1. Mineral Stability
What controls when and where a particular mineral forms?
Commonly referred to as “Rock cycle”
Rock cycle: Mineralogical changes that occur because of variations in geologic environment
Knowing answer provides information about earth history or processes

2. Mineral formation
Why would you want to know earth history or processes:
Find: ore deposits, oil and gas, building materials
Understand engineering hazards, water cycle
Understand how humans effect the earth: climate…

3. A system for organizing mineralogical changes
The Rock Cycle
A system for organizing mineralogical changes
Fig. 5-1

4. Bowen’s reaction series
Fe, Mg - silicates
Ca, Na - silicates
Changing composition
Ca, Na, Fe, Mg - silicates
K-spar
Qtz

5. 3 requirements for mineral stability
Constituents
Available reactants/elements (X)
Correct environmental conditions (energy)
Pressure (P)
Temperature (T)

6. Mineral Stability More stable position is one of lower energy
Minerals may not be stable – e.g. metastable minerals
Mineral contains more energy than expected from their environment
Energy required to overcome metastability – activation energy

7. - energy to shake book off shelf
Activation Energy:
- energy to shake book off shelf
- Energy required to change mineral phases
Fig 5-2

8. How can stability be estimated?
Algebraically:
Physical chemistry/Thermodynamics
Estimates of DG – Gibbs free energy
Graphically – “phase diagrams”:
Essentially figures of solutions to DG problems
Many types, common ones:
One component – P & T variable, X fixed (i.e. the component)
Two (or more) components – T & X variable, P fixed

9. Components and Phases Component – Chemical entity
H2O
Al2SiO5
Phase – physically separable part of a system; e.g.
for H20: ice, water, water vapor
for Al2SiO5: Sillimanite, Kyanite, Andalusite
One and two component phase diagrams
Several types of 2-component diagrams

10. One component diagrams
Fields – where only one phase (mineral) is stable
Lines – where two phases are stable simultaneously
Points – where three phases are stable

11. One component diagrams
If P and/or T changes
One phase converts to another
Examples:
H2O – component; ice, water, and vapor are phases
Al2SiO5 – component; Kyanite, Andalusite, Sillimanite are phases

12. Al2SiO5 Phase diagram DG = f(P,T) Phase with lowest DG is stable
Lines mark boundaries of regions with the lowest DG
Very useful to remember for metamorphic reactions
Fig. 5.3

13. H2O phase diagram
Only component is H2O

14. More complete H2O diagram
There are 15 polymorphs of ice
Ice IX stability:
T < 140 K
2 kbar < P < 4 kbar
tetragonal
Commonly shown P & T conditions
Ice 9: Kurt Vonnegut, Cat’s Cradle, melting T = 45.8ºC at P = 1 Atm

15. Two component phase diagrams
What happens if there are two components in a system?
Example: Plagioclase feldspars – two components with complete solid solution (at high T, otherwise “exsolution”)
Albite– NaAlSi3O8
Anorthite – CaAl2Si2O8
Any composition in between the two end member compositions

16. How does solid (and melt) composition vary during crystallization?
How does composition vary as solids melt melt to form magma?
OR…
If you know the composition of a plagioclase feldspar, can you determine T and P of crystallization?

17. Two component phase diagram with complete solid solution
= Na, Ca, Al, SiO2
= (Na,Ca)xAlySizO8
100% Albite – NaAlSi3O8
Mole % Anorthite
100% Anorthite – CaAl2Si2O8

18. Equilibrium Crystallization
Start
An77
An68
End
An55
100% Albite – NaAlSi3O8
Mole % Anorthite
100% Anorthite – CaAl2Si2O8
(1) The crystals are always in equilibrium with the melt
(2) Minerals have homogeneous compositions throughout
Fig. 5-14a

19. Lever Rule %B = qr/qs %A = rs/qs
Fraction of two components relate to the relative lengths of tie lines
Fig. 5.5

20. Non-equilibrium crystallization
Results in “zoning”
Individual mineral grains may vary in composition from center to edge
Easily observed petrographically
Very common in plagioclase feldspars

21. Zoned Plagioclase crystal
Oscillatory zoning
Other types of zoning include:
Normal zoning (Ca-rich centers)
Reverse zoning (Na-rich centers)
Fig

22. Zoning reflects change in P and T when mineral crystallizes
Crystallizing mineral in disequilibrium with composition of melt
Can be explained by non-equilibrium crystallization using phase diagram

23. Non-Equilibrium Crystallization
Normal Zoning
Start
An77
An77
An68
An77
An55
Mole % Anorthite
Minerals show zoning – heterogeneous compositions
Fig. 5-14b

24. Controls on zoned crystals
Diffusion rate through solid crystal
Time allowed for diffusion to occur
Diffusion is rapid in olivine – few zoned crystals
Mostly equilibrium
Diffusion slow in plagioclase
Commonly zoned

25. Two component phase diagram - No solid solution
Ca, Mg, Al, SiO2 =
At me diopside, anorthite, and melt present
Fig. 5.4
At me, diopside begins xtll, anorthite continues xtll NO HEAT LOST – remains 1237º C – until all solid. Composition is 75% An, 25% Di. When first reach 1237º C, system is 48% anorthite, 52% melt

26. Rates of growth Slowest growing faces are often most prominent
Fast growth causes faces to disappear
This is why minerals have common forms

27. Halite {001} faces parallel to layers of bonded Na and Cl
Face is charge neutral
Weak attraction from this face to either ion

28. {111} faces parallel layers of pure Na and Cl
High surface charge on face
Comes from unsatisfied bonds from element
Strong attraction from this face to oppositely charge ion
Result is {111} face grows faster than {001} face
Thicker layer for a given amount of time

29. Start with octahedral faces
End with cube faces
Boundaries are “time lines”
Fig 5-7