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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
paramorphs of
each other.
Marmolite is a light green pearly serpentine which may show a somewhat laminated fracture pattern.
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.
Localities
The Ma On Shan Mine, Ma On Shan, Sha Tin District, New Territories, Hong Kong, China, is an abandoned
iron mine, with
both underground and open cast workings. The iron ores contain
magnetite as the ore mineral and occur predominantly as masses of all sizes
enclosed in a large skarn body formed by contact metasomatism of
dolomitic limestone at the
margins of a granite intrusion. In parts of the underground workings
magnetite is also found in
marble in contact with the
granite. The skarn rocks
consist mainly of tremolite,
actinolite, diopside and
garnet.
Serpentine is commonly found as veins cutting the skarn rocks; the
veins consist of parallel layers of serpentine, and also other minerals including
fluorite, calcite and
quartz
(Hong Kong Minerals (1991). Peng, C J. Hong Kong Urban Council)
Alteration
cummingtonite-grunerite (amphibole) and
H2O to serpentine and quartz
6(Fe,Mg)7Si8O22(OH)2 + 22H2O ⇌
7(Fe,Mg)6Si4O10(OH)8 + 20SiO2
(DHZ 2A p138)
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
(DHZ 3 p185).
enstatite-ferrosilite (pyroxene) and
H2O to serpentine and quartz
6(Mg,Fe2+)SiO3 + 4H2O ⇌
(Fe,Mg)6Si4O10(OH)8 +2SiO2
(DHZ p138)
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)
(WJ).
forsterite, SiO3 and H2O to
serpentine
3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5
(OH)4
This reaction is highly exothermic
(Wiki Serpentinite).
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
(R&M 90.6.521).
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
(R&M 90.6.521).
serpentine to forsterite,
talc and H2O
5Mg3Si2O5(OH)4 ⇌ 6Mg2SiO4 +
Mg3Si4O10(OH)2 + 9H2O
At a pressure of 10 kbar the equilibrium temperature is about 600oC. Increasing temperature favours the forward reaction
(SERC).
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
(R&M 90.6.521).
serpentine, aegirine and
quartz to magnesio-riebeckite and
H2O
Mg3Si2O5(OH)4 + 2NaFe3+Si2O6 +
2SiO2 →
Na2(Mg3Fe3+2)Si8O22(OH)2 + H2O
(DHZ 2A p511)
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
(DHZ 1A p259).
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
(DHZ 2A p271).
serpentine and diopside to
tremolite,
forsterite and H2O
5Mg3Si2O5(OH)4 + 2CaMgSi2O6 =
Ca2Mg5Si8O22(OH)2 + 6Mg2SiO4 +
9H2O
(DHZ 2A p271)
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