Mineral StabilityMineral Stability

What controls when and where a What controls when and where a particular mineral forms?particular mineral forms? Commonly referred to as “Rock cycle”Commonly referred to as “Rock cycle” Rock cycle: Mineralogical changes that Rock cycle: Mineralogical changes that

occur because of variations in geologic occur because of variations in geologic environmentenvironment

Knowing answer provides information Knowing answer provides information about earth history or processesabout earth history or processes

Mineral formationMineral formation

Why would you want to know earth Why would you want to know earth history or processes:history or processes: Find: ore deposits, oil and gas, building Find: ore deposits, oil and gas, building

materialsmaterials Understand engineering hazards, water Understand engineering hazards, water

cyclecycle Understand how humans effect the Understand how humans effect the

earth: climate…earth: climate…

Fig. 5-1Fig. 5-1

A system for A system for organizing organizing mineralogicamineralogical changesl changes

The Rock CycleThe Rock Cycle

Bowen’s reaction seriesBowen’s reaction series

Fe, Mg - silicates

Ca, Na - silicates

Ca, Na, Fe, Mg - silicates

K-spar

Qtz

Changin

g c

om

posi

tion

3 requirements for 3 requirements for mineral stabilitymineral stability

Constituents Constituents Available reactants/elements (X)Available reactants/elements (X)

Correct environmental conditions Correct environmental conditions (energy)(energy) Pressure (P)Pressure (P) Temperature (T)Temperature (T)

Mineral StabilityMineral Stability

More stable position is one of lower More stable position is one of lower energyenergy

Minerals may not be stable – e.g. Minerals may not be stable – e.g. metastable mineralsmetastable minerals Mineral contains more energy than Mineral contains more energy than

expected from their environmentexpected from their environment Energy required to overcome Energy required to overcome

metastability – metastability – activation energyactivation energy

Fig 5-2Fig 5-2

Activation Energy:Activation Energy:

- energy to shake book off - energy to shake book off shelfshelf- Energy required to - Energy required to change mineral phaseschange mineral phases

How can stability be How can stability be estimated?estimated?

Algebraically:Algebraically: Physical chemistry/ThermodynamicsPhysical chemistry/Thermodynamics Estimates of Estimates of G – Gibbs free energyG – Gibbs free energy

Graphically – “phase diagrams”:Graphically – “phase diagrams”: Essentially figures of solutions to Essentially figures of solutions to G G

problemsproblems Many types, common ones:Many types, common ones:

One component – P & T variable, X fixed (i.e. One component – P & T variable, X fixed (i.e. the component)the component)

Two (or more) components – T & X variable, P Two (or more) components – T & X variable, P fixedfixed

Components and PhasesComponents and Phases ComponentComponent – Chemical entity – Chemical entity

HH22OO AlAl22SiOSiO55

PhasePhase – physically separable part of a – physically separable part of a system; e.g.system; e.g. for Hfor H220: ice, water, water vapor0: ice, water, water vapor for Alfor Al22SiOSiO55: Sillimanite, Kyanite, Andalusite: Sillimanite, Kyanite, Andalusite

One and two component phase diagramsOne and two component phase diagrams Several types of 2-component diagramsSeveral types of 2-component diagrams

One component diagramsOne component diagrams

FieldsFields – where only one phase – where only one phase (mineral) is stable(mineral) is stable

LinesLines – where two phases are stable – where two phases are stable simultaneouslysimultaneously

PointsPoints – where three phases are – where three phases are stablestable

One component diagramsOne component diagrams

If P and/or T changesIf P and/or T changes One phase converts to anotherOne phase converts to another Examples:Examples:

HH22O – component; ice, water, and vapor are O – component; ice, water, and vapor are phasesphases

AlAl22SiOSiO55 – component; Kyanite, Andalusite, – component; Kyanite, Andalusite, Sillimanite are phasesSillimanite are phases

AlAl22SiOSiO55 Phase Phase diagramdiagram

G = f(P,T)G = f(P,T)Phase with lowest Phase with lowest G G is stableis stable

Lines mark Lines mark boundaries of regions boundaries of regions with the lowest with the lowest GG

Very useful to Very useful to remember for remember for metamorphic metamorphic reactionsreactions

Fig. 5.3

HH22O phase diagramO phase diagram

Only Only component is component is HH22OO

More complete HMore complete H22O diagramO diagram

Commonly shown P & T conditions

There are 15 There are 15 polymorphs of polymorphs of iceice

Ice IX stability:Ice IX stability:

T < 140 KT < 140 K2 kbar < P < 4 2 kbar < P < 4 kbarkbar

tetragonaltetragonal

Ice 9: Kurt Vonnegut, Cat’s Cradle, melting T = 45.8ºC at P = 1 Atm

Two component phase Two component phase diagramsdiagrams

What happens if there are two What happens if there are two components in a system?components in a system?

Example: Plagioclase feldspars – two Example: Plagioclase feldspars – two components with complete solid solution components with complete solid solution (at high T, otherwise “exsolution”)(at high T, otherwise “exsolution”) Albite– NaAlSiAlbite– NaAlSi33OO88

Anorthite – CaAlAnorthite – CaAl22SiSi22OO88

Any composition in between the two end Any composition in between the two end member compositionsmember compositions

How does solid (and melt) composition How does solid (and melt) composition vary during crystallization?vary during crystallization?

How does composition vary as solids How does composition vary as solids melt melt to form magma?melt melt to form magma?

OR…OR… If you know the composition of a If you know the composition of a

plagioclase feldspar, can you plagioclase feldspar, can you determine T and P of crystallization?determine T and P of crystallization?

Two component phase diagram Two component phase diagram with complete solid solutionwith complete solid solution

100% Albite – NaAlSi3O8

100% Anorthite – CaAl2Si2O8

Mole % Anorthite

= Na, Ca, Al, SiO2

= (Na,Ca)xAlySizO8

Fig. 5-14aFig. 5-14a

Equilibrium Crystallization

Start

End

Mole % Anorthite

An77

An68

An55

(1) The crystals are always in equilibrium with the melt(1) The crystals are always in equilibrium with the melt(2) Minerals have homogeneous compositions (2) Minerals have homogeneous compositions throughoutthroughout

100% Albite – NaAlSi3O8

100% Anorthite – CaAl2Si2O8

Lever Lever RuleRule

%B = qr/qs %A = rs/qs

Fig. 5.5

Fraction of two components relate to the relative lengths of tie lines

Non-equilibrium Non-equilibrium crystallizationcrystallization

Results in “zoning”Results in “zoning” Individual mineral grains may vary in Individual mineral grains may vary in

composition from center to edgecomposition from center to edge Easily observed petrographicallyEasily observed petrographically

Very common in plagioclase Very common in plagioclase feldsparsfeldspars

Fig. 12-12Fig. 12-12

Zoned Plagioclase crystalZoned Plagioclase crystal

Oscillatory zoning

Other types of zoning include: (1)Normal zoning (Ca-rich centers)(2)Reverse zoning (Na-rich centers)

Zoning reflects change in P and T Zoning reflects change in P and T when mineral crystallizeswhen mineral crystallizes Crystallizing mineral in disequilibrium Crystallizing mineral in disequilibrium

with composition of meltwith composition of melt Can be explained by non-equilibrium Can be explained by non-equilibrium

crystallization using phase diagramcrystallization using phase diagram

Fig. 5-14bFig. 5-14b

Non-Equilibrium Crystallization

Minerals show zoning – Minerals show zoning – heterogeneous heterogeneous compositionscompositions

Start

Mole % Anorthite

An77

An68

An55An77

An77

Normal Zoning

Controls on zoned crystalsControls on zoned crystals

Diffusion rate through solid crystalDiffusion rate through solid crystal Time allowed for diffusion to occurTime allowed for diffusion to occur

Diffusion is rapid in olivine – few Diffusion is rapid in olivine – few zoned crystalszoned crystals Mostly equilibriumMostly equilibrium

Diffusion slow in plagioclaseDiffusion slow in plagioclase Commonly zonedCommonly zoned

Two component phase diagram - No solid solutionTwo component phase diagram - No solid solution

Fig. 5.4

At mAt mee, diopside begins xtll, anorthite continues xtll NO HEAT LOST , diopside begins xtll, anorthite continues xtll NO HEAT LOST – remains 1237º C – until all solid. Composition is 75% An, 25% Di. – remains 1237º C – until all solid. Composition is 75% An, 25% Di. When first reach 1237º C, system is 48% anorthite, 52% meltWhen first reach 1237º C, system is 48% anorthite, 52% melt

At me diopside, anorthite, and melt present

Ca, Mg, Al, SiO2 =

Rates of growthRates of growth

Slowest growing faces are often most Slowest growing faces are often most prominentprominent

Fast growth causes faces to Fast growth causes faces to disappeardisappear

This is why minerals have common This is why minerals have common formsforms

HaliteHalite

{001} faces parallel {001} faces parallel to layers of bonded to layers of bonded Na and ClNa and Cl Face is charge Face is charge

neutralneutral Weak attraction from Weak attraction from

this face to either this face to either ionion

{111} faces parallel layers of pure Na {111} faces parallel layers of pure Na and Cland Cl High surface charge on faceHigh surface charge on face Comes from unsatisfied bonds from elementComes from unsatisfied bonds from element Strong attraction from this face to oppositely Strong attraction from this face to oppositely

charge ioncharge ion Result is {111} face grows faster than Result is {111} face grows faster than

{001} face{001} face Thicker layer for a given amount of timeThicker layer for a given amount of time

Fig 5-7Fig 5-7

Start with octahedral faces

End with cube faces

Boundaries are “time lines”