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Mineral speciesFormula
 GoethiteFeOOH 
Mineralogy
Family: Gibbsite - Doyleite series 
Space group: Orthorhombic - Dipyramidal HM symbol (2/m 2/m 2/m) Space Group: Pbnm 
Dana class: 6.1.1.2VI - 6 Hydroxides and Oxides Containing Hydroxyl ; 6.1 Hydroxides and Oxides Containing Hydroxyl where X+++O OH ; 6.1.1 Diaspore group (Orthorhombic, Pnma or Pnmd) 
Geological context
High temperature: In the presence of pure water goethite is stable at all temperatures between OC to 100C. Well crystallized goethite gives a d.t.a. curve with a single endothermic peak at a temperature of 350C to 390C. 
Geological context: Goethite is a very common mineral typically occurring as a weathering product of iron-bearing minerals such as siderite, magnetite, pyrite, etc. It is normally formed under oxidizing conditions. 
Thermodynamics
Individual properties
Molar weight: 88.854 g/mol  ΔGºf-488,506 J/mol ref.  Sº: 58.02 J/mol/K ref. 
  ΔHºf-560,000 J/mol ref.  Cp: 74.31 J/mol/K ref. 
Reaction properties
Chemical reaction:  Goethite + 3H+ = Fe+3 + 2H2O 
log10(Keq) = A + B.T + C.T-1 + D.log10(T) + E.T−2 (with T expressed in Kelvin)
A B C D E
 -4.96873346e+2   -8.12700949e-2   2.77465439e+4   1.78978717e+2   -1.28606340e+6 
Van t'Hoff
@ 25°C, 1 bar: log 10(Keq)0.36
ΔH°r38,222 J/mol ref.
T (ºC) : 0 25 60 100 150 200 250 300
log 10(Keq):  1.33   0.36   -0.75   -1.77   -2.79   -3.66   -4.47   -5.29 
Maier Kelley
Cp(T) = a + b*10-3T + C*105T-2 (with T expressed in Kelvin)  ref.
a:1.25 J/mol/K 
b:233.20 J/mol/K2 
c:3.14 J.K/mol 

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