(Earth Science Teachers’ Association) 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 iron- rich crystals in ferromagnesium minerals (e.g. olivine, pyroxene). The example here is for the PLAGIOCLASE FELDSPARS.

The conventions for the phase diagram include the following: The following relates to the 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.

As we expect from Bowen's 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.

The solidus and liquidus 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.

(numbers on phase diagram correspond with numbers below) USING THE GRAPH (numbers on phase diagram 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. 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. 1 2 3 4

As the temperature drops 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

5. Thus as the temperature 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 3 5 5 4

Thus at any temperature the combined composition of both the crystals and melt must equal the original composition of the melt. QUESTIONS: (i) What will be the composition of the last drop of melt? (ii) At what temperature will it crystallise? (iii) What will be the composition of the last crystal from the melt? 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. 1 2 3 5 5 4

EXPLANATION 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 3 5 5 4

Zoned crystals 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. Ca-rich plagioclase Na-rich plagioclase 5cm 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

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