Figure 8.11. Ternary diagram model of fractional crystallization.

Use this diagram to see how the bulk composition of an igneous rock might change during "perfect" fractional crystallization of some common minerals. Minerals are removed as formed from a liquid in 2 weight percent solidified (PCS) steps.

 (1) Select a "Starting Magma Composition" using the pulldown list (center right).
 (2) Enter the weight percents of your choice for the fractionating minerals. You can enter any positive numbers and the software will normalize to 100%. You may need to reenter all values when you change your choices.
 (3) Move the slider to change the percent solidification (PCS) value (at 2 PCS steps until an oxide drops below 0 wt.%).
 (4) Use the "Choose axis" tools to select different oxides plotting variables.
 (5) Change the exchange coefficients if you wish. Then move the slider to see the effect.

The Fe/Mg compositions of the olivine (Mg,Fe)₂SiO₄ (KD=0.33), orthopyroxene (Mg,Fe)SiO₃ (KD=0.19), and clinopyroxene Ca(Mg,Fe)Si₂O₆ (KD=0.19) being removed are adjusted for changes in the liquid Fe/Mg compositions using the exchange coefficients (KD's) listed. The An-composition of the plagioclase (KD=0.52) is adjusted for changes in the liquid normative Ab/An using the KD listed. Ilmenite and magnetite are subtracted as pure end member compositions. The calculation will stop if the weight percent of one of the oxides in the liquid drops below zero. If you view the fractionation steps, variables that end in "m" are mole not mass units. Exchange coefficients may depend on the presence of other minerals, pressure, water fugacity and more. For more information about the KD's, see Morse (2000, 2004, 2015).

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