Editing Hot spring (section)

==Chemistry==
[[File:GM Guelma Hammam Challala01.jpg|thumb|right|[[Hammam Maskhoutine]] in [[Algeria]], an example of a bicarbonate hot spring]]
Because heated water can hold more [[solvation|dissolved]] [[solid]]s than cold water, the water that issues from hot springs often has a very high [[mineral]] content, containing everything from [[calcium]] to [[lithium]] and even [[radium]]. The overall chemistry of hot springs varies from ''alkaline chloride'' to ''acid sulfate'' to ''bicarbonate'' to ''iron-rich'', each of which defines an end member of a range of possible hot spring chemistries.<ref>{{cite journal |last1=Drake |first1=Bryan D. |last2=Campbell |first2=Kathleen A. |last3=Rowland |first3=Julie V. |last4=Guido |first4=Diego M. |last5=Browne |first5=Patrick R.L. |last6=Rae |first6=Andrew |title=Evolution of a dynamic paleo-hydrothermal system at Mangatete, Taupo Volcanic Zone, New Zealand |journal=Journal of Volcanology and Geothermal Research |date=August 2014 |volume=282 |pages=19–35 |doi=10.1016/j.jvolgeores.2014.06.010|bibcode=2014JVGR..282...19D |hdl=11336/31453 |hdl-access=free }}</ref>{{sfn|Des Marais|Walter|2019}}

Alkaline chloride hot springs are fed by hydrothermal fluids that form when groundwater containing dissolved [[chloride]] salts reacts with silicate rocks at high temperature. These springs have nearly neutral [[pH]] but are saturated with [[silica]] ({{chem2|SiO2}}). The solubility of silica depends strongly upon temperature, so upon cooling, the silica is deposited as [[geyserite]], a form of [[opal]] (opal-A: {{chem2|SiO2·nH2O}}).<ref>{{cite journal |last1=White |first1=Donald E. |last2=Brannock |first2=W.W. |last3=Murata |first3=K.J. |title=Silica in hot-spring waters |journal=Geochimica et Cosmochimica Acta |date=August 1956 |volume=10 |issue=1–2 |pages=27–59 |doi=10.1016/0016-7037(56)90010-2|bibcode=1956GeCoA..10...27W }}</ref> This process is slow enough that geyserite is not all deposited immediately around the vent, but tends to build up a low, broad platform for some distance around the spring opening.{{sfn|Drake|Campbell|Rowland|Guido|2014}}{{sfn|Des Marais|Walter|2019}}<ref>{{cite journal |last1=White |first1=D.E. |last2=Thompson |first2=G.A. |last3=Sandberg |first3=C.H. |title=Rocks, structure, and geologic history of Steamboat Springs thermal area, Washoe County, Nevada |journal=U.S. Geological Survey Professional Paper |series=Professional Paper |date=1964 |volume=458-B |doi=10.3133/pp458B|doi-access=free |bibcode=1964usgs.rept...19W }}</ref>

Acid sulfate hot springs are fed by hydrothermal fluids rich in [[hydrogen sulfide]] ({{chem2|H2S}}), which is oxidized to form [[sulfuric acid]], {{chem2|H2SO4}}.{{sfn|Drake|Campbell|Rowland|Guido|2014}} The pH of the fluids is thereby lowered to values as low as 0.8.<ref>{{cite journal |last1=Cox |first1=Alysia |last2=Shock |first2=Everett L. |last3=Havig |first3=Jeff R. |title=The transition to microbial photosynthesis in hot spring ecosystems |journal=Chemical Geology |date=January 2011 |volume=280 |issue=3–4 |pages=344–351 |doi=10.1016/j.chemgeo.2010.11.022|bibcode=2011ChGeo.280..344C }}</ref> The acid reacts with rock to alter it to [[clay mineral]]s, [[oxide minerals]], and a residue of silica.{{sfn|Des Marais|Walter|2019}}

Bicarbonate hot springs are fed by hydrothermal fluids that form when [[carbon dioxide]] ({{chem2|CO2}}) and groundwater react with [[carbonate rocks]].{{sfn|Drake|Campbell|Rowland|Guido|2014}} When the fluids reach the surface, {{chem2|CO2}} is rapidly lost and carbonate minerals precipitate as [[travertine]], so that bicarbonate hot springs tend to form high-relief structures around their openings.{{sfn|Des Marais|Walter|2019}}

Iron-rich springs are characterized by the presence of microbial communities that produce clumps of oxidized iron from iron in the hydrothermal fluids feeding the spring.<ref>{{cite journal |last1=Parenteau |first1=M. N. |last2=Cady |first2=S. L. |title=Microbial biosignatures in iron-mineralized phototrophic mats at Chocolate Pots Hot Springs, Yellowstone National Park, United States |journal=PALAIOS |date=2010-02-01 |volume=25 |issue=2 |pages=97–111 |doi=10.2110/palo.2008.p08-133r|bibcode=2010Palai..25...97P |s2cid=128592574 }}</ref>{{sfn|Des Marais|Walter|2019}}

Some hot springs produce fluids that are intermediate in chemistry between these extremes. For example, mixed acid-sulfate-chloride hot springs are intermediate between acid sulfate and alkaline chloride springs and may form by mixing of acid sulfate and alkaline chloride fluids. They deposit geyserite, but in smaller quantities than alkaline chloride springs.{{sfn|Drake|Campbell|Rowland|Guido|2014}}