + 4 HCO3− + H4SiO4
The hydrolysis of feldspar to yield clay minerals is a classic example of chemical weathering of rocks by hydrolysis.
The reaction is as follows:
2 KAlSi3O8 (aluminosilicate feldspar) + 2 H2CO3 + 9 H2O ⇌ Al2Si2O5(OH)4 (Kaolinite, a clay mineral) + 4 H4SiO4 + 2 K+ + 2 HCO3−
Certain chemical constituents of rocks like sodium chloride directly dissolve in water. The dissolution of such substances by water results in softening the rock, eventually breaking it down to pieces. The acidification of water speeds up this process.
Hydration is also one of the processes by which chemical weathering takes place. In hydration, the mineral reacts with water resulting in the rigid attachment of H+ and OH- ions derived from water to the molecules and atoms of the mineral. The absorption of water by the mineral acts to increase the volume of the rock giving birth to physical stresses within the rock and ultimately leads to the breaking down of the rock. Iron oxide, aluminum oxide, anhydrite, etc., are some of the rock minerals that undergo hydration. Two examples of such reaction are provided below:
2Fe2O3 (hematite) + 3H2O → 2Fe2O3.3H2O (limonite)
Al2O3 (bauxite) + 3H2O → Al2O3.3H2O (Hyd. aluminum Oxide)
Carbonation also brings about the chemical weathering of rocks, soils, and minerals. Carbonation refers to the binding of carbon dioxide to substrates via a two-step reaction. First, carbon-dioxide reacts with water to form carbonic acid. Next, carbonic acid reacts with minerals in rocks to produce carbonates or bicarbonates. Carbonation of rocks containing calcium carbonate (limestone) is a common process of chemical weathering which leads to the formation of calcium bicarbonate that is highly soluble in water. Such reactions lead to the formation of hollow spaces in limestone that ultimately form limestone caves. Carbonation reactions are faster in cold temperatures because cold water dissolves more carbon dioxide than warmer water.
The carbonation reaction of limestone is presented below:
The first step in the reaction: formation of carbonic acid by the reaction of water and carbon dioxide.
CO2 + H2O → H2CO3
The second step in the reaction: formation of calcium carbonate by reaction between carbonic acid and calcium carbonate.
H2CO3 + CaCO3 → Ca(HCO3)2
The removal of O2 from a rock mineral resulting in the production of a reduced secondary mineral is also one of the processes of chemical weathering. Reduction reactions are common under waterlogged conditions as the absence or low levels of oxygen in such environments triggers reduction reactions in rock minerals. An example of such a reaction is provided below:
2Fe2O3 (Hematite) - O2 → 4FeO (Ferrous oxide)
End Result Of Chemical Weathering Of Rocks
Chemical weathering by the above processes serves to change the chemical composition of rocks. It also makes the rocks more brittle or renders the rock minerals more soluble in water. Thus, the rocks start to degrade and are ultimately broken down into smaller pieces over a period of time following which the erosional forces remove the pieces of rock from their site of origin and carry them away to far away places with further degradation and dissociation.