Editing Weathering (section)

===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