Chemical Weathering

Chemical weathering involves changes that alter both the chemical and mineralogical composition of rocks, as minerals are attacked by water and dissolved atmospheric gases - chiefly oxygen and carbon dioxide - which cause some mineral components to dissolve and others to recombine into new mineral phases in place. ^[1] These chemical changes, acting alongside physical weathering, disrupt the fabric of the rock and ultimately produce both a residual accumulation of resistant grains and a loose residue of secondary minerals. ^[1]

Because some water is present in almost every environment, chemical weathering processes are commonly far more important than physical weathering, even in arid climates. ^[1] The low temperatures of the weathering environment - generally below 30°C - mean that reactions proceed slowly, but slow does not mean unimportant. Over geological timescales, chemical weathering reshapes entire landscapes and determines the chemical composition of the ocean. ^[1]

Simple Solution

Simple solution - also called congruent dissolution - occurs when a mineral dissolves completely without precipitation of other substances. ^[1] Highly soluble minerals such as calcite, dolomite, gypsum, and halite, as well as less soluble minerals such as quartz, are dissolved when exposed to rainwater. Chemical bonds between ions in the minerals are broken, releasing constituent ions into solution in surface and groundwater. ^[1]

When carbon dioxide dissolves in rainwater through interaction with atmospheric or soil CO₂ - the normal situation in the weathering environment - the solubilizing ability of the water is enhanced. Dissolved CO₂ forms carbonic acid (H₂CO₃), which dissociates to produce hydrogen ions and bicarbonate ions: CO₂ + H₂O ⇆ H₂CO₃ ⇆ H⁺ + HCO₃^-. ^[1] The increase in H⁺ ions makes meteoric waters more acidic and thus more aggressive dissolution agents, particularly for carbonate minerals. ^[1] Simple solution is especially important in moderately wet climates where carbonate rocks or evaporites are present near the surface or at the water table. ^[1]

Hydrolysis

Hydrolysis is an extremely important chemical reaction between silicate minerals and acids that leads to breakdown of the silicate minerals and release of metal cations and silica, but does not lead to complete dissolution of the minerals. ^[1] Because the dissolved ions do not correspond in proportion to the formula of the original mineral, this type of breakdown is called incongruent dissolution. ^[1] When aluminum is present in the dissolving mineral, clay minerals such as kaolinite, illite, and smectite form as by-products. For example, orthoclase feldspar can break down to yield kaolinite or illite, and albite can decompose to kaolinite or smectite. ^[1] Hydrolysis is the primary process by which silicate minerals decompose during weathering. ^[1]

Oxidation and Reduction

Oxidation of iron and manganese in silicate minerals such as biotite and pyroxenes - driven by oxygen dissolved in water - is an important weathering process because iron is so abundant in the common rock-forming silicate minerals. ^[1] During oxidation, an electron is lost from iron (Fe²⁺ to Fe³⁺), which causes loss of other cations such as Si⁴⁺ from crystal lattices to maintain electrical neutrality. Cation loss leaves vacancies in the crystal lattice that either bring about the collapse of the lattice or make the mineral more susceptible to attack by other weathering processes. ^[1] A geologically important example is pyrite (FeS₂), which is oxidized to form hematite (Fe₂O₃), with the release of soluble sulfate ions. ^[1]

Under conditions where material undergoing weathering is water-saturated, the oxygen supply may be low and the oxygen demand by organisms high. These conditions can bring about reduction of iron - a gain of an electron - from Fe³⁺ to Fe²⁺. Ferrous iron (Fe²⁺) is more soluble, and thus more mobile, than ferric iron (Fe³⁺), and may be lost from the weathering system in solution. ^[1] This reduction-mobility relationship explains why waterlogged soils can lose their iron content while well-drained soils accumulate iron oxides. The colour difference - gleyed grey-green in waterlogged soils versus red-brown in oxidised soils - directly reflects this chemistry.

Hydration, Dehydration, Ion Exchange, and Chelation

Several additional chemical processes can facilitate weathering under certain conditions. Hydration is the addition of water molecules to a mineral to form a new mineral. Common examples are the addition of water to hematite to form goethite, and to anhydrite to form gypsum. Hydration is accompanied by volume changes that may lead to physical disruption of rocks. ^[1] Under some conditions, hydrated minerals may lose their water - a process called dehydration - and revert to their anhydrous forms with an accompanying decrease in mineral volume. Dehydration is relatively uncommon in the weathering environment because water is generally present. ^[1]

Ion exchange is a process whereby ions in a mineral are exchanged with ions in solution - for example, the exchange of sodium for calcium. Most ion exchange involves cations, though anion exchange also occurs. This reaction converts one mineral to another and releases soluble ions into solution. Ion exchange is particularly important in the alteration of one clay mineral to another, such as the alteration of smectite to illite, and in the alteration of one zeolite to another, such as the alteration of heulandite (a Ca-zeolite) to analcime (a Na-zeolite). ^[1]

Chelation involves the bonding of metal ions to organic substances to form organic molecules with a ring structure. ^[1] During weathering, chelation performs the dual role of removing cations from mineral lattices and keeping those cations in solution until they are removed from the weathering site. Chelated metal ions remain in solution under pH conditions and concentrations at which unchelated ions would normally precipitate. ^[1] A natural example is provided by lichens growing on rock surfaces, which increase the rate of chemical weathering by secreting organic chelating agents. Plants also enhance chemical weathering by retaining soil moisture and by acidifying waters through the release of CO₂ and organic acids during decay. ^[1]

Products: Secondary Minerals and Chemical Enrichment

The secondary minerals that develop at the weathering site are dominantly clay minerals, iron oxides or hydroxides, and aluminum hydroxides. The common secondary iron minerals include goethite, limonite, and hematite. ^[1] The specific clay minerals that form depend on the intensity of weathering: illites or smectites form under moderately intense conditions, kaolinite forms under more prolonged and intense leaching, and aluminum hydroxides such as gibbsite and diaspore form under extremely intense weathering. These last minerals are aluminum ores. ^[1]

Comparing the chemical composition of unweathered silicate rocks with their weathering products reveals a net loss of all major cations except aluminum and iron. In their oxidized state, aluminum and ferric iron (Fe³⁺) are both relatively insoluble, so they remain behind while Mg, Ca, Na, and K are lost comparatively much more. As a result, the relative abundance of silica, aluminum, and ferric iron in particulate weathering residues is greater than in the parent source rocks. ^[1] This chemical enrichment of the residue in Al and Fe³⁺ is precisely what produces the characteristic red-brown and grey-white colours of deeply weathered lateritic and bauxitic soils.

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