Serpentine

phyllosilicate

chrysotile

The serpentine subgroup is a group of phyllosilicates (sheet silicates) including lizardite, chrysotile and antigorite. These three minerals all have the same formula, Mg3Si2O5(OH)4, but different structures, ie they are polymorphs of each other.
Environments

Metamorphic environments

Serpentine is a common mineral and widely distributed, frequently in disseminated particles, in places in such quantity as to make up practically the entire rock mass.
It may be found in kimberlite and skarn.
It is usually formed by alteration of magnesium silicates, especially olivine, pyroxene and amphibole.
It is frequently associated with magnesite, chromite, and magnetite.

Alteration

cummingtonite-grunerite (amphibole) and H2O to serpentine and quartz
6(Fe,Mg)7Si8O22(OH)2 + 22H2O ⇌ 7(Fe,Mg)6Si4O10(OH)8 + 20SiO2

enstatite and H2O to serpentine and talc
3Mg2Si2O6 + 3H2O → Mg3Si2O5(OH)4 + Mg3Si4O10(OH)2
Talc may occur as a pseudomorph after enstatite by the above reaction.

enstatite-ferrosilite (pyroxene) and H2O to serpentine and quartz
6(Mg,Fe2+)SiO3 + 4H2O ⇌ (Fe,Mg)6Si4O10(OH)8 +2SiO2

forsterite and H2O to serpentine and brucite
2Mg2SiO4 + 3H2O ⇌ Mg3Si2O5(OH)4 + Mg(OH)2
The forward reaction is highly exothermic. At 5 kbar pressure the equilibrium temperature is about 420°C (greenschist facies).

forsterite, SiO3 and H2O to serpentine
3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5 (OH)4
This reaction is highly exothermic

forsterite, enstatite and H2O to serpentine
2Mg2SiO4 + Mg2Si2O6 + 4H2O → 2Mg3Si2O5(OH)4
Serpentine (chrysotile) is not stable in the presence of carbon dioxide, and may further react with it to form talc and magnesite.

forsterite, fayalite, H2O and CO2 to serpentine, magnetite and methane
18 Mg2SiO4 + 6Fe2SiO4 + 26H2O + CO2 → 12Mg3Si2O5(OH)4 + 4Fe3O4 + CH4

olivine and H2O to serpentine, magnetite and H2
6(Mg1.5Fe0.5)SiO4 + 7H2O → 3Mg3Si2O5(OH)4 + Fe2+Fe3+2O4 + H2
The iron Fe in olivine does not enter into the serpentine, but recrystallises as magnetite.

serpentine (chrysotile) and CO2 to talc, magnesite and H2O
2Mg3Si2O5(OH)4 + 3CO2 → Mg3Si4O10(OH)2 + 3MgCO3 + 3H2O
Serpentine (chrysotile) is not stable in the presence of carbon dioxide and reacts with it according to the above equation.

serpentine, aegirine and quartz to magnesio-riebeckite and H2O
Mg3Si2O5(OH)4 + 2NaFe3+Si2O6 + 2SiO2 → Na2(Mg3Fe3+2)Si8O22(OH)2 + H2O

serpentine and brucite to forsterite and H2O
Mg3Si2O5(OH)4 + Mg(OH)2 → 2Mg2SiO4 + 3H2O
This reaction can proceed in either direction, depending on the external conditions. Early formation of forsterite.

serpentine and diopside to tremolite, forsterite and H2O
5Mg3Si2O5(OH)4 + 2CaMgSi2O6 ⇌ Ca2Mg5Si8O22(OH)2 + 6Mg2SiO4 + 9H2O + H2O
In lower grade assemblages associated with contact and regional metamorphism serpentine may form tremolite and forsterite according to the above reaction.

serpentine and diopside to tremolite, forsterite and H2O
5Mg3Si2O5(OH)4 + 2CaMgSi2O6 = Ca2Mg5Si8O22(OH)2 + 6Mg2SiO4 + 9H2O

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