Chemical weathering involves the chemical breakdown of bedrock and the formation of new mineral products. A detailed treatment of the processes in chemical weathering is found in Anderson and Anderson (2010, p. 183–202), and a very brief overview is provided here.
The main chemical weathering processes are:
1. Water acts as a solvent in the process of solution. For example, water easily dissolves gypsum (CaSO₄). The equilibrium solubility of a mineral expresses its tendency to dissolve in water. Temperature and pH of the local environment affect this solubility.
For example, quartz (SiO2) has a low but finite solubility below a pH of 10, and is highly soluble in very alkaline water above this value. Alumina (Al2O3) is only soluble in conditions seldom found in nature, below a pH of 4 and above a pH of 9.
As a result, weathering causes alumina to accumulate as a residue, while silica may slowly leach away. In contrast, the solubility of calcium carbonate (CaCO₃) steadily decreases in alkaline waters.
However, the natural environment rarely exhibits the low solubility of CaCO₃ in pure water because dissolved CO₂ in water replaces CaCO₃ with calcium bicarbonate Ca(HCO₃)₂, which is highly soluble (see ‘Acid hydrolysis’)
Other ions from various sources commonly affect the solubility of a particular mineral. This complicates the calculation of the equilibrium solubility of mixtures of minerals and solutes, as seen in the chemical weathering of common aluminosilicates that make up much of the continental crust.
2. Oxidation and reduction involve the gain or loss of charge by the addition (reduction) or loss (oxidation) of negatively charged electrons. The oxygen dissolved in water is the most common oxidizing agent.
Oxidation forms oxides and hydroxides, as when sulfides such as iron pyrite (FeS₂) oxidize under anaerobic conditions, producing sulfuric acid and iron hydroxide. Bacteria oxidize organic matter in soils, producing CO₂ and generating acidity.
The hydrolysis of minerals then utilizes the acidity (see ‘Acid hydrolysis’). The redox potential (Eh), measured in millivolts, indicates the tendency for oxidation and reduction to take place.
3. Hydration occurs when the crystal lattice absorbs water, increasing its porosity and making it more susceptible to weathering. A common example is when the iron oxide hematite transforms into the hydrated iron hydroxide limonite.
4. Acid hydrolysis is the reaction of a mineral with acidic weathering agents, where the acidity is mainly derived from the dissociation of atmospheric CO2 in rainwater and soil zones by respiration of plant roots and bacterial decomposition of plants, producing in both cases carbonic acid (H2CO3).
In hydrolysis, the hydrogen or hydroxyl ions of water replace metal cations like K+, Na+, Ca2+, and Mg2+ in the crystal lattice. The process releases these cations, which then combine with additional hydroxyl ions, often forming clay minerals.
Examples are the hydrolysis of albite (plagioclase feldspar NaAlSi3O8) to kaolinite (Al4Si4)O10(OH)8, and the hydrolysis of orthoclase feldspar (KAlSi3O8) to illite (K2Al4(Si6Al2O20)(OH4)).
Such reactions, in general, produce a clay mineral residue plus the release of silica, metal cations, and bicarbonate ions in solution. Acid hydrolysis involving CO2 is commonly termed carbonation. Carbonation dominates the weathering of limestones.
