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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

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

  • 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

  • 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

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


Phase Rule Application

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


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

  • 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 – Intermediate Compositions


Binary Eutectic Crystallization


Binary Eutectic Melting


Melting A Rock With A 30/70 Eutectic


Fractionation


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

  • 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

  • L = Liquid

  • Fo is fosterite

  • En is enstatite

  • Qtz is quartz


Reaction

  • MgSiO3 + SiO2 = Mg2SiO4

  • En + Qtz = Fo


Fo- Qtz


Forsterite – Quartz at Higher T

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


Binary Minimum Melting Point


Albite - Orthoclase


Lever


Application of Lever Rule

  • C = 65 weight% β


Point 2

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


Point 3

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


Point 3 Calculations

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

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


Point 4

  • C3 = 48 weight% β

  • C4 = 87 weight% β


Point 4 Calculations

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

  • Fraction of liquid at eutectic = 56 weight%


Point 5

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


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

  • 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 5 kbars


Reading a Ternary Diagram


Points on Diagram Edge

  • Points on the edge have only two components


Reading Sample Points

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


Ternary Rock Composition Diagram

  • Ternary diagrams can be used to plot any three things


Texture Diagram

  • Another use of ternary diagrams


Ternary System

  • Sample ternary diagram


Silica

0

100

90

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80

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70

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% Silica ↑

% Kalsilite ↓

50

50

60

40

70

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80

20

90

10

0

100

90

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Nepheline

Kalsilite

← % Nepheline


Real Plot


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