Mineral Stability Phase Diagrams

1
Mineral StabilityPhase Diagrams

• Klein, 2002, p. 115-134

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Mineral Stability

• Phase diagrams / stability diagrams
• Diagram with compositional temperature
  variation
• Showing the stability of the different phases
• This is a phase diagram.
• Time to introduce
• Phase
• Phase diagram

temperature
composition
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Mineral Stability

• A PHASE is a homogenous substance with a
  well-defined set of physical chemical
  properties.
• Looks like the definition of mineral but
• well-defined (so no variations)
• substance (so not only crystalline solid)
• Examples
• Low quartz is a low-temperature phase in system
  SiO2
• Kyanite is a high pressure phase in system Al2SiO5

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Mineral Stability

• Solid solution is called phase region
• Eg olivine forsterite (Mg-rich) to fayalite
  (Fe-rich)
• Phase can be solid, liquid gaseous
• Easy example
• H2O ice (solid) water (liquid) steam (gas)

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Mineral Stability

• H2O phase diagram
• Pressure Temperature phase diagram
• aka P-T diagram
• At 1 atmosphere
• At 0o C ice/water
• At 100o C water/steam

liquid
Pressure
solid
vapor
Temperature
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Mineral Stability

• H2O phase diagram
• Along the curves two phases can coexist in
  equilibrium
• Ice liquid water
• Liquid water vapor

liquid
Pressure
solid
vapor
Triple Point
Temperature
TRIPLE POINT All 3 phases co-exist
7
Mineral Stability

• This system of H2O (solid-liquid-vapor) is called
  a one-component system.
• Al2SiO5 is a one-component system too.

8
Phase Rule
What is it used for?

• How to study the formation of minerals in igneous
  rocks?
• Most processes take place deep in the crust
  outside our realm of observation
• The origin of rocks and minerals involves the
  application of physical chemistry to the
  formation of igneous or metamorphic rocks.

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Phase Rule

• How to study the formation of minerals in igneous
  rocks?
• Laboratory experiments
• Difficult because high pressure, high
  temperature, corrosive substance, and many
  elements involved.
• Geologic observation
• Direct observation of volcanic rocks forming
• Observation of cooled plutonic rocks
• Extrapolation of simple experiments
• Laboratory experiments with only a few phases

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Phase Rule

• Watching volcanic rocks form

Hawaii
Iceland
Hawaii
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Phase Rule

• Direct observation of volcanic rocks forming
• Lava is a silicate melt
• Viscous
• Emitting gasses
• Gradually loses mobility
• Freezing to glass mass of crystals or both
• Measuring
• The cooled rock composition (melt without gas)
• Rate of temperature loss study of how melt
  becomes rock
• Change in viscosity with cooling and composition
• These observations can be extrapolated to
  plutonic rocks

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• Laboratory experiments
• 2 approaches
• 1. Melt an igneous rock in the lab and then cool
• Glass forms from rapidly cooling lava
• Crystals form from slow cooling lava
• Silica-rich magmas more viscous than magma with
  higher MgO and FeO contents
• This approach did not lead to all the desired
  results
• 2. Experiment with simple silicate systems
• This approach was more successful
• IMPORTANT The guiding principle in working with
  simple silicate melts is Gibbs Phase Rule

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Phase Rule
Josiah Willard Gibbs (1839-1903) 

• Gibbs was a theoretical physicist who derived the
  Phase Rule
• F C-P2
• where F degrees of freedom C no. of
  components P no. of phases
• Here we are going to use the phase rule in the
  one component system H2O

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Phase Rule

• H2O Pressure Temperature phase diagram
• Water vapor at constant pressure, decrease the
  temperature (cooling)
• Suddenly liquid water appears
• But the temperature does not decrease until all
  the vapor has disappeared

liquid
solid
vapor
Temperature
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Phase Rule
Log Pressure (MPa)

• H2O Pressure Temperature phase diagram
• If all the liquid (water) has formed, temperature
  can decrease further to freezing point
• Then ice starts forming, the temperature remains
  the same until all water is turned into ice

liquid
solid
vapor
Temperature
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Phase Rule

• H2O Pressure Temp. phase diagram
• When only ice left, then temperature can drop
  further.

liquid
vapor
solid
Temperature
17
Phase Rule

• Increasing pressure lowering melting point

Freezing point curve water and ice in equilibrium
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Phase Rule

• Example 1 We know from ice that increase of
  pressure will cause melting
• EG Copper wire over ice block will move through
  ice block without cutting it.
• The pressure causes melting, whereas the water
  refreezes above the wire

Melting as result of pressure
ice
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Phase Rule

• Example 2 Lower pressure, water boils at lower
  temperatures.
• On Mount Kilimanjaro (6000 m) eggs take longer
  to boil and tea does not get very hot.

lower pressure lower boiling point
see Vapor pressure curve
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Phase Rule
Critical point

• H2O P-T diagram
• Ice and water in equilibrium
• Water and vapor in equilibrium
• Ice and vapor in equilibrium
• Triple point is where ice, water vapor are all
  in equilibrium
• Critical point no difference between liquid and
  gas.

liquid
Pressure
solid
vapor
Temperature
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Phase Rule

• H2O P-T diagram
• One component
• 3 phases ice (solid), water (liquid), and steam
  (gas).
• In the field of vapor at point X, we can vary
  pressure, P and temperature T and still have 1
  phase vapor
• So 1 Phase 2 degrees of freedom

liquid
Pressure
solid
vapor
X
Temperature
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Phase Rule

• H2O P-T diagram
• In the field of liquid at point X
• Pressure and temperature can be varied
  independently and still only one phase liquid
  water
• Or 1 component, 1 phase, and 2 degrees of freedom

X
liquid
Pressure
solid
vapor
Temperature
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Phase Rule

• H2O P-T diagram
• Point X liquid
• Increase T
• Liquid up to where line reaches red curve
• Here liquid and vapor (2 phases)
  co-exist
• To keep 2 phases we would have to change P and T
  together

liquid
Pressure
X
solid
vapor
So now 2 phases, but only 1 variable, because P
and T have to be varied together
Temperature
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Phase Rule

• H2O P-T diagram
• Point X liquid, solid and vapor together
• Triple point
• 1 component 3 phases
• To keep the 3 phases we cannot change any of the
  variables (P or T), so 1 component, 3 phases and
  no variables (no degrees of freedom)

liquid
Pressure
solid
vapor
X
Temperature
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Phase Rule

• Summary
• 1component 1 phase 2 degrees of freedom
• 1component 2 phases 1 degree of freedom
• 1component 3 phases 0 degrees of freedom

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Phase Rule

• Reminder Gibbs Phase Rule F C P 2
• degrees of Freedom Components Phases 2

Gibbs Phase Rule
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Phase Rule

• 2 component or Binary systems
• Two components so C2
• Then FC-P2 or F2-P2
• or when F1 then P3
• 3-dimensional diagram
• But if we keep the pressure constant we have 1
  degree of freedom less
• So FC-P1

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Phase Rule

• 2 component or Binary systems
• if FC-P1
• We can draw TX diagrams (where TX temperature
  composition)
• Temperature - composition diagrams depreciate the
  importance of pressure, but sometimes effects of
  pressure will be discussed.

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Phase rule

• 2 component or Binary systems
• Both components mix completely
• Solid solution system (eg plagioclase feldspar)
• Both components do not mix
• Eutectic relationship (eg diopside-anorthite)
• NOTE The eutectic point is the minimum
  temperature point of the liquid field.

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Phase Rule

• 2 component or Binary systems
• If 2 components mix completely - Solid solution
  series
• Example Plagioclase system
• NaAlSi3O8 to CaAl2Si2O8 2 components C2
• Remember C minimum number of components to
  describe the system NaO, Al2O3, SiO2, and CaO
  are more than the minimum necessary
• So Ab (albite) for NaAlSi3O8 and
• An (anorthite) for CaAl2Si2O8

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Phase rule

• 2 component or Binary systems (Plagioclase)
• Remember
• Coupled substitution of Na1Si4 for Al3Ca2
• In constant AlSi2O8 reference frame

IMPORTANT LIQUIDUS - points above which all of
mixture is melt. Also, it is a line or surface
along which compositions of melt are in
equilibrium with a crystalline phase SOLIDUS -
Points below which all of mixture is crystalline
solid. Also, it is a line along which
compositions of a crystalline phase are in
equilibrium with the melt NOTE All points
between liquidus solidus are mixture of melt
crystals
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Phase rule

• Plagioclase system
• Isobaric temperature composition diagram

liquidus
Same pressure
Here 0.1 MPa (atmospheric pressure)
solidus
NOTE All diagrams are experimentally determined
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Phase rule

• Plagioclase system
• TX diagram
• In 1 component system, pure Ab melts at 1118oC
• In 1 component system, pure An melts at 1553oC
• Adding Ab to pure An lowers the melting
  temperature

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Phase rule
a Xbulk

• Plagioclase system
• Composition
• a Xbulk or bulk composition
• Here 60 by weight of An or 40 wt Ab.
• At a T 1600o C
• C 2, P1 so F2

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Phase rule
a Xbulk

• Plagioclase system
• Composition a Xbulk
• 60wt An or 40 wt Ab.
• Cooling to about 1487o first crystal to form is
  plagioclase with An 87. Composition of the
  liquid is An60

87
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Phase Rule
a Xbulk

• Plagioclase system
• Composition a Xbulk or bulk composition
• 1487o plagioclase with An 87. liquid is An60
• F2-211 or 1 degree of freedom, so when T
  changes Xliq and Xsolid will have to change
  dependently
• Composition of solid changes according to
  solidus, whereas liquid changes following the
  liquidus

An60
An87
liquidus
solidus
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Phase rule
a Xbulk

• Plagioclase system
• Composition a Xbulk or bulk composition
• Continuing cooling, continuous reaction between
  crystal and liquid, change in composition.
• Solid will change composition following solidus
  until the solid has reached composition An60
• At this point all liquid is used up and T keeps
  falling but X remains constant (An60)
• The last drop of liquid, in equilibrium with the
  solid is An20

60
liquidus
20
solidus
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Phase rule
a Xbulk

• Plagioclase system
• Equilibrium melting is the opposite
• Partial melting
• Start with solid composition An60
• First melt An20 at 1330o
• When T 1400o, melt escapes and intrudes country
  rock. This melt has composition of An35.
• So partial melt is more felsic than mother (or
  parent) rock

35
liquidus
20
60
solidus
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Phase rule

• Plagioclase solid solution
• With equilibrium melting, there is a continuous
  reaction between crystal liquid
• Rather than react with the crystals in
  equilibrium, the liquid deposits a layer of new
  composition around the crystals
• Compositional zoning

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Phase rule

• Binary eutectic system - Anorthite Diopside

• Pure anorthite An crystallizes at 1553o
  (CaAlSi3O8)
• Pure diopside Di crystallizes at 1392o
  (CaMgSi2O6)
• Hence, adding diopside to anorthite will lower
  the melting/crystallization temperature
• Adding anorthite to diopside will raise the
  melting/crystallization temperature

An
Di
These curves meet at the eutectic point. REMEMBER
that the eutectic point is the minimum temp. in
the liquid field.
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Phase rule
Z

• Binary eutectic system
• At Pt Z, Bulk composition of melt is
• 70 Anorthite 30 diopside (C2)
• Equilibrium cooling
• 1st crystal (anorthite) forms at 1450o
• Cooling continues to 1274o then also diopside
  crystallizes
• Pdi an liquid3
• F2-310 so no degrees of freedom
• Cooling will continue when all liquid
  crystallized, so P2 and F1
• Final drop of melt 42An and 58 di
• Final solid 70 An and 30 di

Final crystal
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Phase rule

• Binary eutectic system
• Bulk melt comp 20 An 80 Di
• 1st crystal of diopside _at_ 1360o
• Cooling to 1274o now anorthite starts
  crystallizing
• Liquiddian C3 so F0
• Cooling continues when all liquid used up, final
  drop 42 An and 58 Di
• Final solid 20 An and 80 Di

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Phase Rule

• Binary eutectic system
• Equilibrium melting
• Initial rock is at Z, rising Temp.
• First melt is always the eutectic composition
• Assume initial rock composition is 20An-80Di
• First melt is eutectic (42 An and 58 Di)
• Temperature will not increase until all anorthite
  is molten
• When all anorthite in the melt has crystallized,
  temperature will rise with diopside melting.
• Until at about 1360oC the entire rock is molten
  and melt is 80 Di - 20 An

Z
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Phase rule

• Binary eutectic system
• Equilibrium melting
• Initial rock composition 80An and 20 Di
• First melt is always the eutectic composition
  (42 An, 58 Di)
• Temperature will not rise until all diopside is
  molten.
• Melting will continue until all the rock is
  molten at 1500 C

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Phase Rule

• Binary eutectic system
• Partial melting
• Initial rock composition at Z is 80 An 20 Di
• First melt is always the eutectic composition
  (42 An, 58 di)
• When about 10 melt is formed it can escape
• At 1350oC the melt escapes
• Melt composition 55 An 45 Di
• When this melt cools at some other place it will
  finally crystallize as a rock with 45 Diopside
  and 55 Anorthite

Z
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Phase rule

• Binary eutectic system Partial melting
• 1/ A melt can be formed with a lower temperature
  composition than the mother rock
• 2/ This melt will cool and form a new rock with a
  different composition than the initial rock
  composition