Binary Phase Diagrams
GLY 4200
Fall, 2012
1
Binary Diagrams
• Binary diagrams have two components
• We therefore usually choose to plot both T
(temperature) and X (composition) with
pressure held constant
• P-X (T fixed) or P-T (X fixed) are also
possible
2
Binary System Examples
• Binary solid solution - olivine, plagioclase
feldspar
• Binary eutectic with congruent melting potassium feldspar - silica
• Binary peritectic and eutectic with solid to solid
conversion - leucite - Potassium feldspar - silica
• Binary minimum melting point - potassium
feldspar - albite
• Binary minimum melting point with solvus potassium feldspar - albite
3
Phase Rule for Binary Systems
 f=c-p+2=2-p+2=4-p
• If two phases are present, there are two
degrees of freedom (both T and X)
• If three phases are present, there is one
degree of freedom (either T or X)
4
Binary
Solid
Solution
• Plagioclase
Feldspar
series, 39% of
the earth’s
crust
5
Phase Rule
Application
• How many
degrees of
freedom are
present at points
1,2, and 3?
6
Binary Eutectic
Diagram Conventions
• Two immiscible components: such as CaAl2Si2O8
(calcic plagioclase) and FeMgSiO4 (olivine) plotted
along the horizontal axis, OR olivine (isolated
tetrahedra) and pyroxene (single chain tetrahedra),
which are immiscible because they have different
crystal structures
• One variable, temperature, plotted along the vertical
axis.
• Pressure is held constant at 1 atmosphere.
• Three phases, crystal A, crystal B, and melt.
• Complete miscibility of the melt (magma)
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Binary Eutectic
Diagram Assumptions
• The system remains in equilibrium
throughout its history, so that all reactions
can take place and everything can come to
stability
• Everything in the original melt remains in
communication throughout the
crystallization process
8
Binary
Eutectic
Diagram
9
Binary
Eutectic
Diagram –
Intermediate
Compositions
10
Binary Eutectic
Crystallization
11
Binary
Eutectic
Melting
12
Melting
A Rock
With A
30/70
Eutectic
13
F
r
a
c
t
i
o
n
a
t
i
o
n
14
Congruent Melting
• The previous case is an example of
congruent melting
• Congruent melting means melting of a
substance directly to a liquid that is of the
same composition as the solid
15
Incongruent Melting
• Melting accompanied by decomposition or
by reaction with the liquid, so that one solid
phase is converted into another
• Melting to give a liquid different in
composition from the original solid
• One example occurs in the forsterite-quartz
system
16
Binary
Eutectic
with
Incongruent
Melting
•
•
•
•
L = Liquid
Fo is fosterite
En is enstatite
Qtz is quartz
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Reaction
• MgSiO3 + SiO2 = Mg2SiO4
• En + Qtz = Fo
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Fo- Qtz
19
Forsterite –
Quartz at
Higher T
• Two liquid
phases are
present on
the right
side of the
diagram
20
Binary Minimum Melting Point
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Albite Orthoclase
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Lever
23
Application of Lever Rule
• C = 65 weight% β
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Point 2
• Solid phase B starts to form with a composition of
96 weight% β
25
Point 3
• C1 = 58 weight% β and C2 = 92 weight% β
26
Point 3 Calculations
• Fraction of solid b =
(65 - 58) / (92 - 58) = 20 weight%
• Fraction of liquid =
(92 - 65) / (92 - 58) = 80 weight%
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Point 4
• C3 = 48 weight% β
• C4 = 87 weight% β
28
Point 4 Calculations
• Fraction of solid with composition C4 =
(65 - 48) / (87 - 48) = 44 weight%.
• Fraction of liquid at eutectic = 56 weight%
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Point 5
• C5 = 9 weight% β and C6 = 91 weight% β
30
Point 5 Calculations
• Fraction of solid with composition C6 =
(65 - 9) / (91 - 9) = 68 weight%
• Fraction of solid with composition C5 =
(91 - 65) / (91 - 9) = 32 weight%
31
Effects of
Pressure
• With increased pressure, from 1 atm to 7 kbar, En changes
from incongruent to congruent melting behavior
• The field of liquid immiscibility, the 2L field on the 1 atm
diagram, disappears at slightly elevated pressures and is no
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longer evident in the 3 kbar diagram.
Orthoclase
– Albite at
Low P
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Orthoclase
– Albite at
5 kbars
34
Reading a Ternary Diagram
35
Points on Diagram Edge
• Points on the edge have only two components
36
Reading
Sample
Points
• Note that
the scales
are plotted
increasing
in a CCW
directions
37
Ternary
Rock
Composition
Diagram
• Ternary
diagrams can
be used to
plot any
three things
38
Texture
Diagram
• Another use
of ternary
diagrams
39
Ternary
System
• Sample
ternary
diagram
40
Silica
0
100
90
10
80
20
70
30
60
% Silica ↑
40
50
% Kalsilite ↓
50
60
40
70
30
80
20
90
10
0
100
Nepheline
100
90
80
70
60
50
40
← % Nepheline
30
20
10
0
Kalsilite
41
Real
Plot
42