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Mineral speciesFormula
 Cristobalite(alpha)SiO2 
Mineralogy
Family: Quartz Group 
Space group: Tetragonal - Trapezohedral H-M Symbol (4 2 2) Space Group: P 41212 
Dana class: 75.1.1.1 (75) Tectosilicate Si Tetrahedral Frameworks (75.1)SiO2 with [4] coordinated Si (75.1.1) Dana Group 
Geological context
High temperature: Above approximately 1470C to the melting point of silica round 1713C.Alpha cristobalite undergoes a first phase transition at 200C-275C, from cubic to tetragonal phase beta cristobalite. 
Geological context: Present in volcanic rocks, it form from molten rock at temperatures above approximately 1470 degrees Celsius to the melting point of silica (around 1713C) 
Thermodynamics
Individual properties
Molar weight: 60.084 g/mol  ΔGºf-852,989 J/mol ref.  Sº: 46.06 J/mol/K ref. 
  ΔHºf-906,030 J/mol ref.  Cp: 44.32 J/mol/K ref. 
Reaction properties
Chemical reaction:  Cristobalite(alpha) + 2H2O = H4SiO4 
log10(Keq) = A + B.T + C.T-1 + D.log10(T) + E.T−2 (with T expressed in Kelvin)
A B C D E
 -2.09934027e+1   -3.43780449e-3   3.67618092e+1   7.70978315e+0   -3.06969672e+4 
Van t'Hoff
@ 25°C, 1 bar: log 10(Keq)-3.16
ΔH°r16,496 J/mol ref.
T (ºC) : 0 25 60 100 150 200 250 300
log 10(Keq):  -3.43   -3.16   -2.86   -2.57   -2.28   -2.06   -1.87   -1.73 
Maier Kelley
Cp(T) = a + b*10-3T + C*105T-2 (with T expressed in Kelvin)  ref.
a:33.47 J/mol/K 
b:49.28 J/mol/K2 
c:-3.42 J.K/mol 

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