Equilibrium Crystallization of Composition X

Point X has a bulk composition of 65% An - 35% Ab, which for equilibrium crystallization remains unchanged, so our final solid must be mixture of 65% An and 35% Ab. This can be determined by drawing the isopleth from our starting composition to the base of the diagram.

1. From X1 to X2 the liquid simply cools down to the liquidus surface.
2. At X2, on the liquidus surface.
   • Plagioclase begins to crystallize.
   • This plagioclase has a composition represented by Pl2 and is in equilibrium with L2.
   • Note that the an An-rich plagioclase is in equilibrium with a more Ab-rich liquid.
3. From X2 to X3
   • The liquid composition moves down the liquidus from L2 to L3, becoming richer in the Ab component.
   • At the same time the plagioclase composition moves down the solidus from Pl2 to Pl3, becoming more Ab rich.
   • Again note that the liquid composition is still more Ab-rich than the solid.
   • In this type of system the horizontal lines from the liquidus to the solidus are isotherms, which join a liquid and solid phase which are in equilibrium with each other.
   • In order to maintain equilibrium between the solid and liquid as the temperature drops the composition of the solid and liquid phases is constantly changing.
4. At X4, on the solidus.
   • The last droplet of liquid, with a composition represented by L4, is in equilibrium with plagioclase Pl4, which has a composition of 65% An - 35% Ab - the bulk composition of the initial liquid.

Note in this system the composition of the plagioclase is always more enriched in the An component than the composition of the liquid which is in equilibrium with the solid.