Editing Serpentinization (section)

===Formation of magnetite and hydrogen===
{{See also|Schikorr reaction}}
In most crustal rock, the chemical activity of oxygen is prevented from dropping to very low values by the [[Mineral redox buffer#Common redox buffers and mineralogy|fayalite-magnetite-quartz (FMQ) buffer]].{{sfn|Moody|1976|p=129}} The very low chemical activity of silica during serpentinization eliminates this buffer, creating highly [[Reduction (chemistry)|reducing]] conditions<ref name=FrostBeard2007/> that allow water to oxidize ferrous ({{chem|Fe|2+}}) ions in fayalite. This reaction modifies minerals and [[Hydrogen_cycle#Sources|liberates hydrogen gas]]:<ref name=":0" /><ref>{{cite web| title = Methane and hydrogen formation from rocks – Energy sources for life| url = http://www.lostcity.washington.edu/science/chemistry/methane.html| access-date = 2011-11-06}}</ref><ref>{{Cite journal| last = Sleep| first = N.H.| author2 = A. Meibom, Th. Fridriksson, R.G. Coleman, D.K. Bird| year = 2004| title = H<sub>2</sub>-rich fluids from serpentinization: Geochemical and biotic implications| journal = Proceedings of the National Academy of Sciences of the United States of America| volume = 101| issue = 35| pages = 12818–12823| doi = 10.1073/pnas.0405289101|bibcode = 2004PNAS..10112818S| pmid=15326313| pmc=516479| doi-access = free}}</ref>

{{NumBlk|:
|{{overset|[[Fayalite]]|3 {{chem|Fe|2|SiO|4}}}} + {{overset|water|2 {{chem|H|2|O}}}} → {{overset|[[magnetite]]|2 {{chem|Fe|3|O|4}}}} + {{overset|silicon dioxide|3 {{chem|SiO|2}}}} + {{overset|hydrogen|2 {{chem|H|2}}}}
|{{EquationRef|Reaction 5}}}}

Studies of serpentinites suggest that in nature iron minerals are first converted to [[wikt:ferroan|ferroan]] brucite, that is, brucite containing {{chem2|Fe(OH)2}},<ref>{{cite journal |last1=Bach |first1=Wolfgang |last2=Paulick |first2=Holger |last3=Garrido |first3=Carlos J. |last4=Ildefonse |first4=Benoit |last5=Meurer |first5=William P. |last6=Humphris |first6=Susan E. |title=Unraveling the sequence of serpentinization reactions: petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15°N (ODP Leg 209, Site 1274) |journal=Geophysical Research Letters |date=2006 |volume=33 |issue=13 |pages=L13306 |doi=10.1029/2006GL025681|bibcode=2006GeoRL..3313306B |hdl=1912/3324 |s2cid=55802656 |hdl-access=free }}</ref> which then undergoes the [[Schikorr reaction]] in the anaerobic conditions of serpentinization:<ref name=Esource>{{cite journal |doi=10.1111/j.1472-4669.2010.00249.x|title=Serpentinization as a source of energy at the origin of life|year=2010|last1=Russell|first1=M. J.|last2=Hall|first2=A. J.|last3=Martin|first3=W.|journal=Geobiology|volume=8|issue=5|pages=355–371|pmid=20572872|s2cid=41118603 }}</ref><ref>{{cite journal |doi=10.2138/rmg.2013.75.18|title=Serpentinization, Carbon, and Deep Life|year=2013|last1=Schrenk|first1=M. O.|last2=Brazelton|first2=W. J.|last3=Lang|first3=S. Q.|journal=Reviews in Mineralogy and Geochemistry|volume=75|issue=1|pages=575–606|bibcode=2013RvMG...75..575S}}</ref>

{{NumBlk|:
|{{underset|ferrous hydroxide|6 {{chem|Fe|(OH)|2}}}} → {{underset|magnetite|2 {{chem|Fe|3|O|4}}}} + {{underset|water|4 {{chem|H|2|O}}}}  + {{underset|hydrogen|2 {{chem|H|2}}}}
|{{EquationRef|Reaction 6}}}}

Maximum reducing conditions, and the maximum rate of production of hydrogen, occur when the temperature of serpentinization is between {{convert|200 and 315|C||sp=us}}<ref>{{cite journal |last1=McCollom |first1=Thomas M. |last2=Bach |first2=Wolfgang |title=Thermodynamic constraints on hydrogen generation during serpentinization of ultramafic rocks |journal=Geochimica et Cosmochimica Acta |date=February 2009 |volume=73 |issue=3 |pages=856–875 |doi=10.1016/j.gca.2008.10.032|bibcode=2009GeCoA..73..856M }}</ref> and when fluids are carbonate undersaturated.<ref name=":0" /> If the original ultramafic rock (the ''[[protolith]]'') is peridotite, which is rich in olivine, considerable magnetite and hydrogen are produced. When the protolith is pyroxenite, which contains more pyroxene than olivine, iron-rich talc is produced with no magnetite and only modest hydrogen production. Infiltration of silica-bearing fluids during serpentinization can suppress both the formation of brucite and the subsequent production of hydrogen.<ref>{{cite journal |last1=Klein |first1=Frieder |last2=Bach |first2=Wolfgang |last3=McCollom |first3=Thomas M. |title=Compositional controls on hydrogen generation during serpentinization of ultramafic rocks |journal=Lithos |date=September 2013 |volume=178 |pages=55–69 |doi=10.1016/j.lithos.2013.03.008|bibcode=2013Litho.178...55K }}</ref>

Chromite present in the protolith will be altered to chromium-rich magnetite at lower serpentinization temperatures. At higher temperatures, it will be altered to iron-rich chromite (ferrit-chromite).{{sfn|Moody|1976|p=128}} During serpentinization, the rock is enriched in [[chlorine]], [[boron]], [[fluorine]], and sulfur. Sulfur will be reduced to hydrogen sulfide and sulfide minerals, though significant quantities are incorporated into serpentine minerals, and some may later be reoxidized to sulfate minerals such as [[anhydrite]].<ref>{{Cite journal |last1=Debret |first1=Baptiste |last2=Andreani |first2=Muriel |last3=Delacour |first3=Adélie |last4=Rouméjon |first4=Stéphane |last5=Trcera |first5=Nicolas |last6=Williams |first6=Helen |date=2017-05-15 |title=Assessing sulfur redox state and distribution in abyssal serpentinites using XANES spectroscopy |url=https://www.sciencedirect.com/science/article/pii/S0012821X17300973 |journal=Earth and Planetary Science Letters |language=en |volume=466 |pages=1–11 |doi=10.1016/j.epsl.2017.02.029 |bibcode=2017E&PSL.466....1D |issn=0012-821X|doi-access=free |hdl=20.500.11850/207239 |hdl-access=free }}</ref> The sulfides produced include nickel-rich sulfides, such as [[mackinawite]].<ref>{{cite journal |last1=Delacour |first1=Adélie |last2=Früh-Green |first2=Gretchen L. |last3=Bernasconi |first3=Stefano M. |title=Sulfur mineralogy and geochemistry of serpentinites and gabbros of the Atlantis Massif (IODP Site U1309) |journal=Geochimica et Cosmochimica Acta |date=October 2008 |volume=72 |issue=20 |pages=5111–5127 |doi=10.1016/j.gca.2008.07.018|bibcode=2008GeCoA..72.5111D }}</ref>