Thermodynamics in petrology
Thermodynamics = heat + work
We can study energy transfer during reactions in
chemical systems.
Reactions
• reversible
• irreversible
Cautions and limitations
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Monday, September 28, 2009
Thermodynamics in petrology
Want to answer questions such as:
• How are phase diagrams made?
• How do we evaluate phase equilibria?
• How do mineral compositions relate to P & T?
• What do we need to know to make a P-T or T-X diagram?
• What are the thermodynamic properties of minerals?
Shortly we will apply these principles to:
• Metamorphic mineral reactions
• Activity models
• Thermobarometry
• P-T-t paths
Monday, September 28, 2009
Le Châtelier’s Principle
If a chemical system at equilibrium experiences a change in
concentration, temperature, volume, or partial pressure, then
the equilibrium shifts to counter-act the imposed change.
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phase stable at
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• relation of T & H
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Monday, September 28, 2009
energy input
• discontinuity of ∆H
shows energy
gained by system
at constant T
during fusion
reaction
Configurational entropy (S)
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Monday, September 28, 2009
∴
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∴ A is stable
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Monday, September 28, 2009
Clapeyron example:
if we know one point on a reaction equilibrium,
can we determine the whole thing?
grossular + corundum
=
anorthite + gehlenite
If we know one point
(760-780°C @ 1 kbar),
how can we get the rest?
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How do we interpret the Clapeyron slope?
good barometer
good
thermometer
• solid-solid reaction
• equilibrium is a straight line
because ΔS/ΔV = constant for
reactions with no compressible
fluid phase
• devolatilization or melting reaction
• at low T, initial devolatilization and low ΔS/ΔV
(low slope)
• fluids are highly expandable, so ΔV is high when
going to higher T at low P
• ΔS is low because just beginning to make
volatiles
• at high T, have high ΔS/ΔV
• more volatiles are released, so have high ΔS
• ΔV is low because fluids compress at high P
Monday, September 28, 2009