Editing Weathering (section)

=={{anchor|Chemical weathering}}Chemical==
[[File:Weathering Limestone State College PA.jpg|thumb|Comparison of unweathered (left) and weathered (right) limestone]]
Most rock forms at elevated temperature and pressure, and the minerals making up the rock are often chemically unstable in the relatively cool, wet, and oxidizing conditions typical of the Earth's surface. Chemical weathering takes place when water, oxygen, carbon dioxide, and other chemical substances react with rock to change its composition. These reactions convert some of the original ''primary'' minerals in the rock to ''secondary'' minerals, remove other substances as solutes, and leave the most stable minerals as a chemically unchanged ''resistate''. In effect, chemical weathering changes the original set of minerals in the rock into a new set of minerals that is in closer equilibrium with surface conditions. True equilibrium is rarely reached, because weathering is a slow process, and leaching carries away solutes produced by weathering reactions before they can accumulate to equilibrium levels. This is particularly true in tropical environments.{{sfn|Blatt|Middleton|Murray|1980|pp=245-246}}

Water is the principal agent of chemical weathering, converting many primary minerals to clay minerals or hydrated oxides via reactions collectively described as [[hydrolysis]]. Oxygen is also important, acting to [[Redox|oxidize]] many minerals, as is carbon dioxide, whose weathering reactions are described as [[carbonation]].{{sfn|Blatt|Middleton|Murray|1980|pp=246}}

The process of mountain block uplift is important in exposing new rock strata to the atmosphere and moisture, enabling important chemical weathering to occur; significant release occurs of Ca<sup>2+</sup> and other ions into surface waters.<ref>Hogan, C. Michael (2010) [http://www.eoearth.org/article/Calcium?topic=49557 "Calcium"], in A. Jorgenson and C. Cleveland (eds.) ''[[Encyclopedia of Earth]]'', National Council for Science and the Environment, Washington DC</ref>

===Dissolution===

[[File:Weathered limestone cores.jpg|thumb|[[Limestone]] [[core samples]] at different stages of chemical weathering, from very high at shallow depths (bottom) to very low at greater depths (top). Slightly weathered limestone shows brownish stains, while highly weathered limestone loses much of its carbonate mineral content, leaving behind clay. Limestone drill core taken from the carbonate West Congolian deposit in [[Kimpese]], [[Democratic Republic of Congo]].]]

Dissolution (also called ''simple solution'' or ''congruent dissolution'') is the process in which a mineral dissolves completely without producing any new solid substance.<ref>{{cite book |last1=Birkeland |first1=Peter W. |title=Soils and geomorphology |date=1999 |publisher=Oxford University Press |location=New York |isbn=978-0195078862 |page=59 |edition=3rd}}</ref> Rainwater easily dissolves soluble minerals, such as [[halite]] or [[gypsum]], but can also dissolve highly resistant minerals such as [[quartz]], given sufficient time.<ref>{{cite book |last1=Boggs |first1=Sam |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, N.J. |isbn=0131547283 |edition=4th |page=7}}</ref> Water breaks the bonds between atoms in the crystal:<ref name="Environmental Studies in Sedimentar">{{cite journal|jstor=43418626|title=Environmental Studies in Sedimentary Geochemistry|last1=Nicholls|first1=G. D.|journal=Science Progress (1933- )|year=1963|volume=51|issue=201|pages=12–31}}</ref>

[[File:SiO H2O.jpg|Hydrolysis of a silica mineral]]

The overall reaction for dissolution of quartz is
:{{chem2|SiO2 + 2 H2O -> H4SiO4}}

The dissolved quartz takes the form of [[silicic acid]].

A particularly important form of dissolution is carbonate dissolution, in which atmospheric [[carbon dioxide]] enhances solution weathering. Carbonate dissolution affects rocks containing [[calcium carbonate]], such as [[limestone]] and [[chalk]]. It takes place when rainwater combines with carbon dioxide to form [[carbonic acid]], a [[weak acid]], which dissolves calcium carbonate (limestone) and forms soluble [[calcium bicarbonate]]. Despite a slower [[reaction kinetics]], this process is thermodynamically favored at low temperature, because colder water holds more dissolved carbon dioxide gas (due to the retrograde [[solubility]] of gases). Carbonate dissolution is therefore an important feature of glacial weathering.<ref>{{cite journal |last1=Plan |first1=Lukas |title=Factors controlling carbonate dissolution rates quantified in a field test in the Austrian alps |journal=Geomorphology |date=June 2005 |volume=68 |issue=3–4 |pages=201–212 |doi=10.1016/j.geomorph.2004.11.014|bibcode=2005Geomo..68..201P }}</ref>

Carbonate dissolution involves the following steps:

:CO<sub>2</sub> + H<sub>2</sub>O → H<sub>2</sub>CO<sub>3</sub>
:carbon dioxide + water → carbonic acid

:H<sub>2</sub>CO<sub>3</sub> + CaCO<sub>3</sub> → Ca(HCO<sub>3</sub>)<sub>2</sub>
:carbonic acid + calcium carbonate → calcium bicarbonate

Carbonate dissolution on the surface of well-jointed limestone produces a dissected [[limestone pavement]]. This process is most effective along the joints, widening and deepening them.<ref name="Geology and geomorphology">{{cite web|url=http://www.limestone-pavements.org.uk/geology.html|title=Geology and geomorphology|last=Anon|work=Limestone Pavement Conservation|publisher=UK and Ireland Biodiversity Action Plan Steering Group|accessdate=30 May 2011|archive-url=https://web.archive.org/web/20110807234809/http://www.limestone-pavements.org.uk/geology.html|archive-date=7 August 2011|url-status=dead}}</ref>

In unpolluted environments, the [[pH]] of rainwater due to dissolved carbon dioxide is around 5.6.  [[Acid rain]] occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere.  These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. [[Sulfur dioxide]], SO<sub>2</sub>, comes from volcanic eruptions or from fossil fuels, and can become [[sulfuric acid]] within rainwater, which can cause solution weathering to the rocks on which it falls.<ref>{{cite journal |last1=Charlson |first1=R. J. |last2=Rodhe |first2=H. |title=Factors controlling the acidity of natural rainwater |journal=Nature |date=February 1982 |volume=295 |issue=5851 |pages=683–685 |doi=10.1038/295683a0|bibcode=1982Natur.295..683C |s2cid=4368102 }}</ref>

===Hydrolysis and carbonation===
[[File:Iddingsite.JPG|thumb|right|[[Olivine]] weathering to [[iddingsite]] within a [[Mantle (geology)|mantle]] [[xenolith]]]]
[[Hydrolysis]] (also called ''incongruent dissolution'') is a form of chemical weathering in which only part of a mineral is taken into solution. The rest of the mineral is transformed into a new solid material, such as a [[clay mineral]].{{sfn|Boggs|2006|pp=7-8}} For example, [[forsterite]] (magnesium [[olivine]]) is hydrolyzed into solid [[brucite]] and dissolved silicic acid:

:Mg<sub>2</sub>SiO<sub>4</sub> + 4 H<sub>2</sub>O ⇌ 2 Mg(OH)<sub>2</sub> + H<sub>4</sub>SiO<sub>4</sub>
:forsterite + water ⇌ brucite + silicic acid

Most hydrolysis during weathering of minerals is ''acid hydrolysis'', in which protons (hydrogen ions), which are present in acidic water, attack chemical bonds in mineral crystals.{{sfn|Leeder|2011|p=4}} The bonds between different cations and oxygen ions in minerals differ in strength, and the weakest will be attacked first. The result is that minerals in igneous rock weather in roughly the same order in which they were originally formed ([[Bowen's Reaction Series]]).{{sfn|Blatt|Middleton|Murray|1980|p=252}} Relative bond strength is shown in the following table:<ref name="Environmental Studies in Sedimentar"/> 
{| class="wikitable" style="text-align: left;"
|+<!--Bond strengths between oxygen and common cations-->
! Bond
! Relative strength
|-
| Si&ndash;O
|2.4
|-
| Ti&ndash;O
|1.8
|-
| Al&ndash;O
| 1.65
|- 
| Fe<sup>+3</sup>&ndash;O
|1.4
|- 
| Mg&ndash;O
|0.9
|- 
| Fe<sup>+2</sup>&ndash;O
|0.85
|- 
| Mn&ndash;O
|0.8
|- 
| Ca&ndash;O
|0.7
|- 
| Na&ndash;O
|0.35
|-
| K&ndash;O
|0.25
|}

This table is only a rough guide to order of weathering. Some minerals, such as [[illite]], are unusually stable, while silica is unusually unstable given the strength of the [[silicon–oxygen bond]].{{sfn|Blatt|Middleton|Murray|1980|p=258}}

Carbon dioxide that dissolves in water to form carbonic acid is the most important source of protons, but organic acids are also important natural sources of acidity.{{sfn|Blatt|Middleton|Murray|1980|p=250}} Acid hydrolysis from dissolved carbon dioxide is sometimes described as ''carbonation'', and can result in weathering of the primary minerals to secondary carbonate minerals.<ref name="thornbury-1969">{{cite book |last1=Thornbury |first1=William D. |title=Principles of geomorphology |date=1969 |publisher=Wiley |location=New York |isbn=0471861979 |pages=303–344 |edition=2d}}</ref> For example, weathering of forsterite can produce [[magnesite]] instead of brucite via the reaction:

:Mg<sub>2</sub>SiO<sub>4</sub> + 2 CO<sub>2</sub> + 2 H<sub>2</sub>O ⇌ 2 MgCO<sub>3</sub> + H<sub>4</sub>SiO<sub>4</sub> 
:forsterite + carbon dioxide + water ⇌ magnesite + silicic acid in solution

[[Carbonic acid]] is consumed by [[silicate]] weathering, resulting in more [[alkaline]] solutions because of the [[bicarbonate]]. This is an important reaction in controlling the amount of CO<sub>2</sub> in the atmosphere and can affect climate.<ref>{{cite journal |last1=Berner |first1=Robert A. |editor1-first=Arthur F |editor1-last=White |editor2-first=Susan L |editor2-last=Brantley |title=Chapter 13. CHEMICAL WEATHERING AND ITS EFFECT ON ATMOSPHERIC CO2 AND CLIMATE |journal=Chemical Weathering Rates of Silicate Minerals |date=31 December 1995 |pages=565–584 |doi=10.1515/9781501509650-015|isbn=9781501509650 }}</ref>

[[Aluminosilicate]]s containing highly soluble cations, such as sodium or potassium ions, will release the cations as dissolved bicarbonates during acid hydrolysis:

:2 KAlSi<sub>3</sub>O<sub>8</sub> + 2 H<sub>2</sub>CO<sub>3</sub> + 9 H<sub>2</sub>O ⇌ Al<sub>2</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub> + 4 H<sub>4</sub>SiO<sub>4</sub> + 2 K<sup>+</sup> + 2 HCO<sub>3</sub><sup>−</sup>
:[[orthoclase]] (aluminosilicate feldspar) + carbonic acid + water ⇌ [[kaolinite]] (a clay mineral) + silicic acid in solution + potassium and bicarbonate ions in solution

===Oxidation===
[[File:GoldinPyriteDrainage acide.JPG|thumb|A [[pyrite]] cube has dissolved away from host rock, leaving [[gold]] particles behind.]]
[[File:PyOx.JPG|thumb|Oxidized [[pyrite]] cubes]]
Within the weathering environment, chemical [[oxidation]] of a variety of metals occurs. The most commonly observed is the oxidation of Fe<sup>2+</sup> ([[iron]]) by oxygen and water to form Fe<sup>3+</sup> oxides and hydroxides such as [[goethite]], [[limonite]], and [[hematite]]. This gives the affected rocks a reddish-brown coloration on the surface which crumbles easily and weakens the rock. Many other metallic ores and minerals oxidize and hydrate to produce colored deposits, as does sulfur during the weathering of [[sulfide mineral]]s such as [[chalcopyrite]]s or CuFeS<sub>2</sub> oxidizing to [[copper hydroxide]] and [[iron oxide]]s.{{sfn|Boggs|2006|p=9}}

===Hydration===
[[Mineral hydration]] is a form of chemical weathering that involves the rigid attachment of water molecules or H+ and OH- ions to the atoms and molecules of a mineral. No significant dissolution takes place. For example, [[iron oxide]]s are converted to [[iron hydroxide]]s and the hydration of [[anhydrite]] forms [[gypsum]].{{sfn|Boggs|1996|p=8}}

Bulk hydration of minerals is secondary in importance to dissolution, hydrolysis, and oxidation,{{sfn|Boggs|2006|p=9}} but hydration of the crystal surface is the crucial first step in hydrolysis. A fresh surface of a mineral crystal exposes ions whose electrical charge attracts water molecules. Some of these molecules break into H+ that bonds to exposed anions (usually oxygen) and OH- that bonds to exposed cations. This further disrupts the surface, making it susceptible to various hydrolysis reactions. Additional protons replace cations exposed on the surface, freeing the cations as solutes. As cations are removed, silicon-oxygen and silicon-aluminium bonds become more susceptible to hydrolysis, freeing silicic acid and aluminium hydroxides to be leached away or to form clay minerals.{{sfn|Blatt|Middleton|Murray|1980|p=258}}{{sfn|Leeder|2011|pp=653-655}} Laboratory experiments show that weathering of feldspar crystals begins at dislocations or other defects on the surface of the crystal, and that the weathering layer is only a few atoms thick. Diffusion within the mineral grain does not appear to be significant.<ref>{{cite journal |last1=Berner |first1=Robert A. |last2=Holdren |first2=George R. |title=Mechanism of feldspar weathering: Some observational evidence |journal=Geology |date=1 June 1977 |volume=5 |issue=6 |pages=369–372 |doi=10.1130/0091-7613(1977)5<369:MOFWSO>2.0.CO;2|bibcode=1977Geo.....5..369B }}</ref>

[[File:Weathering 9039.jpg|thumb|A freshly broken rock shows differential chemical weathering (probably mostly oxidation) progressing inward. This piece of [[sandstone]] was found in [[Moraine|glacial drift]] near [[Angelica, New York]].]]

===Biological===
Mineral weathering can also be initiated or accelerated by soil microorganisms. Soil organisms make up about 10&nbsp;mg/cm<sup>3</sup> of typical soils, and laboratory experiments have demonstrated that [[albite]] and [[muscovite]] weather twice as fast in live versus sterile soil. [[Lichens]] on rocks are among the most effective biological agents of chemical weathering.{{sfn|Blatt|Middleton|Murray|1980|p=250}} For example, an experimental study on hornblende granite in New Jersey, US, demonstrated a 3x – 4x increase in weathering rate under lichen covered surfaces compared to recently exposed bare rock surfaces.<ref>{{cite journal|doi=10.1016/j.chemgeo.2011.10.009|title=Effect of lichen colonization on chemical weathering of hornblende granite as estimated by aqueous elemental flux|date=2012|last1=Zambell|first1=C.B.|last2=Adams|first2=J.M.|last3=Gorring|first3=M.L.|last4=Schwartzman|first4=D.W.|journal=Chemical Geology|volume=291|pages=166–174|bibcode=2012ChGeo.291..166Z}}</ref>

[[File:lava z14.jpg|thumb|right|Biological weathering of [[basalt]] by [[lichen]], [[La Palma]]]]
The most common forms of biological weathering result from the release of [[chelating]] compounds (such as certain organic acids and [[siderophore]]s) and of carbon dioxide and organic acids by plants. Roots can build up the carbon dioxide level to 30% of all soil gases, aided by adsorption of {{CO2}} on clay minerals and the very slow diffusion rate of {{CO2}} out of the soil.<ref>{{cite journal |last1=Fripiat |first1=J. J. |title=Interlamellar Adsorption of Carbon Dioxide by Smectites |journal=Clays and Clay Minerals |date=1974 |volume=22 |issue=1 |pages=23–30 |doi=10.1346/CCMN.1974.0220105|bibcode=1974CCM....22...23F |s2cid=53610319 |url=http://www.clays.org/journal/archive/volume%2022/22-1-23.pdf |url-status=dead |archive-url=https://web.archive.org/web/20180603022725/http://www.clays.org/journal/archive/volume%2022/22-1-23.pdf |archive-date= Jun 3, 2018 }}</ref> The {{CO2}} and organic acids help break down [[aluminium]]- and [[iron]]-containing compounds in the soils beneath them. Roots have a negative electrical charge balanced by protons in the soil next to the roots, and these can be exchanged for essential nutrient cations such as potassium.{{sfn|Blatt|Middleton|Murray|1980|pp=251}} [[Bacterial decay|Decaying]] remains of dead plants in soil may form organic acids which, when dissolved in water, cause chemical weathering.<ref>{{cite book|last=Chapin III|first=F. Stuart|author2=Pamela A. Matson |author3=Harold A. Mooney |title=Principles of terrestrial ecosystem ecology |url=https://books.google.com/books?id=OOH1H779-7EC&pg=PA54 |date=2002|publisher=Springer|location=New York|isbn=9780387954431|pages=54–55|edition=[Nachdr.]}}</ref> Chelating compounds, mostly low molecular weight organic acids, are capable of removing metal ions from bare rock surfaces, with aluminium and silicon being particularly susceptible.{{sfn|Blatt|Tracy|1996|p=233}} The ability to break down bare rock allows lichens to be among the first colonizers of dry land.{{sfn|Blatt|Middleton|Murray|1980|pp=250-251}} The accumulation of chelating compounds can easily affect surrounding rocks and soils, and may lead to [[podsol]]isation of soils.<ref>{{Cite journal|last1=Lundström|first1=U. S.|last2=van Breemen|first2=N.|last3=Bain|first3=D. C.|last4=van Hees|first4=P. A. W.|last5=Giesler|first5=R.|last6=Gustafsson|first6=J. P.|last7=Ilvesniemi|first7=H.|last8=Karltun|first8=E.|last9=Melkerud|first9=P. -A.|last10=Olsson|first10=M.|last11=Riise|first11=G.|date=2000-02-01|title=Advances in understanding the podzolization process resulting from a multidisciplinary study of three coniferous forest soils in the Nordic Countries|url=http://www.sciencedirect.com/science/article/pii/S0016706199000774|journal=Geoderma|language=en|volume=94|issue=2|pages=335–353|doi=10.1016/S0016-7061(99)00077-4|bibcode=2000Geode..94..335L|issn=0016-7061|url-access=subscription}}</ref><ref>{{cite book|last=Waugh|first=David|title=Geography : an integrated approach|date=2000|publisher=[[Nelson Thornes]]|location=Gloucester, U.K.|isbn=9780174447061|page=272|edition=3rd}}</ref>

The symbiotic [[Mycorrhiza|mycorrhizal fungi]] associated with tree root systems can release inorganic nutrients from minerals such as apatite or biotite and transfer these nutrients to the trees, thus contributing to tree nutrition.<ref>{{cite journal|author=Landeweert, R. |author2=Hoffland, E. |author3=Finlay, R.D. |author4=Kuyper, T.W. |author5=van Breemen, N. |author-link5=Nico van Breemen|pmid=11301154|date=2001|title=Linking plants to rocks: Ectomycorrhizal fungi mobilize nutrients from minerals|volume=16|issue=5|pages=248–254|journal=Trends in Ecology & Evolution|doi=10.1016/S0169-5347(01)02122-X}}</ref>  It was also recently evidenced that bacterial communities can impact mineral stability leading to the release of inorganic nutrients.<ref>{{cite journal|author=Calvaruso, C.|author2=Turpault, M.-P.|author3=Frey-Klett, P.|doi=10.1128/AEM.72.2.1258-1266.2006|title=Root-Associated Bacteria Contribute to Mineral Weathering and to Mineral Nutrition in Trees: A Budgeting Analysis|date=2006|journal=Applied and Environmental Microbiology|volume=72|issue=2|pages=1258–66|pmid=16461674|pmc=1392890 |bibcode=2006ApEnM..72.1258C}}</ref> A large range of bacterial strains or communities from diverse genera have been reported to be able to colonize mineral surfaces or to weather minerals, and for some of them a plant growth promoting effect has been demonstrated.<ref>{{cite journal|author=Uroz, S.|author2=Calvaruso, C.|author3=Turpault, M.-P.|author4=Frey-Klett, P.|title=Mineral weathering by bacteria: ecology, actors and mechanisms|journal=Trends Microbiol|date= 2009|volume=17|issue=8|pages=378–87|doi=10.1016/j.tim.2009.05.004|pmid=19660952}}</ref>  The demonstrated or hypothesised mechanisms used by bacteria to weather minerals include several oxidoreduction and dissolution reactions as well as the production of weathering agents, such as protons, organic acids and chelating molecules.