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Mineral Stability

Mineral Stability

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Mineral Stability

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

  4. Bowen’s reaction series Fe, Mg - silicates Ca, Na - silicates Changing composition K-spar Ca, Na, Fe, Mg - silicates 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. metastableminerals • Mineral contains more energy than expected from their environment • Energy required to overcome metastability – activation energy

  7. 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. Fig. 5-14a 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

  19. %A = rs/qs Fraction of two components relate to the relative lengths of tie lines Lever Rule %B = qr/qs 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. Fig. 12-12 Zoned Plagioclase crystal Oscillatory zoning • Other types of zoning include: • Normal zoning (Ca-rich centers) • Reverse zoning (Na-rich centers)

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

  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. Ca, Mg, Al, SiO2 = At me diopside, anorthite, and melt present Two component phase diagram - No solid solution 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. Fig 5-7 Start with octahedral faces End with cube faces Boundaries are “time lines”