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Formula: Al2Si4O10(OH)2
Phyllosilicate (sheet silicate)
Crystal System: Triclinic
Specific gravity: 2.65 to 2.9 g/cm3 measured 2.81 calculated
Hardness: 1 to 2
Streak: White
Colour: White, gray, pale blue, pale green, pale yellow, brownish green
Solubility: Slightly soluble in sulphuric acid
Environments:
Metamorphic environments
Hydrothermal environments
Pyrophyllite is characteristic of advanced alteration resulting from the the reaction of low pH (highly acidic)
hydrothermal fluids with
rock, forming at temperatures from about 200oC to 350oC. It is found both in hydrothermal
veins and in bedded deposits in schistose
metamorphic rocks, where is is
often associated with kyanite
(AofA).
It can be pseudomorphous after
kyanite,
feldspar and
pyroxene
(DHZ 3 page 119).
Pyrophyllite is a mineral of the
prehnite-pumpellyite,
greenschist and
albite-epidote-hornfels facies.
Localities
At the Witwatersrand goldfield, South Africa, pyrophyllite is a relatively common
secondary metamorphic mineral and occurs as finely
disseminated
golden-yellow crystals less than 1 mm in size. Micaceous rosettes of pyrophyllite 2 to 3 mm in diameter
are found rarely, associated with quartz
(R&M 96.4.339).
Alteration
diaspore and quartz to pyrophyllite
2AlO(OH) + 4SiO2 ⇌ Al2Si4O10(OH)2
The equilibrium temperature for this reaction at 5 kbar pressure is about 160oC
(zeolite facies), and at 10 kbar it is
about 300oC (blueschist facies).
The equilibrium is to
the right at higher temperatures, and to the left at lower temperatures
(SERC).
kaolinite to andalusite, pyrophyllite
and H2O
3Al2Si2O5(OH)4 ⇌ 2Al2OSiO4 +
Al2Si4O10(OH)2 + 5H2O
At 1 kbar pressure the equilibrium temperature for the reaction is about 320oC (albite-epidote-hornfels facies), with the equilibrium to the right
at higher temperatures and to the left at lower temperatures
(SERC, AM61.699-709).
kaolinite to kyanite, pyrophyllite
and H2O
3Al2Si2O5(OH)4 ⇌ 2Al2OSiO4 +
Al2Si4O10(OH)2 + 5H2O
At 5 kbar pressure the equilibrium temperature for the reaction is about 375oC (greenschist facies), with the equilibrium to the right
at higher temperatures and to the left at lower temperatures
(SERC, AM61.699-709).
kaolinite to
pyrophyllite, diaspore and H2O
2Al2Si2O5(OH)4 →
Al2Si4O10(OH)2 + 2AlO(OH) + 2H2O
In the absence of quartz, kaolinite
breaks down on heating according to the above reaction
(KB p431).
At 5 kbar pressure the equilibrium temperature for the reaction is about 320oC
(prehnite-pumpellyite facies), and at 9 kbar it
is about 380oC
(greenschist facies)
(SERC, AM61.699-709).
kaolinite and quartz to
pyrophyllite and H2O
Al2Si2O5(OH)4 + 2SiO2 →
Al2Si4O10(OH)2 + H2O
This reaction represents the breakdown of kaolinite in the presence of
quartz.
(If quartz is absent, diaspore is formed as well as
pyrophyllite).
(KB p432 )
At 5 kbar pressure the equilibrium temperature is about 340oC
(prehnite-pumpellyite facies), and at 10 kbar
it is about 300oC (blueschist facies)
(SERC, AM61.699-709, AM 63.664-676).
lawsonite and kaolinite
to margarite, pyrophyllite and H2O
CaAl2(Si2O7)(OH)2.H2 +
2Al2Si2O5(OH)4 ⇌
CaAl2(Al2Si2O10)(OH)2 +
Al2Si4O10(OH)2 + 4H2O
The equilibrium temperature for this reaction at 5 kbar pressure is about 360oC
(greenschist facies), with
the equilibrium to
the right at higher temperatures, and to the left at lower temperatures
(AM61.699-709).
lawsonite and kyanite to
margarite, pyrophyllite and H2O
3CaAl2(Si2O7)(OH)2.H2 + 4Al2OSiO4
⇌ 3CaAl2(Al2Si2O10)(OH)2 +
Al2Si4O10(OH)2 + 2H2O
The equilibrium temperature for this reaction at 6.5 kbar pressure is about 390oC
(greenschist facies), with the equilibrium to
the right at higher temperatures, and to the left at lower temperatures
(AM61.699-709).
lawsonite and pyrophyllite to
margarite and
quartz and H2O
CaAl2(Si2O7(OH)2.H2O + Al2Si4O10(OH)2 ⇌
CaAl2Si2Al2O10(OH)2 + 4SiO2 + 2H2O
Increasing temperature favours the forward reaction
(AM61.699-709).
pyrophyllite to kyanite, quartz and
H2O
Al2Si2O10(OH)2 ⇌ Al2OSiO4 + 3SiO2
+ H2O
At 9 kbar pressure the equilibrium temperature is about 425oC (greenschist facies)
(AM61.699-709).
pyrophyllite to andalusite or
kyanite,
quartz and H2O
Al2Si4O10(OH)2 ⇌ Al2OSiO4 +
3SiO2 + H2O
The equilibrium temperature for this reaction at 1.8 kbar pressure is 414oC
(greenschist facies), with the equilibrium to
the right at higher temperatures, and to the left at lower temperatures. The pressure determines whether
kyanite or andalusite is formed. Above about 2.5 kbar it is
kyanite, and andalusite at lower pressure
(JVW p539, Tulane, SERC).
pyrophyllite and H2O to kaolinite and aqueous SiO2
Al2Si2O10(OH)2 + H2O →
Al2Si2O5(OH)4 + 2SiO2
(JVW p 363)
At 1 kbar pressure kaolinite is stable at temperatures less than
300oC; it can be in equilibrium with
quartz and water in solutions both saturated and undersaturated with
quartz. Pyrophyllite is stable at
temperatures up to 450oC and above, but except for a very narrow band of temperature and composition, it is
stable only with solutions supersaturated with quartz
(KB p 93).
pyrophyllite and diaspore to andalusite
and H2O
Al2Si4O10(OH)2 + 6AlO(OH) ⇌ 4Al2OSiO4 +
4H2O
(JVW p 553)
This reacton is a low pressure reaction, occurring below about 1.9 kbar. Increasing temperature favours the
forward reaction
(SERC).
pyrophyllite and diaspore to kyanite
and H2O
Al2Si4O10(OH)2 + 6AlO(OH) ⇌ 4Al2OSiO4 +
4H2O
(JVW p 553)
This reacton is a higher pressure reaction, occurring above about 1.9 kbar. Increasing temperature favours the
forward reaction. At 9 kbar pressure the equilibrium temperature is 380oC
(greenschist facies)
(SERC, AM61.699-709).
wairakite and pyrophyllite to
margarite, quartz and H2O
Ca(Si4Al212.2H2O + Al2Si4O10(OH)2 ⇌
CaAl2Si2Al2O10(OH)2 + 6SiO2 + 2H2O
Increasing temperature favours the forward reaction
(SERC).
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