Pyrophyllite

phyllosilicate

feldspar

Formula: Al2Si4O10(OH)2 phyllosilicate (sheet silicate)
Specific gravity: 2.65 to 2.90
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 200 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.

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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