Chemical Weathering Definition
Chemical weathering is the process of chemical alteration to rocks and minerals due to exposure to air, water, or acid, resulting in dissolution of minerals into water or formation of a new mineral. Although this chemical alteration commonly results in a product that may be more easily transported than the original, transportation is not part of the process of chemical weathering. Chemical weathering is most likely to occur in evaporite, aluminosilicate, and carbonate minerals, as well as metallic metals containing iron.
Examples of Chemical Weathering
One of the most common examples of chemical weathering is the formation of rust on products made of steel. Rust, also called iron oxide, is a crumbly or chalky, red-orange colored mineral that forms when materials containing iron are exposed to water in its liquid form or as water vapor. Once iron oxide begins to form, the chemical reaction (known as oxidation) will continue until the entire piece of steel has rusted. In fact all steel, given enough time, will become entirely composed of iron oxide. Since rust is the result of infiltration of water into a piece steel, the only way to prevent rust is to create a perfect seal around the metal so water in any form cannot reach the mineral components and start a chemical reaction.
In areas where limestone is a large component of the bedrock, cities must plan and mitigate for the destruction caused by sinkholes. If you are not familiar with the term sinkhole, follow the link to the video below.
Groundwater assists in the breakdown of limestone (calcium carbonate) into its components, Calcium and bicarbonate. When the subsurface limestone breaks down it leaves behind empty space, like an underground cave. The sinkhole forms when eventually the surface collapses down into the cave, leaving a deep hole on the surface.
Types of Chemical Weathering
Water is a necessary component in most kinds of chemical weathering, due to its ability to facilitate in chemical reaction. Hydrolysis is one such type of chemical reaction in which ions such as Ca2+, Na2+, or Si2+ are released into solution, ionizing the water and resulting in transformation of one mineral to another. Several types of aluminosilicates, through the transfer of ions during hydrolysis produce clay minerals as a product of chemical weathering. Potassium feldspar, for example, yields the common clay mineral kaolinite through hydrolysis, while pyroxene will break down into chlorite or smectite, and olivine to serpentine.
Other minerals can entirely dissolve into water to be transported in solution. Evaporite minerals (minerals that are deposited by evaporation of water out of solution) such as halite, gypsum, anhydrite, and sylvite are water-soluble. This characteristic makes them particularly mobile, as the process of dissolution, transportation, and precipitation can be ongoing.
Acids, commonly weak acids like carbonic acid or sulfuric acid, assist in the weathering of minerals by facilitating the chemical reactions that result in the dissolution of minerals. Carbonic acid (H2CO3) is introduced to the environment when carbon dioxide (CO2) in the atmosphere dissolves into rainwater (H20), and reacts with rocks that contain calcium carbonate such as limestone and dolomite (as described above in the sinkhole example). Processes such as this where the presence of carbon dioxide assists in chemical weathering are called carbonation.
Acid rain forms in a similar process, when atmospheric sulfur dioxide or nitrogen oxide dissolves into rainwater. The resulting aqueous solutions are sulfuric acid (H2SO4) and nitric acid (HNO3), respectively. Acid rain rapidly increases the rate of weathering on rocks containing carbonate minerals as well as other types of rock, but also has adverse effects on manmade structures and the environment. The introduction of sulfuric or nitric acid causes corrosion to steel structures such as skyscrapers and bridges, and a lower pH from increased acid can damage soils and decrease biodiversity in shallow aquatic ecosystems.
Oxidation occurs when dissolved ions bond with atmospheric oxygen through a chemical reaction facilitated by water, resulting in a new mineral. The most common form of oxidation weathering is likely the production of iron oxide from iron-including minerals such as amphibole, biotite, olivine, pyroxene, etc. In this chemical reaction (which is the same as the rust-forming reaction involving steel mentioned above), the ion Fe2+ is released into solution and bonds with oxygen to form iron oxide (Fe2O3), which is the same as the mineral hematite.
When oxidation takes place in the presence ferrosulfide minerals such as pyrite (FeS2), the chemical reaction produces sulfuric acid (H2SO4) as a byproduct. As described in the acid rain example above, this is a huge environmental concern when present in large quantities. The term acid rock drainage describes this event as it occurs at metal mines where even small amounts of pyrite can produce a dangerous amount of sulfuric acid. The results can be detrimental to local ecosystems and the damage can have a wide reach when acid is transported via rivers, streams and groundwater.
Causes of Chemical Weathering
The effects of chemical weathering are likely to be more intense in warm, wet climates, where atmospheric conditions accelerate the rate of chemical reactions. Chemical weathering can also be accelerated in conditions where physical weathering is also present, which helps to expose bedrock as well as break it down into pieces that can be dissolved more quickly.