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Mineral speciesFormula
 HalloysiteAl2Si2O5(OH)4 
Mineralogy
Family: Kaolinite-Serpentine Group 
Space group: Monoclinic - Domatic H-M Symbol (m) Space Group: Cc 
Dana class: 71.1.1.4 (71) Phyllosilicate Sheets of Six-Membered Rings (71.1) with 1:1 layers (71.1.1) Kaolinite group 
Geological context
High temperature: Formed by dehydration of endellite [Al2Si2O5(OH)4.2(H2O)] above 110C. Stability range <120C. 
Geological context: A product of hydrothermal alteration or surface weathering of aluminosilicate minerals, as feldspars. 
Thermodynamics
Individual properties
Molar weight: 258.160 g/mol  ΔGºf-3,771,950 J/mol ref.  Sº: 203.30 J/mol/K ref. 
  ΔHºf-4,092,600 J/mol ref.  Cp: 246.30 J/mol/K ref. 
Reaction properties
Chemical reaction:  Halloysite + 6H+ = 2Al+3 + 2H4SiO4 + H2O 
log10(Keq) = A + B.T + C.T-1 + D.log10(T) + E.T−2 (with T expressed in Kelvin)
A B C D E
 -9.99049164e+2   -1.58874365e-1   6.03807607e+4   3.57528359e+2   -2.70746695e+6 
Van t'Hoff
@ 25°C, 1 bar: log 10(Keq)10.32
ΔH°r-192,418 J/mol ref.
T (ºC) : 0 25 60 100 150 200 250 300
log 10(Keq):  13.40   10.32   6.79   3.56   0.34   -2.31   -4.65   -6.84 
Maier Kelley
Cp(T) = a + b*10-3T + C*105T-2 (with T expressed in Kelvin)  ref.
a:246.30 J/mol/K 
b:0.00 J/mol/K2 
c:0.00 J.K/mol 

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