When a rock is brought to the surface millions or billions of years after it has formed, the original minerals that were crystallized deep in the crust under high pressures and temperatures are
unstable in the surface environment and eventually break down. The primary agents in chemical weathering are water, oxygen, and acids. These react with surface rocks to form new minerals that are stable in, or in
equilibrium with, the physical and chemical conditions present at the earth's surface. Any excess ions left over from the chemical reactions are carried away in the acidic water. For example, feldspar minerals will weather to clay minerals, releasing silica, potassium, hydrogen, sodium, and calcium. These elements remain in solution and are commonly found in surface water and groundwater. Newly deposited sediments are often cemented by calcite or quartz that is precipitated between the sediment grains from calcium‐ and silicabearing water, respectively.
How quickly chemical weathering breaks a rock down is directly proportional to the area of rock surface exposed. Thus, it is also related to mechanical weathering, which creates more exposed surface area by breaking the rock down into pieces, and those pieces into smaller pieces. The greater the number of pieces, the greater the total amount of surface area exposed to chemical weathering.
Water. Chemical weathering is most intense in areas that have abundant water. Different minerals weather at different rates that are climate dependent. Ferromagnesian minerals break down quickly, whereas quartz is very resistant to weathering. In tropical climates, where rocks are intensely weathered to form soils, quartz grains are typically the only component of the rock that remains unchanged. Alternatively, in dry desert climates, minerals normally susceptible to weathering in wet environments (such as calcite) can be much more resistant.
Acids. Acids are chemical compounds that decompose in water to release hydrogen atoms. Hydrogen atoms frequently substitute for other elements in mineral structures, breaking them down to form new minerals that contain the hydrogen atoms. The most abundant natural acid is carbonic acid, a weak acid that consists of dissolved carbon dioxide in water. Rainwater usually contains some dissolved carbon dioxide and is slightly acidic. The burning of coal, oil, and gasoline releases carbon dioxide, nitrogen, and sulfur into the atmosphere, which react with rainwater to form much stronger carbonic, nitric, and sulfuric acids that damage the environment (acid rain).
Other acids that can affect the formation of minerals in the nearsurface weathering environment are organic acids derived from plant and humus material. Strong acids that are naturally occurring in the environment are rare—they include the sulfuric acids and hydrofluoric acids released during volcanic and hot spring activity.
Solution weathering is the process by which certain minerals are dissolved by acidic solutions. For example, calcite in limestone is dissolved easily by carbonic acid. Rain that percolates through cracks and fissures in limestone beds dissolves calcite, making wider cracks that can ultimately develop into cave systems.
Oxygen. Oxygen is present in air and water and is an important part of many chemical reactions. One of the more common and visible chemical weathering reactions is the combination of iron and oxygen to form iron oxide (rust). Oxygen reacts with iron‐bearing minerals to form the mineral hematite (Fe2O3) , which weathers a rusty brown. If water is included in the reaction, the resultant mineral is called Iimonite (Fe2O3· nH2O) , which is yellow‐brown. These minerals often stain rock surfaces a reddish‐brown to yellowish color.