Phase
Diagrams
2004 GEOLOGY
INSET PROGRAMME
Supporting ESTA
(Earth Science Teachers’ Association)
Solid Solution
Phase Diagrams
The solid solution phase diagram explains
the behaviour of chemical solid solution
series, such as the transition from high
temperature, calcium-rich plagioclase to
low temperature sodium-rich plagioclase,
or the transition from high temperature
magnesium-rich to low temperature ironrich crystals in ferromagnesium minerals
(e.g. olivine, pyroxene). The example here
is for the PLAGIOCLASE FELDSPARS.
Phase
The following relates to the
Diagrams
behaviour of plagioclase under
complete equilibrium conditions
during crystallization.
The conventions for the phase
diagram include the following:
1. Two components: high
temperature CaAl2Si2O8
(anorthite) and low temperature
NaAl2Si2O8 (albite) plotted along
the horizontal axis.
Low temperature, sodic
plagioclase (Albite) is on the left;
high temperature calcic
plagioclase (anorthite) is on the
right.
Composition is in % anorthite. A
30% anorthite melt has 30%
calcium and 70% sodium.
Phase
As we expect from Bowen's
Diagrams
reaction series, Ca-rich anorthite
crystallizes at a much higher
temperature than Na-rich albite.
2. One variable - Temperature is
plotted along the vertical axis.
3. Pressure is held constant at 1
atmosphere.
4. Two phases: crystal and liquid
melt. The diagram is divided into
three fields, ‘all liquid’, ‘liquid +
crystal’, ‘all crystal’.
The liquidus line separates the
‘all liquid’ phase from the ‘liquid
+ crystal’ phase. The solidus
line separates the ‘liquid +
crystal’ phase from the ‘all
crystal’ phase.
Phase
The solidus and liquidusDiagrams
lines
are experimental; they have
been determined by melting
and cooling many melts at
different anothite percentages.
5. Complete miscibility (mixing)
occurs in both liquid (magma)
and crystal phases.
The assumptions are:
The system remains in
equilibrium throughout its
history so that all reactions can
take place and everything can
come to stability.
Early formed crystals
completely react back with the
melt as temperature falls to
produce the feldspar most
stable at that temperature.
Phase
USING THE GRAPH
Diagrams
(numbers on phase diagram
1
correspond with numbers below)
1. As an example, take a hot melt
(over 1500 degrees) of 30%
anorthite (70% Albite).
2. Cool melt to liquidus line. First
crystal begins to form at about
1380o.
2
3
3. To determine the composition
of the first crystal move
horizontally across to the solidus
line. The solidus always indicates
crystal composition.
4. Then drop from the solidus
straight down to the bottom scale.
The first crystal is 72% anorthite
(28% Albite). The diagram is
always read in this manner,
‘down-across-down’ regardless of
starting composition.
4
Phase
As the temperature drops Diagrams
and
crystallization continues, Ca is
removed from the melt faster than
Na. (a 30% anorthite melt yields a
first crystal with a composition
ratio of 72% Ca:28% Na)
Thus as crystallization proceeds
the Na concentration of the melt
increases and Ca gets lower.
1
2
3
4
Phase
5. Thus as the temperature
Diagrams
lowers the composition of the
melt migrates down the liquidus
line. But at the same time the
composition of the crystals
forming are moving down the
solidus line (that is, decreasing
in Ca.)
This is because the system
cools slowly enough to stay in
equilibrium at all times.
The earlier formed crystals
react with the melt, exchanging
Ca for Na, to come to a
composition in equilibrium with
the temperature at the moment.
The composition of the melt and
crystals move down in tandem,
exactly opposite each other
along a horizontal line.
1
2
5
3
5
4
Phase
Thus at any temperatureDiagrams
the
combined composition of both
1
the crystals and melt must equal
the original composition of the
melt.
QUESTIONS:
2
(i) What will be the composition of
the last drop of melt?
3
(ii) At what temperature will it
crystallise?
(iii) What will be the composition of
the last crystal from the melt?
5
5
ANSWERS
(i) The last drop will be about 3%
anorthite (97% albite)
(ii) At a temperature of about 1185o.
(iii) The last crystal will be about
30% anorthite.
4
Phase
EXPLANATION Diagrams
If the system stays in
equilibrium at all times the
final composition must end up
where we started, 30%
anorthite:70% albite.
So, find the composition of the
melt on the liquidus line which
is opposite where the 30% line
crosses the solidus line.
This is the composition of the
last drop of melt.
The temperature is read
where this line intersects the
temperature scale.
1
2
5
3
5
4
Phase
Diagrams
Zoned crystals
Ca-rich
plagioclase
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.
5cm
Na-rich
plagioclase
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.
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
Diagrams
The End