8.3 Follow Fractional Crystallization on Binary Phase Diagrams

Those familiar with using phase diagrams to understand the results of laboratory experiments on melting and crystallization of rocks can use those diagrams to predict the chemical changes to a liquid that can be caused by fractional crystallization. Consider the binary system KAlSi₂O₆-SiO₂ shown above as Figure 8.03. Click on the diagram and then on the "Coordinates" and "Show Phase %" buttons to follow the crystallization path of a 20 wt% SiO₂ composition liquid.

Equilibrium Crystallization
Lower the temperature of the 20 wt% SiO₂ liquid and the diagram will show the equilibrium path with leucite starting to crystallize at 1550°C. At 1150°C, with 53 wt% of the original liquid crystallized as leucite and with 47 wt% of the original liquid remaing, the cooling magma reaches the peritectic reaction temperature. At 1150°C all the remaining liquid crystallizes in a reaction with the newly formed leucite to produce a completely solid mixture of 7 wt% leucite and 93 wt% sanidine.

Fractional Crystallization
Return to Figure 8.03 and start again with a 20 wt% SiO₂ liquid and lower the temperature, but this time remove all the crystals as they are formed. As before, leucite will begin to crystallize at 1550°C. However, if all the leucite is removed, the bulk composition changes. The bulk composition will always be the liquid composition. At 1150°C, the peritectic reaction temperature, there is no leucite to react with the liquid, so sanidine begins to crystallize (and is removed) and the liquid (and bulk) composition follows the sanidine saturation curve all the way to the eutectic at 990°C. There, the remaining liquid, which is 30 wt% of the original liquid, crystallizes to sanidine and tridymite. Adding up the all the crystals, 53 wt% are leucite, 34 wt% are sanidine, and 13 wt% are tridymite. The weight percent SiO₂ in the final liquid is more than double the weight percent SiO2 in the starting liquid (54.4% vs 20%). This is an ideal case where all the crystals are removed by fractional crystallization as soon as they form. However, from this example, the power of fractional crystallization to change the chemical composition of a magma is quite evident.

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