MRT 251-3 - Mineral Stability Envirnment.pdf

Minaral testing

1. MINERAL STABILITY
MRT 251-3 Mineralogy and Petrology -II

2. Introduction
•Wide range of externally imposed
conditions (e.g. P, T and Composition of
host material)
•Multitude of minerals from (limited)
chemical constituents
•Growth and persistence of minerals
determined by specific set of conditions
•Governed by thermodynamic principles

3. Introduction
•Stability of a chemical
system related to energy of
the system
•Low-energy state of a
system is the most stable
•Change from “Metastable
state” needs some energy
(activation energy)
•In a chemical system,
energy needed to cause a
chemical reaction to occur

4. Introduction
•Stability determined by
energy difference between
two states (not absolute
energy)
•At steady state (no change
with time) materials exist at
equilibrium
•Mineral systems (rocks)
reach equilibrium in millions
of years
•Minerals may be metastable

5. Thermodynamics Principles
•Thermodynamics allows the quantitative
assessment of stability and “phase
equilibria”
•Minerals and rocks tend toward the lowest
energy state
•Minerals react
•(Try to) Achieve the most stable state for
constituents

6. Thermodynamics Principles
•“All organizations of matter drive toward a
minimal energy state”
–A universal observation
•Minerals/rocks tend toward the lowest energy
state
–Most stable state for their constituents

7. Thermodynamics Principles
•“The internal energy (E) of an isolated system
is constant”
–First law of thermodynamics
•A “system” is any part of the universe being
considered
–A rock hand specimen, a specific mineral
assemblage, a specific chemical mixture

8. Thermodynamics Principles
•Closed system
–No addition/subtraction of material
–Mass remains constant
–Energy may exchange
•Change in internal energy (dE)
–Difference between heat (dQ) added and work
done (W)
•dE= dQ–dW(1
st
law of thermodynamics)

9. Thermodynamics Principles
dE= dQ–dW
work = force x distance
force = pressure x surface area
Hence
work = pressure x surface area x distance
work (W) = pressure (P) x volume (V)

10. Thermodynamics Principles
W= PV
At constant pressure
dW= PdV
Then
dE= dQ–PdV
(familiar form of the 1
st
law of thermodynamics)

11. Thermodynamics Principles
dE= dQ–PdV
For a mineral;
When heat is added internal energy
proportionately changes
Part of the added energy is transformed into
work
Thermal expansion of the mineral takes place

12. Thermodynamics Principles
•“A rigorously ordered structure has a lower
entropy than a disordered structure”
–Second law of thermodynamics
•“Entropy (S)” represents the degree of
disorder/randomness in a system
–Highest order at the lowest temperature
–e.g. water has higher entropy than ice
–Change in thermal energy relates to change in
change in disorder (at constant P/T)
dQ/T = dS

13. Thermodynamics Principles
•“At absolute zero (0 K, -273.15
0
C ) a crystalline
structure approaches perfect order, and the
entropy of such a perfect crystal is zero
–Third law of thermodynamics

14. Thermodynamics Principles
•“Gibbs free energy (G) is the energy in excess
of the internal energy (E)”
–Excess energy needed to drive a chemical reaction
(Recall, W = PV, Q = TS, E = Q -W)
G = E + PV –TS
dG= dE+ VdP+ PdV–SdT–TdS
dG= dE+ VdP–SdT–(TdS–PdV)
TdS= dQ
dQ–PdV= dE
dG= VdP–SdT

15. Thermodynamics Principles
•Stability of a chemical reaction is expressed by
•For a chemical reaction, change in Gibbs free
energy (dG
rx) is expressed in terms of Gibbs
free energies of products and reactants
dG= VdP–SdT
dG
rx=dG
p-dG
r

16. Thermodynamics Principles
•For a system in equilibrium, at constant P/T, G
is minimum
G
r= G
p
dG= G
p–G
r= 0
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T= V
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P= -S
dG= VdP–SdT

17. Thermodynamics Principles
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T= V
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P= -S
Dense phases (smaller volumes)
favored at high P
High disorder states (high
entropy) favored at high T

18. Thermodynamics Principles
•Gibbs free energy (G) is a function of only P
and T
–(G) can be graphically represented as functions of
P and T

19. Thermodynamics Principles

20. Thermodynamics Principles
•G-Tplot is “isobaric”
•G-P Plot is “isothermal”
•Projection of equilibrium curve of Gsurface
onto P-T plane shows mineral (phase) stability
fields

21. Phase Diagrams (Stability Diagrams)
•Behavior of solids, liquids and gases under
variable external conditions (P/T) is shown in
phase diagrams
•A “phase” is a homogeneous substance with
well-defined set of physical and chemical
properties (e.g. minerals, water, gases)
•A mineral should be single-composition and
unzonedto be considered a “phase”

22. Phase Diagrams (Stability Diagrams)

23. Phase Diagrams (Stability Diagrams)
•Components
–Describe phases in a chemical system
–Independent chemical species
–Generally the minimum number of chemical
variables chosen
–Can be actual minerals or theoretical
compositions
–e.g. Al
2SiO
5or Al
2O
3and SiO
2for andalusite-
sillimanite-kyanitesystem
–e.g. CaSiO
3-MgSiO
3-FeSiO
3 for pyroxenes

24. Phase Diagrams (Stability Diagrams)
•One-Component Diagrams
–A single chemical component describes the
chemical system
–e.g. C for graphite-diamond, SiO
2for silica
polymorphs, CaCO
3for calcite-aragonite, Al
2SiO
5
for andalusite-sillimanite-kyanite

25. Phase Diagrams (Stability Diagrams)

26. Phase Diagrams (Stability Diagrams)

27. Phase Diagrams (Stability Diagrams)

28. Phase Diagrams (Stability Diagrams)
•Two-Component Diagrams
–Generally display compositional variation with T
(at constant P)
–T-X diagrams
–Different from P-T diagrams
–Depict complete solid solutions, partial solid
solutions or no solid solutions

29. Phase Diagrams (Stability Diagrams)

30. Phase Diagrams (Stability Diagrams)

31. Phase Diagrams (Stability Diagrams)

32. Phase Diagrams (Stability Diagrams)

33. Phase Diagrams (Stability Diagrams)
•Three (or more)-Component Diagrams
–Most igneous, metamorphic and sedimentary
rocks commonly contain 6-10 components
–One-or Two-component diagrams have limited
applicability in natural systems
–Difficult to represent more than four components
graphically
–Triangular plots commonly used in igneous
petrology
–Melting temperatures for specific temperatures
shown as contours (isotherms)

34. Phase Diagrams (Stability Diagrams)

35. Phase Diagrams (Stability Diagrams)
•Three (or more)-Component Diagrams
–Compositional variation among members of a
mineral group can be shown in a triangular
diagram
–e.g. pyroxene, olivine, amphibole groups
–“Tielines” connect coexisting compositions across
miscibility gaps

36. Phase Diagrams (Stability Diagrams)

37. Phase Diagrams (Stability Diagrams)
•Assemblage Diagrams
–Coexistence of possible mineral pairs or groups of
three minerals can be shown in triangular
diagrams
–Known as “assemblage diagrams”
–e.g. orthoclase-albite-quartz-biotite
–Only the assemblage in equilibrium considered
–Alteration products are not considered

38. Phase Diagrams (Stability Diagrams)

39. Phase Diagrams (Stability Diagrams)

40. Geologic Processes and Mineral
Stability