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Stable mineral assemblages

Stable mineral assemblages. At equilibrium. Phase rule: F=C-P+2 F=degrees of freedom (T, P) C=components (chemical constituents) P=phases For equilibrium, # phases = or < than # components. Liquid. Plagioclase. plus. Liquid. Plagioclase. Phase rule. Phases < C

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Stable mineral assemblages

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  1. Stable mineral assemblages

  2. At equilibrium • Phase rule: F=C-P+2 • F=degrees of freedom (T, P) • C=components (chemical constituents) • P=phases • For equilibrium, # phases = or < than # components

  3. Liquid Plagioclase plus Liquid Plagioclase Phase rule • Phases < C • Common with solid solution

  4. Phase rule • Consider with C=1 • P = 1 common • P = 2 rare • P = 3 very specific P-T conditions which is an invariant point

  5. Consider MgO-H2O system • Possible phases are periclase (MgO), water, brucite (Mg(OH)2) • Is water mobile or necessary to drive reaction? • MgO + H2O  Mg(OH)2 Per + Fluid = Bru • Prograde or retrograde reaction?

  6. Consider MgO-H2O system • MgO + H2O  Mg(OH)2 Per + Fluid = Bru • Is water a necessary component or mobile (present but not necessary)?

  7. Consider MgO-H2O system • MgO + H2O  Mg(OH)2 Per + Fluid = Bru • Water is limiting reactant • How can periclase exist in the blue part of the diagram?

  8. Consider MgO-H2O system • Mg(OH)2 MgO + H2O Bru = Per + water • Is water a necessary component or mobile?

  9. Presence or absence of water and CO2 can greatly change stability fields of minerals!!!!

  10. Chemographics • Graphical representation of chemistry of mineral assemblages • A simple example: the olivine system as a linear C = 2 plot:

  11. Chemographic diagrams • 3 components plotted on triangle diagrams

  12. Phases that coexist at equilibrium in a rock are connected by tie-lines

  13. Any point in the diagram represents a specific bulk composiiton • The digram determines the stable mineral assemblage • Shift in bulk composition varies mineral assemblage

  14. What happens if you pick a composition that falls directly on a tie-line, such as point (f)? • Only minerals xyz and z in this bulk composition

  15. Some minerals exhibit solid solution • X(y,z) and x2(yz) show limited solid solution of y and z • Which mineral has more solid solution? • Minerals xyz and z have solid solution of all 3 components

  16. Bulk composition of F • Two phases coexisting

  17. Any bulk composition within a 3-phase triangle will have 3 phases in it

  18. Chemographic Diagrams for Metamorphic Rocks • Most common natural rocks contain the major elements: SiO2, Al2O3, K2O, CaO, Na2O, FeO, MgO, MnO and H2O such that C = 9 • Three components is the maximum number that we can easily deal with in two dimensions • What is the “right” choice of components? • We turn to the following simplifying methods:

  19. 1) Simply “ignore” components • Trace elements • Elements that enter only a single phase (we can drop both the component and the phase without violating the phase rule) • Perfectly mobile components

  20. 2) Combine components • Components that substitute for one another in a solid solution: (Fe + Mg) 3) Limit the types of rocks to be shown • Only deal with a sub-set of rock types for which a simplified system works 4) Use projections • I’ll explain this shortly

  21. ACF diagram for mafic rocks • A: Al2O3 + Fe2O3 - Na2O and K2O • C: CaO+3.3P2O5 • F: FeO+MgO+MnO • Reduce 8 compnents to 3

  22. ACF example • Anorthite CaAl2Si2O8 • A = 1 + 0 - 0 - 0 = 1, C = 1 - 0 = 1, and F = 0 • 1:1 ratio of A and C A = 0.5 C = 0.5 F = 0

  23. ACF example • Anorthite CaAl2Si2O8 • A = 1 + 0 - 0 - 0 = 1, C = 1 - 0 = 1, and F = 0 • 1:1 ratio of A and C A = 0.5 C = 0.5 F = 0

  24. ACF diagram • Quartz-bearing assemblages in kyanite zone • Green field is bulk composition • Which mineral will more calcic rocks be void of?

  25. AKF diagram for more pelitic sediments • High in Al2O3 and K2O • Low in CaO

  26. AKF diagram • Silica-saturated, low P met rocks in Finland • What minerals are present in rock of composition represented by yellow dot?

  27. CMS diagram • For calc-silicates Fo - Mg2SiO4 Per - MgO En - MgSiO3 Qtz - SiO2 Di - CaMgSi2O6 Cc - CaCO3

  28. AKF is useful, but Fe and Mg do not always partition equally in most minerals • Alternative: AFM diagram

  29. A(K)FM diagram A = Al2O3 K = K2O F = FeO M = MgO Can project onto a face of the pyramid to avoid 3D

  30. Projecting from perspective of muscovite since common in pelites • Can’t do it from K since no real mineral sits here • Biotite now outside AFM face projecting below F-M tieline.

  31. AFM projection from muscovite for pelitic rocks • What minerals are stable for the composition shown by the yellow dot?

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