binary ternary

8/8/2019 Binary Ternary

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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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binary ternary

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