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Formula: Al2Si2O5(OH)4
Phyllosilicate (sheet silicate), polytype of
dickite and nacrite
Crystal System: Triclinic
Specific gravity: 2.68 measured, 2.63 calculated
Hardness: 2 to 2½
Streak: white
Colour: white, brownish white, greyish white, yellowish white, greyish green
Solubility: Insoluble in water, nitric and sulphuric acid; soluble with decomposition in hydrochloric acid
Common impurities: Fe,Mg,Na,K,Ti,Ca,H2O
Environments:
Pegmatites
(USGS bulletin 445 Pegmatites of Maine)
Sedimentary environments
Metamorphic environments
Hydrothermal environments
Kaolinite is always a secondary mineral derived from
weathering or hydrothermal alteration of alumino-silicate minerals such as
feldspar. It is a mineral of the
zeolite facies, where
clay minerals transform to
illite, kaolinite and
vermiculite
(MM 242).
It is also a mineral of the
prehnite-pumpellyite,
greenschist and
albite-epidote-hornfels facies.
At the Blue Points mine and at the Thunder Bay Amethyst mine, Thunder Bay, Ontario, Canada, kaolinite
is found within cavities bearing amethyst
(R&M 94.4.323, 333).
Alteration
anorthite, H2SO4 and H2O to
gypsum and
kaolinite
CaAl2 Si2O8 + H2SO4 + 3H2O →
CaSO4.2H2O + Al2Si2O5(OH)4
(DHZ 5B p65)
anorthite to calcite and
kaolinite in the early Earth's atmosphere
CO2 + H2O + anorthite →
calcite +
kaolinite
CO2 + 2H2O + CaAl2Si2O8 → CaCO3 +
Al2Si2O5(OH)4
(JVW p634)
anorthite, H2O and CO2 ⇌ kaolinite and
calcite
2CaAl2 Si2O8 + 4H2O + 2CO2 ⇌
Al4Si4O10(OH)8 + 2CaCO3
calcite is found as a low-temperature, late-stage alteraation product according to the above reaction
(DHZ 5B p128).
bertrandite and kaolinite to euclase,
beryl and H2O
4Be4Si2O7(OH)2 + 7Al2Si2O5(OH)4 ⇌
10BeAlSiO4(OH) + 2Be3Al2Si6O18 + 13H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
bertrandite and kaolinite to euclase,
quartz and H2O
Be4Si2O7(OH)2 + 2Al2Si2O5(OH)4 ⇌
4BeAlSiO4(OH) + 2SiO2 + 3H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
bertrandite, kaolinite and quartz to
beryl and H2O
3Be4Si2O7(OH)2 + 4Al2Si2O5(OH)4 +
10SiO2 ⇌ 4Be3Al2Si6O18 + 11H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
chrysoberyl, bertrandite and kaolinite to
euclase and H2O
2BeAl2O4 + 2Be4Si2O7(OH)2 +
3Al2Si2O5(OH)4 ⇌ 10BeAlSiO4(OH) + 3H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
euclase and kaolinite to chrysoberyl,
quartz and H2O
2BeAlSiO4(OH) + Al2Si2O5(OH)4 ⇌ 2BeAl2O4 +
4SiO2 + 3H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
euclase and quartz to
beryl, kaolinite and H2O
6BeAlSiO4(OH) + 8SiO2 ⇌ 2Be3Al2Si6O18 +
Al2Si2O5(OH)4 + H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
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 diaspore, SiO2 and H2O
Al2Si2O5(OH)4 ⇌ 2AlO(OH) + 2SiO2 (aqueous) +
H2O
At 10 kbar pressure the equilibrium temperature is about 300oC
(blueschist facies).
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. Diaspore is stable at temperatures
less than 400oC but only in solutions undersaturated with quartz.
High temperature and low quartz
saturation favours the forward reaction
(KB p 93).
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 H2O to gibbsite and
quartz
Al2Si2O5(OH)4 + H2O ⇌ 2Al(OH)3 +
2SiO2
(JVW p144)
kaolinite and diaspore to andalusite and
H2O
Al2Si2O5(OH)4 + 2AlO(OH) ⇌ 2Al2OSiO4 +
3H2O
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 and diaspore to kyanite and
H2O
Al2Si2O5(OH)4 + 2AlO(OH) ⇌ 2Al2OSiO4 +
3H2O
At 5 kbar pressure the equilibrium temperature for the reaction is about 370oC
(greenschist facies), with the equilibrium to the right
at higher temperatures and to the left at lower temperatures
(SERC, AM61.699-709).
kaolinite,
dolomite,
quartz and H2O to chlorite,
calcite and CO2
Al2Si2O5(OH)4 + 5CaMg(CO3)2 + SiO2
+ 2H2O ⇌ Mg5Al(AlSi3O10)(OH)8 + 5CaCO3 +
5CO2
Chlorite often forms in this way from reactions between clay minerals such
as kaolinite and carbonates such as
dolomite
(KB p377).
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).
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 kaolinite to
margarite,
quartz and H2O
CaAl2(Si2O7)(OH)2.H2 +
Al2Si2O5(OH)4 ⇌
CaAl2(Al2Si2O10)(OH)2 + 2SiO2
+3H2O
The equilibrium temperature for this reaction at 2 kbar pressure is about 300oC
(prehnite-pumpellyite facies), with the equilibrium to
the right at higher temperatures, and to the left at lower temperatures
(AM61.699-709).
lawsonite and kaolinite to
margarite and
wairakite and H2O
2CaAl2(Si2O7(OH)2.H2O +
Al2Si2O5(OH)4 ⇌
CaAl2Si2Al2O10(OH)2 + Ca(Si4Al2)O10.2H2O
+ 3H2O
Increasing temperature favours the forward reaction
(AM61.699-709).
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).
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).
kaolinite and quartz to pyrophyllite and H2O
Al2Si2O5(OH)4 + 2SiO2 ⇌
Al2Si4O10(OH)2 + H2O
Increasing temperature favours the forward reaction
(AM 63.664-676).
muscovite, H+ and H2O to kaolinite and K+
2KAl2(AlSi3O10)(OH)2 + 2H+ + 3H2O ⇌
3Al2Si2O5(OH)4 + 2K+
Low temperature and a low K+/H+ ratio favour the forward reaction
(KB p99).
wairakite and kaolinite to
margarite and quartz and H2O
Ca(Si4Al2)O10.2H2O + Al2Si2O5(OH)4 ⇌
CaAl2Si2Al2O10(OH)2 + 4SiO2 + 3H2O
Increasing temperature favours the forward reaction
(AM61.699-709).
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