8.2 Chemical Weathering
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, and are reversible if the solvent is removed. A household example would be dissolving a teaspoon of table salt (the mineral halite) in a glass of water. The halite will separate into Na+ and Cl– ions. If the water in the glass is allowed to evaporate, there will not be enough water molecules to hold the Na+ and Cl– ions apart, and the ions will come together again to form halite. Gypsum and anhydrite are other minerals that will dissolve in water alone.
Other minerals, such as calcite, will dissolve in acidic water. Acidic water is common in nature, because carbon dioxide (CO2) in the atmosphere reacts with water vapour in the atmosphere, and with water on land and in the oceans to produce carbonic acid (Figure 8.9).
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, as well as adding organic acids. 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%). In the presence of acidic groundwater, limestone can dissolve 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 8.10, downstream of the Mosul Dam in Iraq.
Although the sinkhole in Figure 8.9 might appear minor, it indicates 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. Ongoing measures to fill gaps with grout are required, or else 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. The results can be seen by comparing weathered and unweathered surfaces of the same sample of granite (Figure 8.11). On the recently broken unweathered surface (Figure 8.11, left) feldspar is visible as bright white crystals. On a weathered surface (right) 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 (CaSO4) is transformed into gypsum (CaSO4·2H2O). 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 applied force to an overlying limestone layer, breaking it into pieces. While unbroken limestone is a strong enough material upon which to build a foundation, 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. Olivine reacts with carbonic acid, leaving dissolved iron, bicarbonate, and silicic acid:
Fe2SiO4 + 4H2CO3 → 2Fe2+ + 4HCO3– + H4SiO4
Iron and oxygen dissolved in water react in the presence of bicarbonate to produce hematite and carbonic acid:
2Fe2+ + ½ O2 + 2H2O + 4HCO3– → Fe2O3 + 4H2CO3
When the olivine in basalt is oxidized, the basalt takes on a reddish colour that is distinct from the dark grey or black of unweathered basalt (Figure 8.12).
The oxidation reaction would be similar for other iron-containing silicate minerals such as pyroxene, amphibole, and biotite. Iron in sulphide minerals such as pyrite (FeS2) can also be oxidized in this way.
Hematite is not the only mineral that can result from oxidation. In fact, a wide range of iron oxide minerals that can form in this way, In granite, for example, biotite and amphibole can be altered to form the iron oxide and iron hydroxyoxide minerals that are referred to in combination as limonite (orange material in Figure 8.13).
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. This is because when oxygen and water react with pyrite, sulphuric acid is produced:
2FeS2 + 7O2 + 2H2O → 2FeSO4 + 2H2SO4
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 be toxic to aquatic life 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.
For each of the following reactions, indicate which chemical weathering process—dissolution, hydrolysis, hydration, or oxidation—is the primary mechanism.
- Pyrite → hematite
- Calcite → calcium and bicarbonate ions
- Feldspar → clay
- Olivine → serpentine
- Pyroxene → iron oxide
- Anhydrite → gypsum