4 Types and Examples of Chemical Weathering

What is Chemical Weathering?

Chemical weathering is caused by rainwater reacting with the mineral grains in rocks to form new minerals (clays) and soluble salts. These reactions occur particularly when the water is slightly acidic.

It is a decomposition of rocks due to chemical reactions occurring between the minerals in rocks and the environment. The examples below illustrate chemical weathering.

Where does Chemical Weathering occur?

Chemical weathering occurs when rocks undergo chemical reactions to form new minerals. Water, acids, and oxygen are just a few of the chemicals that lead to geological change. Over time, chemical weathering can produce dramatic results.

Chemical weathering (especially hydrolysis and oxidation) is the first stage in the production of soils.

How Chemical Weathering Happens

Chemical weathering does not break rocks into smaller fragments through wind, water, and ice (that’s physical weathering). Nor does it break rocks apart through the action of plants or animals (that’s biological weathering). Instead, it changes the chemical composition of the rock, usually through carbonation, hydration, hydrolysis or oxidation.

Chemical weathering alters the composition of the rock material toward surface minerals, such as clays. It attacks minerals that are relatively unstable in surface conditions, such as the primary minerals of igneous rocks like basalt, granite or peridotite. It can also occur in sedimentary and metamorphic rocks and is an element of corrosion or chemical erosion.

Water is especially effective at introducing chemically active agents by way of fractures and causing rocks to crumble piecemeal. Water may also loosen thin shells of material (in spheroidal weathering). Chemical weathering may include shallow, low-temperature alteration.

Let’s take a look at the four main types of chemical weathering that were mentioned earlier. It should be noted that these are not the only forms, just the most common.

Types of Chemical Weathering

There are different types of chemical weathering processes, such as solution, hydration, hydrolysis, carbonation, oxidation, reduction, and chelation. Some of these reactions occur more easily when the water is slightly acidic.

There are different types of chemical weathering processes, such as solution, hydration, hydrolysis, carbonation, oxidation, reduction, and chelation. Some of these reactions occur more easily when the water is slightly acidic.

Carbonation

Carbonation occurs when rain, which is naturally slightly acidic due to atmospheric carbon dioxide (CO2), combines with calcium carbonate (CaCO3), such as limestone or chalk. The interaction forms calcium bicarbonate, or Ca(HCO3)2.

Rain has a normal pH level of 5.0-5.5, which alone is acidic enough to cause a chemical reaction. Acid rain, which is unnaturally acidic from atmospheric pollution, has a pH level of 4 (a lower number indicates greater acidity while a higher number indicates greater basicity).

Carbonation, sometimes referred to as dissolution, is the driving force behind the sinkholes, caverns and underground rivers of karst topography.

Hydration

Hydration occurs when water reacts with an anhydrous mineral, creating a new mineral. The water is added to the crystalline structure of a mineral, which forms a hydrate.

Anhydrite, which means “waterless stone,” is a calcium sulfate (CaSO4) that is usually found in underground settings. When exposed to water near the surface, it quickly becomes gypsum, the softest mineral on the Mohs hardness scale.  

Hydrolysis

Hydrolysis is the opposite of hydration; in this case, water breaks down the chemical bonds of a mineral instead of creating a new mineral. It is a decomposition reaction.

The name makes this one particularly easy to remember: The prefix “hydro-” means water, while the suffix “-lysis” means decomposition, breakdown or separation.

Oxidation

Oxidation refers to the reaction of oxygen with metal elements in a rock, forming oxides. An easily recognizable example of this is rust. Iron (steel) reacts easily with oxygen, turning into reddish-brown iron oxides. This reaction is responsible for the red surface of Mars and the red color of hematite and magnetite, two other common oxides.

Acidification

You’ve probably heard of acid rain. However, most people don’t know what it is or how it contributes to chemical weathering. Acid rain is water with sulfuric and nitric acids from the burning of coal and fossil fuels, along with volcano eruptions.

The acids create a reaction when they hit stone, causing the surface to wear and the composition to soften. Acidification can also be caused by organisms like lichens, which are created from algae and fungi.

One well-known case of rapid weathering and blackening of stone is the weathering on the 1,000-year-old Leshan Giant Buddha in China. The 232-foot-tall Buddha required six months of repair for weathering after only 12 years due, in part, to acidification.

What type of chemical weathering is enhanced by acid rain?

Carbonation is a type of chemical weathering that is aided by acid rain. When rocks and minerals are altered by hydrolysis, acids may be produced. Acids may also be produced when water reacts with the atmosphere, so acidic water can react with rocks.

The effect of acids on minerals is an example of solution weathering. Solution weathering also covers other types of chemical solutions, such as basic rather than acidic ones.

One common acid is carbonic acid, a weak acid that is produced when carbon dioxide reacts with water. Carbonation is an important process in the formation of many caves and sinkholes. Calcite in limestone dissolves under acidic conditions, leaving open spaces.

Carbonation is the form of chemical weathering.

  • When carbon dioxide is released into the environment by automobiles, it can be converted to carbonic acid.
  • These acids then descend to earth as rain, removing important compounds from the crystalline structures that make up rocks.
  • Simultaneously, sulfur dioxide is transformed to sulphuric acid, and nitrogen dioxide is turned to nitric acid as a result of fossil fuel burning.
  • At high temperatures, these acids continue as vapor, but when the temperature drops, they begin to condense.
  • These acids mingled with rain on their way down to the ground, causing it to become acidic.
  • These acids are particularly harmful to marble, chalk, and limestone, and they cause damage to tombstones and statutes that are made with these kinds of materials.
  • The case study of the yellowing of the Taj Mahal gives the perfect example of this.