Chemical weathering results from chemical changes to minerals that become unstable when they are exposed to surface conditions. The kinds of changes that take place are specific to the mineral and the environmental conditions. Some minerals, like quartz, are virtually unaffected by chemical weathering. Others, like feldspar, are easily altered.
Types of Chemical Weathering Reactions
Dissolution reactions produce ions, but no minerals. A household example would be dissolving a teaspoon of table salt (the mineral halite) in a glass of water. Some minerals will dissolve in water alone. In addition to halite these minerals include gypsum and anhydrite.
Other minerals, such as calcite, will dissolve in acidic water. Acidic water is easier to come by than you might think, because carbon dioxide (CO2) reacts with water in the atmosphere, on land, and in the oceans to produce carbonic acid as follows (Figure 8.8):
While rainwater and atmospheric CO2 can combine to create carbonic acid, the amount of CO2 in the air is enough to make only very weak carbonic acid. In contrast, biological processes acting in soil can result in a much higher concentration of CO2 within soil. Water that percolates through the soil can become significantly more acidic.
Calcite is a major component of the sedimentary rock called limestone (typically more than 95%), which means that limestone dissolves at relatively shallow depths underground. Over time the dissolution can remove enough of the calcite to form caves.
If dissolution of limestone or other materials removes enough rock to undermine support near the surface, the surface may collapse, creating a sinkhole such as the one in Figure 8.9, downstream of the Mosul Dam in Iraq.
Although the sinkhole in Figure 8.9 might appear minor, it reflects a serious problem. The dam itself is constructed on limestone supported by beds of gypsum and anhydrite. Gypsum and anhydrite are soluble in water, and the gypsum and anhydrite beneath the dam are rapidly dissolving away. This was the case prior to construction of the dam. However, once the dam was filled the increased water pressure began to force water through the formations much faster, accelerating dissolution. At present there is a grave risk of catastrophic failure, placing nearly 1.5 million people at risk.
The term hydrolysis combines the prefix hydro, referring to water, with lysis, which is derived from a Greek word meaning to loosen or dissolve. Thus, you can think of hydrolysis as a chemical reaction where water loosens the chemical bonds within a mineral. This might sound the same as dissolution but the difference is that hydrolysis produces a different mineral in addition to ions. An example of hydrolysis is when water reacts with potassium feldspar to produce clay minerals and ions:
Hydrolysis Reaction of Potassium Feldspar
2KAlSi3O8 + 3H2O → Al2Si2O5(OH)4 + 4SiO2 + 2K+ + 2OH–
Potassium feldspar is broken down by water to produce kaolinite (a clay mineral), quartz, and ions.
The hydrolysis of feldspar to clay is illustrated in Figure 8.10, which shows two surfaces of the same granite sample. On the left is a recently broken unweathered surface, where feldspar is visible as white crystals. On the right is a weathered surface where the feldspar has been altered to the chalky-looking clay mineral kaolinite.
Silicate minerals other than feldspar can undergo hydrolysis, but with different end results. For example, pyroxene can be converted to the clay minerals chlorite or smectite. Olivine can be converted to the clay mineral serpentine.
Hydration reactions involve water being added to the chemical structure of a mineral. An example of a hydration reaction is when anhydrite is transformed into gypsum. A consequence of hydration is that the resulting mineral has a greater volume than the original mineral. In the case of the Mosul Dam, hydration of anhydrite has important consequences. The increase in volume put force on the overlying limestone layer, breaking it into pieces. While unbroken limestone is a strong enough material upon which to build a foundation, the broken limestone is too weak to provide a safe foundation.
Oxidation happens when free oxygen (i.e., oxygen not bound up in molecules with other elements) is involved in chemical reactions. Oxidation reactions provide valuable insight into Earth’s early surface conditions because there is a clear transition in the rock record from rocks containing no minerals that are products of oxidation reactions, to rocks containing abundant minerals produced by oxidation. This reflects a transition from an oxygen-free atmosphere to an oxygenated one.
In iron-rich minerals such as olivine, the oxidation reaction begins with taking iron out of the mineral and putting it into solution as an ion. Once it’s an ion, oxygen reacts with it. The steps in the reaction for olivine are shown below.
Oxidation Reaction of Olivine to Hematite
Fe2SiO4 + 4H2CO3 → 2Fe2+ + 4HCO3– + H4SiO4
Olivine reacts with carbonic acid, leaving dissolved iron, bicarbonate, and silicic acid.
2Fe2+ + ½ O2 + 2H2O + 4HCO3– → Fe2O3 + 4H2CO3
Iron and oxygen dissolved in water react in the presence of bicarbonate to produce hematite and carbonic acid.
The oxidation reaction would be similar for other iron-containing silicate minerals such as pyroxene, amphibole, and biotite. Iron in sulphide minerals (e.g., pyrite, FeS2) can also be oxidized in this way.
The mineral hematite is not the only possible end result of oxidation. There is a wide range of iron oxide minerals that can form in this way. For example, Figure 8.11 shows granite in which some of the biotite and amphibole have been altered to form the iron oxide minerals in limonite  (the yellowish colour).
Oxidation Reactions and Acid Rock Drainage
Oxidation reactions can pose an environmental problem in areas where rocks have elevated levels of sulphide minerals such as pyrite, because a by-product of the reaction is sulphuric acid.
The runoff from areas where this process is taking place is known as acid rock drainage (ARD), and even a rock with 1% or 2% pyrite can produce significant ARD. Some of the worst examples of ARD are at metal mine sites, especially where pyrite-bearing rock and waste material have been mined from deep underground and then piled up and left exposed to water and oxygen. In these cases the problem is referred to as acid mine drainage. One example is the Mt. Washington Mine near Courtenay on Vancouver Island (Figure 8.12), but there are many similar sites across Canada and around the world.
At many ARD sites, the pH of the runoff water is less than 4 (very acidic). Under these conditions, metals such as copper, zinc, and lead easily dissolve in water, which can lead to toxicity for aquatic and other organisms. For many years, the river downstream from the Mt. Washington Mine had so much dissolved copper in it that it was toxic to salmon. Remediation work has since been carried out at the mine and the situation has improved.
Exercise 8.2 Chemical Weathering
For each of the reactions in the table below, indicate which of the chemical weathering processes- dissolution, hydrolysis, hydration, or oxidation- is primarily responsible.
|Pyrite to hematite|
|Calcite to calcium and bicarbonate ions|
|Feldspar to clay|
|Olivine to serpentine|
|Anhydrite to gypsum|
|Pyroxene to iron oxide|
Controls on Weathering Processes and Rates
Weathering does not happen at the same rate in all environments. The same types of weathering do not happen in all environments. There are a variety of factors that determine what kinds of weathering will occur, and how fast the processes will proceed.
Water and temperature are key factors controlling both weathering rates and the types of weathering that happen. For example:
- Water is required for chemical weathering reactions to occur.
- Water must be present for ice wedging to happen.
- Higher temperatures speed up chemical reactions.
- Climate will determine whether water is present mostly in liquid form, solid form (ice), or as both.
- Climate will determine what plant life is available to force rocks apart with their roots, and to contribute organic acids to soils to aid in chemical weathering.
This means, for example, that chemical weathering will be faster in a tropical rainforest than in the Antarctic, a cold desert. It means physical weathering will be the predominant form of weathering in the Antarctic.
Oxygen and Carbon Dioxide
The presence and abundance of oxygen and carbon dioxide affect chemical weathering rates. Surface environments on Earth almost all have some oxygen, permitting oxidation reactions to take place. Exceptions are in settings such as deep lakes or swamps where oxygen cannot easily mix into the water, or biological processes use it up rapidly.
Carbon dioxide, which acidifies water and contributes to chemical weathering, is everywhere, but more concentrated in some settings than others. For example, soils can have very high concentrations of carbon dioxide, whereas carbon dioxide concentrations will be lower on surfaces free of soils and exposed to the atmosphere.
The minerals making up a rock will determine what kinds of chemical weathering reactions are possible. Under the same conditions, dissolution reactions happening to calcite making up limestone will occur more rapidly than hydrolysis reactions happening to feldspar in granite.
Some minerals are very resilient to chemical weathering in general compared to others. For example, quartz is very resilient to chemical weathering whereas calcite is not. Under the same conditions, a rock with grains cemented together with calcite will weather faster than a rock with grains cemented together with quartz.
When rocks in an outcrop weather at different rates, the result is differential weathering. In Figure 8.13 some of the beds are recessed further into the outcrop than others. The recessed beds are weathering faster than the surrounding beds.
Weathering Makes Weathering Go Faster
Weathering accelerates weathering. Physical weathering breaks rocks into smaller pieces or forms cracks. Newly exposed surfaces can be acted upon by chemical weathering, and physical weathering processes such as wedging can expand cracks further. Chemical weathering weakens rock making it more susceptible to physical weathering processes.
- Limonite isn't technically a mineral. It is a way to refer to iron oxide minerals which occur together in a mass, without being specific about what those minerals are. ↵