Binary phase diagrams
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Binary Phase Diagrams. GLY 4200 Fall, 2012. 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. Binary System Examples.

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Binary Phase Diagrams

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Binary phase diagrams

Binary Phase Diagrams

GLY 4200

Fall, 2012


Binary diagrams

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


Binary system examples

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


Phase rule for binary systems

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)


Binary solid solution

Binary Solid Solution

  • Plagioclase Feldspar series, 39% of the earth’s crust


Phase rule application

Phase Rule Application

  • How many degrees of freedom are present at points 1,2, and 3?


Binary eutectic diagram conventions

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)


Binary eutectic diagram assumptions

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


Binary eutectic diagram

Binary Eutectic Diagram


Binary eutectic diagram intermediate compositions

Binary Eutectic Diagram – Intermediate Compositions


Binary eutectic crystallization

Binary Eutectic Crystallization


Binary eutectic melting

Binary Eutectic Melting


Melting a rock with a 30 70 eutectic

Melting A Rock With A 30/70 Eutectic


Fractionation

Fractionation


Congruent melting

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


Incongruent melting

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


Binary eutectic with incongruent melting

Binary Eutectic with Incongruent Melting

  • L = Liquid

  • Fo is fosterite

  • En is enstatite

  • Qtz is quartz


Reaction

Reaction

  • MgSiO3 + SiO2 = Mg2SiO4

  • En + Qtz = Fo


Fo qtz

Fo- Qtz


Forsterite quartz at higher t

Forsterite – Quartz at Higher T

  • Two liquid phases are present on the right side of the diagram


Binary minimum melting point

Binary Minimum Melting Point


Albite orthoclase

Albite - Orthoclase


Lever

Lever


Application of lever rule

Application of Lever Rule

  • C = 65 weight% β


Point 2

Point 2

  • Solid phase B starts to form with a composition of 96 weight% β


Point 3

Point 3

  • C1 = 58 weight% β and C2 = 92 weight% β


Point 3 calculations

Point 3 Calculations

  • Fraction of solid b = (65 - 58) / (92 - 58) = 20 weight%

  • Fraction of liquid = (92 - 65) / (92 - 58) = 80 weight%


Point 4

Point 4

  • C3 = 48 weight% β

  • C4 = 87 weight% β


Point 4 calculations

Point 4 Calculations

  • Fraction of solid with composition C4 = (65 - 48) / (87 - 48) = 44 weight%.

  • Fraction of liquid at eutectic = 56 weight%


Point 5

Point 5

  • C5 = 9 weight% β and C6 = 91 weight% β


Point 5 calculations

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%


Effects of pressure

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 longer evident in the 3 kbar diagram.


Orthoclase albite at low p

Orthoclase – Albite at Low P


Orthoclase albite at 5 kbars

Orthoclase – Albite at 5 kbars


Reading a ternary diagram

Reading a Ternary Diagram


Points on diagram edge

Points on Diagram Edge

  • Points on the edge have only two components


Reading sample points

Reading Sample Points

  • Note that the scales are plotted increasing in a CCW directions


Ternary rock composition diagram

Ternary Rock Composition Diagram

  • Ternary diagrams can be used to plot any three things


Texture diagram

Texture Diagram

  • Another use of ternary diagrams


Ternary system

Ternary System

  • Sample ternary diagram


Binary phase diagrams

Silica

0

100

90

10

80

20

70

30

60

40

% Silica ↑

% Kalsilite ↓

50

50

60

40

70

30

80

20

90

10

0

100

90

80

70

60

50

40

30

20

10

0

100

Nepheline

Kalsilite

← % Nepheline


Real plot

Real Plot


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