Editing Serpentinization (section)

===Formation of serpentine minerals ===
Olivine is a [[solid solution]] of [[forsterite]], the [[magnesium]] endmember of {{chem2|(Mg(2+), Fe(2+))2SiO4}}, and [[fayalite]], the [[iron]] endmember, with forsterite typically making up about 90% of the olivine in ultramafic rocks.<ref name=SnowDick1995>{{cite journal |last1=Snow |first1=Jonathan E. |last2=Dick |first2=Henry J.B. |title=Pervasive magnesium loss by marine weathering of peridotite |journal=Geochimica et Cosmochimica Acta |date=October 1995 |volume=59 |issue=20 |pages=4219–4235 |doi=10.1016/0016-7037(95)00239-V|bibcode=1995GeCoA..59.4219S }}</ref> [[Serpentine subgroup|Serpentine]] can form from [[olivine]] via several reactions:

{{NumBlk|:
|{{overset|[[Forsterite]]|3 {{chem|Mg|2|SiO|4}}}} + {{overset|silicon dioxide|{{chem|SiO|2}}}} + 4 {{chem|H|2|O}} → {{overset|serpentine|2 {{chem|Mg|3|Si|2|O|5|(OH)|4}}}}
|{{EquationRef|Reaction 1a}}}}

{{NumBlk|:
|{{overset|[[Forsterite]]|2 {{chem|Mg|2|SiO|4}}}} + {{overset|water|3 {{chem|H|2|O}}}} → {{overset|serpentine|{{chem|Mg|3|Si|2|O|5|(OH)|4}}}} + {{overset|[[brucite]]|{{chem|Mg|(OH)|2}}}}
|{{EquationRef|Reaction 1b}}}}

Reaction 1a tightly binds silica, lowering its [[chemical activity]] to the lowest values seen in common rocks of the [[Earth's crust]].<ref name=FrostBeard2007>{{cite journal |last1=Frost |first1=B. R. |last2=Beard |first2=J. S. |title=On Silica Activity and Serpentinization |journal=Journal of Petrology |date=3 April 2007 |volume=48 |issue=7 |pages=1351–1368 |doi=10.1093/petrology/egm021|url=http://petrology.oxfordjournals.org/content/48/7/1351.full.pdf }}</ref> Serpentinization then continues through the hydration of olivine to yield serpentine and brucite (Reaction 1b).<ref name="Coleman77">{{cite book|last=Coleman|first=Robert G.|title=Ophiolites|date=1977|publisher=Springer-Verlag|isbn=978-3540082767|pages=100–101}}</ref> The mixture of brucite and serpentine formed by Reaction 1b has the lowest silica activity in the [[serpentinite]], so that the brucite phase is very important in understanding serpentinization.<ref name=FrostBeard2007/> However, the brucite is often blended in with the serpentine such that it is difficult to identify except with [[X-ray diffraction]], and it is easily altered under surface weathering conditions.{{sfn|Moody|1976|p=127}}

A similar suite of reactions involves [[pyroxene]]-group minerals:

{{NumBlk|:
|{{overset|[[Enstatite]]|3 {{chem|Mg|SiO|3}}}} + {{overset|silicon dioxide|{{chem|SiO|2}}}} + {{chem|H|2|O}} → {{overset|[[talc]]|{{chem|Mg|3|Si|4|O|10|(OH)|2}}}}
|{{EquationRef|Reaction 2a}}}}

{{NumBlk|:
|{{overset|[[Enstatite]]|6 {{chem|Mg|SiO|3}}}} + 3 {{chem|H|2|O}} → {{overset|serpentine|{{chem|Mg|3|Si|2|O|5|(OH)|4}}}} + {{overset|[[talc]]|{{chem|Mg|3|Si|4|O|10|(OH)|2}}}}
|{{EquationRef|Reaction 2b}}}}

Reaction 2a quickly comes to a halt as silica becomes unavailable, and Reaction 2b takes over.{{sfn|Frost|Beard|2007|p=1355}} When olivine is abundant, silica activity drops low enough that talc begins to react with olivine:

{{NumBlk|:
|{{overset|[[Forsterite]]|6 {{chem|Mg|2|SiO|4}}}} + {{overset|[[talc]]|{{chem|Mg|3|Si|4|O|10|(OH)|2}}}} + {{overset|water|9 {{chem|H|2|O}}}} → {{overset|serpentine|5 {{chem|Mg|3|Si|2|O|5|(OH)|4}}}}
|{{EquationRef|Reaction 3}}}}

This reaction requires higher temperatures than those at which brucite forms.{{sfn|Moody|1976|p=127}}

The final mineralogy depends both on rock and fluid compositions, temperature, and pressure. Antigorite forms in reactions at temperatures that can exceed {{convert|600|C|F|abbr=on}} during metamorphism, and it is the serpentine group mineral stable at the highest temperatures. Lizardite and chrysotile can form at low temperatures very near the Earth's surface.{{sfn|Moody|1976|p=125, 127, 131}}