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Lecture 17:Lecture 17:
Binary and Ternary PhaseBinary and Ternary Phase DiagramsDiagrams
Binary diagrams come in three types. We will look at each in turn.
(1) No Solid Solution
(2) Complete Solid Solution
(3) Partial Solid Solution
There are a number of tricks for understanding any of these binary (2 component)
phase diagrams, one is the concept of a:
Tie Line: a line on a phase diagramjoining phases at equilibrium with one
another (dashed and dotted lines on diagrams below)
Liquidus: line that shows the composition of the liquid phase formed ata
given T
ReadRead ChptChpt 88
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Evenutally, youll reach the Eutectic composition: approximately 45%An, 55% Di
So farther from eutectic T, composition gets farther from eutectic composition,
closer to pure system composition
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(2) INCOMPLETE SOLID SOLUTION
Now we have some miscibility (ability of one phase to dissolve in another)
Just like salt is miscible in water, up to the saturation limit.
Solvus: a line that shows the saturation limitof one solid componentinto another
Solidus: a line that gives the composition of a solid solution in equilibrium with a
liquid (analogous to liquidus in the no solid solution case)
Diss + Anss
AnDi
TDiss + L Anss + L
Liquid1450
AnssDiss
X1 X3 X4X2
T*
SolvusSolvus
SolidusSolidus
LiquidusLiquidus
Example: Orthoclase (KAlSi3O8) Albite (NaAlSi3O8)
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Tielines:
At T*
From Di to X1: pure solid Diss, composition evolves from Di to X1
From X1 to X2: pure solid Diss + liquid; composition of Diss => given by X1;
composition of liquid given by X2
From X2 to X3: pure liquid; composition evolves from X2 to X3
From X3 to X4: pure solid Bss + liquid; compositioin of Bss given by X4; composition of
liquid given by X3
From X4
to An: pure solid Bss
; composition evolves from X4
to An
Diss
+ Anss
AnDi
TDiss + L Anss + L
Liquid1450
AnssDiss
X1 X3 X4X2
T*
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(3) COMPLETE SOLID SOLUTION
Plagioclase Feldspars, for example,
NaAlSi3O8 (albite) => CaAl2Si2O8 (anorthite)
melting loop: solidus and liquidus lines join to form a loop
You can still use tielines to understand this diagram!!
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(3) COMPLETE SOLID SOLUTION
At T*: At compositions from A to X*: pure solid solution with composition A to X*
At compositions from X* to Y*: liquid + solid solution; solid solution always has
composition X*; liquid always has composition Y*; as you move from X* to Y* you
change from mostly solid to mostly liquid
At compositions from Y* to B: pure liquid with composition from Y* to B
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How can you use phase diagrams to understand something about petrology?
Well, what happens when you take a melt and cool it down? Depends on the type of
phase diagram and on how the magma forms:
(1) Perfect Equilibrium Crystallization: always at equilibrium, compositionof liquid and solid formed are constantly evolving; start with liquid of composition X,
end with solid of composition X
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(1) Perfect Fractional Crystallization Crystals are removed from the melt (remember Bowen and differentiation of
magmas?), in other words, they sink to the bottom of the melt and stop reacting with
the remaining liquid
So, first crystal to form has the same composition as in the perfect equilibrium
crystallization case, but subsequent crystals have solidus composition, so that the
AVERAGE composition of solids is very different
The liquid becomes more and more pure (following the liquidus), and the liquid lastslonger and goes through more compositions:
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TERNARY PHASE DIAGRAMSTERNARY PHASE DIAGRAMS
What if we add yet another component? Now we cant draw the T-X phase diagram
in 2D, so we either need a 3D diagram or, we can use Projections
Binary eutectic point =>ternary cotectic line
Ternary eutectic: 3 solids and liquid form
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