12.3 Mineral Stability and Phase Diagrams

Mineral stability and phase diagrams are key to understanding how rocks form and change. They show us which minerals can exist together under different conditions of pressure, temperature, and composition. This knowledge helps geologists decode the history of rocks and predict how they'll behave in various environments.

These concepts are crucial for grasping mineral associations and paragenesis. By studying stability fields and reaction boundaries, we can figure out why certain minerals appear together and in what order they formed. This gives us a powerful tool for unraveling the complex stories hidden in rocks.

Mineral Stability and Its Controls

Fundamental Concepts of Mineral Stability

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• Mineral stability defines the range of conditions where a mineral exists without phase change or chemical reaction
• Three primary variables control mineral stability
  • Pressure (P)
  • Temperature (T)
  • Composition (X)
• Gibbs' Phase Rule relates components, phases, and degrees of freedom in equilibrium systems
  • Formula: $F = C - P + 2$
  • F = degrees of freedom
  • C = number of components
  • P = number of phases
• Chemical potential of minerals fluctuates with P, T, and X variations affecting stability
• Polymorphism occurs in minerals with identical chemical composition but different crystal structures based on P-T conditions (diamond and graphite)

Advanced Concepts in Mineral Stability

• Solid solutions influence mineral stability across varying compositions
  • Example: plagioclase feldspar series (albite to anorthite)
• Element partitioning between coexisting minerals impacts stability
  • Distribution coefficient (Kd) quantifies element partitioning
  • $Kd = (X_A/X_B)_{mineral1} / (X_A/X_B)_{mineral2}$
• Ostwald's step rule describes metastable phase formation in some systems
  • Metastable phases may form before stable phases due to kinetic factors
• Reaction kinetics affect the rate of mineral transformations
  • Slow kinetics can preserve metastable minerals (andalusite in high-pressure rocks)

Interpreting Phase Diagrams

Fundamentals of Phase Diagrams

• Phase diagrams graphically represent mineral stability fields as functions of P, T, and/or X
• Binary phase diagrams illustrate two-component systems
  • Example: SiO2-Al2O3 system for ceramics and refractories
• Ternary diagrams depict three-component systems
  • Example: CaO-MgO-SiO2 system for igneous petrology
• Stability fields in phase diagrams separated by reaction lines or curves
  • Reaction lines represent P-T conditions of mineral coexistence in equilibrium
• Invariant points on phase diagrams show conditions where multiple phases coexist in equilibrium
  • Triple point of water (0.01°C, 611.73 Pa) where solid, liquid, and vapor coexist

Advanced Interpretation Techniques

• Lever rule determines phase proportions in two-phase regions of binary systems
  • $\frac{W_A}{W_B} = \frac{X_B - X}{X - X_A}$
  • W = weight fraction, X = composition
• Tie lines in ternary systems connect coexisting phases
  • Used to determine phase compositions and proportions
• Isothermal and isobaric sections of multicomponent systems provide snapshots of phase relations
• Schreinemakers' method constructs and analyzes multivariant systems
  • Determines sequence of mineral reactions
• Reaction bundles in P-T space illustrate related sets of reactions
  • Example: Al2SiO5 polymorphs (andalusite, sillimanite, kyanite)

Predicting Mineral Assemblages

Mineral Reactions and Assemblage Changes

• P, T, or X changes trigger mineral reactions forming new assemblages or altering existing ones
• Reaction boundaries on phase diagrams indicate conditions of specific mineral reactions
• Discontinuous reactions involve complete disappearance or appearance of mineral phases
  • Example: muscovite + quartz → K-feldspar + Al2SiO5 + H2O
• Continuous reactions involve gradual changes in mineral composition
  • Example: Fe-Mg exchange in garnet-biotite pairs
• Mineral zonation in metamorphic rocks interpreted using phase diagrams
  • Reveals P-T-X path of rock during metamorphism
• Pseudosections (P-T-X diagrams for specific bulk compositions) predict mineral assemblages and modal abundances
  • Example: MnNCKFMASH system for metapelites

Advanced Prediction Techniques

• Gibbs method of phase diagram analysis predicts stable assemblages
  • Minimizes Gibbs free energy of the system
• Thermodynamic databases and software (THERMOCALC, Perple_X) enable complex phase equilibria calculations
• Reaction progress variables track extent of reactions in multicomponent systems
• Chemographic projections simplify representation of mineral compositions in complex systems
  • AFM diagrams for metapelitic rocks
• Fractionation effects on mineral assemblages during metamorphism or partial melting
  • Example: garnet fractionation during prograde metamorphism

Phase Diagrams: Applications in Geology

Metamorphic and Igneous Applications

• Metamorphic facies represent mineral assemblages formed under specific P-T conditions
  • Example: greenschist facies (chlorite, actinolite, albite)
• P-T diagrams illustrate stability fields of metamorphic facies and transitions
  • Barrovian metamorphic sequence in pelitic rocks
• Igneous differentiation processes modeled using phase diagrams
  • Fractional crystallization paths in binary and ternary systems
  • Example: olivine fractionation in basaltic magmas
• Magma mixing processes represented on phase diagrams
  • Linear mixing lines between end-member compositions
• Volatile effects (H2O, CO2) on mineral stability and melting behavior
  • H2O lowers melting temperatures in silicate systems
  • CO2 affects calc-silicate mineral stability

Hydrothermal and Environmental Applications

• Hydrothermal alteration processes understood through activity-activity diagrams
  • Mineral stability fields as function of fluid composition
  • Example: K-feldspar-muscovite-kaolinite stability in K+/H+ vs. SiO2 space
• Reaction path modeling predicts mineral assemblage sequences
  • Progressive metamorphism or hydrothermal alteration
  • Example: skarn formation at limestone-intrusion contacts
• Eh-pH diagrams illustrate mineral stability in aqueous environments
  • Applications in ore deposit formation and environmental geochemistry
• Solid solution models in environmental applications
  • Trace element partitioning in minerals (heavy metals in clays)
• Phase diagrams in planetary geology
  • Mineral stability under extreme P-T conditions of planetary interiors