Phase Phase DiagramsDiagrams

2004 GEOLOGY

INSET PROGRAMME

Supporting ESTA

(Earth Science Teachers’ Association)

Solid Solution Solid Solution Phase DiagramsPhase Diagrams

The solid solution phase diagram explains The solid solution phase diagram explains the behaviour of chemical solid solution the behaviour of chemical solid solution series, such as the transition from high series, such as the transition from high temperature, calcium-rich plagioclase to temperature, calcium-rich plagioclase to low temperature sodium-rich plagioclase, low temperature sodium-rich plagioclase, or the transition from high temperature or the transition from high temperature magnesium-rich to low temperature iron-magnesium-rich to low temperature iron-rich crystals in ferromagnesium minerals rich crystals in ferromagnesium minerals (e.g. olivine, pyroxene). The example here (e.g. olivine, pyroxene). The example here is for the PLAGIOCLASE FELDSPARS.is for the PLAGIOCLASE FELDSPARS.

Phase Phase DiagramsDiagramsThe following relates to the The following relates to the

behaviour of plagioclase under behaviour of plagioclase under complete complete equilibriumequilibrium conditions conditions during crystallization.during crystallization.

The conventions for the phase The conventions for the phase diagram include the following:diagram include the following:

1. Two components: high temperature 1. Two components: high temperature CaAlCaAl22SiSi22OO88 (anorthite) and low (anorthite) and low temperature NaAltemperature NaAl22SiSi22OO88 (albite) (albite) plotted along the horizontal axis.plotted along the horizontal axis.

Low temperature, sodic plagioclase Low temperature, sodic plagioclase (Albite) is on the left; high (Albite) is on the left; high temperature calcic plagioclase temperature calcic plagioclase (anorthite) is on the right.(anorthite) is on the right.

Composition is in % anorthite. A Composition is in % anorthite. A 30% anorthite melt has 30% 30% anorthite melt has 30% calcium and 70% sodium.calcium and 70% sodium.

Phase Phase DiagramsDiagrams As we expect from Bowen's As we expect from Bowen's

reaction series, Ca-rich anorthite reaction series, Ca-rich anorthite crystallizes at a much higher crystallizes at a much higher temperature than Na-rich albite. temperature than Na-rich albite.

2. One variable - Temperature is 2. One variable - Temperature is plotted along the vertical axis. plotted along the vertical axis.

3. 3. Pressure is held constant at 1 Pressure is held constant at 1 atmosphere.atmosphere.

4. 4. Two phases: crystal and liquid Two phases: crystal and liquid melt. The diagram is divided into melt. The diagram is divided into three fields, ‘three fields, ‘all liquid’, ‘liquid + all liquid’, ‘liquid + crystal’, ‘all crystal’crystal’, ‘all crystal’. .

The The liquidusliquidus line separates the line separates the ‘all liquid’ phase from the ‘liquid ‘all liquid’ phase from the ‘liquid + crystal’ phase. The + crystal’ phase. The solidussolidus line separates the ‘liquid + line separates the ‘liquid + crystal’ phase from the ‘all crystal’ phase from the ‘all crystal’ phase. crystal’ phase.

Phase Phase DiagramsDiagramsThe solidus and liquidus lines are The solidus and liquidus lines are

experimental; they have been experimental; they have been determined by melting and cooling determined by melting and cooling many melts at different anothite many melts at different anothite percentages. percentages.

5.5. Complete miscibility (mixing) Complete miscibility (mixing) occurs in both liquid (magma) and occurs in both liquid (magma) and crystal phases.crystal phases.

The assumptions are:The assumptions are:

The system remains in The system remains in equilibriumequilibrium throughout its history throughout its history so that all reactions can take place so that all reactions can take place and everything can come to and everything can come to stability. stability.

Early formed crystals completely Early formed crystals completely react back with the melt as react back with the melt as temperature falls to produce the temperature falls to produce the feldspar most stable at that feldspar most stable at that temperature.temperature.

Phase Phase DiagramsDiagramsUSING THE GRAPHUSING THE GRAPH

(numbers on phase diagram (numbers on phase diagram correspond with numbers below)correspond with numbers below)

1. As an example, take a hot melt 1. As an example, take a hot melt (over 1500 degrees) of 30% (over 1500 degrees) of 30% anorthite (70% Albite).anorthite (70% Albite).

2. Cool melt to liquidus line. First 2. Cool melt to liquidus line. First crystal begins to form at about crystal begins to form at about 13801380oo..

3. To determine the composition 3. To determine the composition of the first crystal move of the first crystal move horizontally across to the solidus horizontally across to the solidus line. The solidus always indicates line. The solidus always indicates crystal composition.crystal composition.

4. Then drop from the solidus 4. Then drop from the solidus straight down to the bottom scale. straight down to the bottom scale. The first crystal is 72% anorthite The first crystal is 72% anorthite (28% Albite). The diagram is (28% Albite). The diagram is always read in this manner, always read in this manner, ‘down-across-down’ regardless of ‘down-across-down’ regardless of starting composition.starting composition.

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Phase Phase DiagramsDiagramsAs the temperature drops and As the temperature drops and

crystallization continues, Ca is crystallization continues, Ca is removed from the melt faster than removed from the melt faster than Na. (a 30% anorthite melt yields a Na. (a 30% anorthite melt yields a first crystal with a composition first crystal with a composition ratio of 72% Ca:28% Na) ratio of 72% Ca:28% Na)

Thus as crystallization proceeds Thus as crystallization proceeds the Na concentration of the the Na concentration of the meltmelt increases and Ca gets lower.increases and Ca gets lower.

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Phase Phase DiagramsDiagrams5. Thus as the temperature 5. Thus as the temperature

lowers the composition of the lowers the composition of the melt migrates down the liquidus melt migrates down the liquidus line. But at the same time the line. But at the same time the composition of the crystals composition of the crystals forming are moving down the forming are moving down the solidus line (that is, decreasing solidus line (that is, decreasing in Ca.) in Ca.)

This is because the system This is because the system cools slowly enough to stay in cools slowly enough to stay in equilibriumequilibrium at all times. at all times.

The earlier formed crystals The earlier formed crystals reactreact with the melt, exchanging with the melt, exchanging Ca for Na, to come to a Ca for Na, to come to a composition in equilibrium with composition in equilibrium with the temperature at the moment. the temperature at the moment.

The composition of the melt and The composition of the melt and crystals move down in tandem, crystals move down in tandem, exactly opposite each other exactly opposite each other along a horizontal line.along a horizontal line.

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Phase Phase DiagramsDiagramsThus at any temperature the Thus at any temperature the

combined composition of both combined composition of both the crystals and melt must equal the crystals and melt must equal the original composition of the the original composition of the melt.melt.

QUESTIONS: QUESTIONS:

(i) (i) What will be the composition of What will be the composition of the last drop of melt? the last drop of melt?

(ii) At what temperature will it (ii) At what temperature will it crystallise?crystallise?

(iii)(iii) What will be the composition of What will be the composition of the last crystal from the melt? the last crystal from the melt?

ANSWERSANSWERS

(i)(i) The last drop will be about 3% The last drop will be about 3% anorthite (97% albite)anorthite (97% albite)

(ii)(ii) At a temperature of about 1185At a temperature of about 1185oo..

(iii)(iii) The last crystal will be about The last crystal will be about 30% anorthite.30% anorthite.

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Phase Phase DiagramsDiagrams 1

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EXPLANATIONEXPLANATION

If the system stays in If the system stays in equilibrium at all times the equilibrium at all times the final composition must end up final composition must end up where we started, 30% where we started, 30% anorthite:70% albite.anorthite:70% albite.

So, find the composition of the So, find the composition of the melt on the liquidus line which melt on the liquidus line which is opposite where the 30% line is opposite where the 30% line crosses the solidus line. crosses the solidus line.

This is the composition of the This is the composition of the last drop of melt. last drop of melt.

The temperature is read The temperature is read where this line intersects the where this line intersects the temperature scale.temperature scale.

Phase Phase DiagramsDiagrams

But what if crystallisation of the melt takes place too rapidly and the early formed crystals do not have time to react back fully with the melt?

The early Ca-rich crystal will only partly react back with the melt and leave a crystal remnant behind. It is around this that a later crystal will grow which in turn will become surrounded by later forming crystals as temperature falls.

In this way a ZONED CRYSTAL is formed with a central core of Ca–rich anorthite surrounded by layers (zones) of increasingly Na-rich plagioclase.

Zoned crystals

Ca-rich plagioclase

5cmNa-rich plagioclase

The feldspar crystal illustrated is a phenocryst as it is surrounded by a finer grained groundmass of un-zoned plagioclase crystals that have cooled too quickly to form larger crystals - PORPHYRYTIC texture

Phase Phase DiagramsDiagrams

The End