EQUILIBRIUM CRYSTALLIZATION 2
For an intitial liquid of composition X, the final solid after equilibrium must be a mixture of plagioclase of composition N and Di, in the proportions given by applying the lever rule to the line DiN.
The path followed by the liquid is shown in blue, with the sequence of events as follows:
- Above the liquidus surface we have 100% liquid.
- Cool this liquid to the the liquidus.
- On cooling to the liquidus, the liquid first crystallizes an An-rich plagioclase, represented by Point T. The actual composition of the plagioclase forming from the liquid can not be worked out by examining the diagram, it must be determined experimentally.
- As cooling continues the liquid moves down the liquidus surface from X to S, precipitating a more Ab-rich plagioclase, while the previously formed An-rich plagioclase becomes more sodic by reacting with the liquid.
This double reaction between the liquid and solid causes the liquid to migrate in a curved path to the boundary curve at S. The actual liquid path must be determined experimentally.
- At S, on the boundary curve.
Di begins to crystallize and is in equilibrium with Pl represented by Point R and a liquid of composition S.
To maintain equilibrium the liquid and the plagioclase compositions are constantly changing, such that each successive liquid is more Ab-rich and each new plagioclase grain is more Ab-rich than the previous one.
- From S to Q to O the liquid composition moves down the boundary curve.
Each new liquid is in equilibrium with a new plagioclase of composition R, P and N respectively, linked by appropriate coloured tie lines.
The last drop of liquid is used up at O, yielding a solid mixture of Di and Pl of composition N, in the proportions given by X.