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

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…

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

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

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

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

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

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

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

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

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

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

H2O phase diagram Only component is H2O

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

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

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?

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

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

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

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

Zoned Plagioclase crystal Oscillatory zoning Other types of zoning include: Normal zoning (Ca-rich centers) Reverse zoning (Na-rich centers) Fig. 12-12

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

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

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

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

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

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

{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

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