Chemical weathering occurs when rocks are weakened and broken down through chemical reactions. This process involves a chemical change, which actually alters the rock's or mineral’s chemical composition. Chemical weathering is more common in wet, humid areas than in dry ones, because moisture is an important component of many types of chemical weathering.
Reacting With Oxygen
The reaction between rocks and oxygen is known as oxidation. When elements or compounds in rocks react with oxygen and water, they form substances called oxides. One of the most common examples of oxidation is iron oxide, or rust. Rust is a reddish-brown color, and it is soft and crumbly, which makes the oxidized rock more susceptible to other forms of weathering. A change of color from silvery iron to the reddish-brown iron oxide is a good indicator a chemical change has occurred.
Dissolving in Acid
When carbon dioxide in the air dissolves in water, it forms carbonic acid. While carbonic acid is fairly weak, it can cause some chemical weathering known as carbonation. For example, calcite is a calcium carbonate mineral made up of calcium, carbon and oxygen. When it reacts with carbonic acid, the calcium carbonate breaks down into its components, calcium and bicarbonate. This type of chemical weathering is particularly important in the creation of karst topography, such as caves and sinkholes. Limestone, which is largely made up of calcium carbonate, reacts with underground water. When the water breaks down and dissolves the rock, caves develop in the space left underground. When the space underground gets too big, the land at the surface can collapse, forming a sinkhole.
Mixing With Water
Hydrolysis is a form of chemical weathering caused by water. The weathering of feldspar, which turns into clay when it reacts with water, is one of the most common examples of hydrolysis. Ions in the feldspar, a mineral often found in granite, are dissolved by the water. These ions react with the water to form clay minerals.
Hydration occurs when a mineral absorbs water to form a new substance. Hydration causes the rock to expand its volume, which can put stress on the rock and make it more vulnerable to other types of weathering. Two examples of hydration include the creation of gypsum from anhydrite and the formation of limonite from hematite.
While hydration adds water to form a rock with a new chemical structure, dehydration involves the removal of water from rocks. The addition of water to hematite, or hydration, forms limonite; in reverse, the removal of water from limonite, or dehydration, results in hematite.