EQUILIBRIUM CRYSTAZLLIZATION

The Diopside - Anorthite System represents the simplest type of Binary diagram, and is one that contains a simple eutectic and no solid solution. This system is pertinent to the study of gabbroic rocks, which are composed essentially of clinopyroxene and plagioclase.


The two components which define the system are: On the overhead note the following features:
  1. melting temperatures of the pure end member components;
  2. dilution effect of lowering the melting temperature of one end member by addition of the other;
  3. eutectic - lowest temperature point on the liquidus surface, where three phases coexist.

Equilibrium Crystallization

Composition X

The bulk composition of X is 75% An and 25% Di. For equilibrium crystallization the final solid must have the same composition as the initial liquid, therefore the final solid will be a mixture of 75% An and 25% Di.
  1. At 1600° X1 is 100% liquid.
    As the temperature drops from X1 to X2 the composition reaches the liquidus surface at a temperature of ~ 1500°C.
  2. At X2
    On the liquidus surface the liquid now begins to crystallize Pure An (of fixed composition) which is in equilibrium with liquid. At the temperature of the liquidus, for this composition, or system consists of ~ 1% An and 99% liquid.
  3. At X3
    Drop the temperature to 1400°C Point X3.
    At this point the system consists of An in equilibrium with Liquid - X3 lies in the field of An + L.
  4. At X4
    The system has reached the temperature of the eutectic 1270°C

    The liquid composition has moved down the liquidus surface from L3 to the eutectic. The composition of the eutectic liquid (LE) is 42% An and 58% Di. Notice that the liquid composition has changed from having 75% An to only having 42% An. This results from the fact that as the temperature dropped from 1500° to 1270°C An continuously crystallized from the liquid, thus depleting the liquid in the An component. Overall the bulk composition has remained the same, only the proportions and compositions of the phases involved have changed.

    At X4 the proportion of An:L is 56:44, applying the Lever rule to the line from An to E.

    The liquid remains at TE (1270°C) and now begins to crystallize Di along with An, in the proportions corresponding to the composition of the eutectic (42% An and 58% Di). Crstallization of An and Di continues until all of the liquid is used up. With the crystallization of the last droplet of liquid into grains of An and Di, the temperature of the system can now fall below TE.


  5. At X5, T = 1250°C We have two solids An and Di in equilibrium with each other. The relative proportions of the two solids is the same as the bulk compositions X which we began with.

For equilibrium crystallization the bulk composition we end up with is the same as the bulk composition we began with. The crystallization of a solid phase depletes the liquid in the components of that phase, thus changing the composition of the liquid in equilibrium with the solid.