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Oxidation Reaction using Oxygen Free energy fugacity fugacity buffers
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DM Sherman, University of Bristol
Geochemistry, DM Sherman University of Bristol
Consider a redox reaction that can occur in an igneous rock: 3Fe 2 SiO 4 + O 2 = 2Fe 3 O 4 + 3SiO 2 €
( aSiO 2 ) 3 ( aFe 3 O 4 ) 2 ( aFe 2 SiO 4 ) 3 aO 2
( XFe 2 ) 3 aO 2 We can define the redox state in terms of aO2 , but how do we define aO2?
DM Sherman, University of Bristol
Recall how we calculated G of a gas as a function of pressure:
At constant T, € dG = VdP € G ( P ) − G ( P 0 ) = VdP P 0 P
which we can integrate to get
For an ideal gas, € PV = nRT or V = nRT P € G ( P ) − G ( P 0 ) = VdP P 0 P ∫ =^ nRT^ 1 P dP P 0 P ∫ So the integral can be evaluated € G ( P ) = G ( P 0 ) + nRT ln P P 0
DM Sherman, University of Bristol
Fugacity is like a corrected pressure: Fugacity has units of pressure and equals the partial pressure in the limit of ideal gas behavior. €
Note that the activity of component i in a mixture is simply: € ai = fi fi 0
where fi^0 is the fugacity of i at a reference state (taken to be pure i at 1 bar). Activity only has meaning relative to a standard state while fugacity has an absolute meaning. Geologists have a habit of saying “fugacity” when they should be saying “activity”. If we use a standard state of 1 bar, they are numerically equal.
DM Sherman, University of Bristol
(( γ Fe 3 O 4 XFe 2 O 3 Hm ) 2 ) 6 ( aO 2 )( γ Fe 3 O 4 XFe 3 O 4 Mt ) 4 In igneous and metamorphic rocks, coexisting solid solutions Fe 2 O 3 (hematite)-FeTiO 3 (ilmenite) and Fe 3 O 4 (magnetite)-Fe 2 TiO 3 (ulvospinel) can be used to determine aO2 and T: 4Fe 3 O 4 + O 2 = 6Fe 2 O 3
