The Forsterite - Anorthite - Silica System

This system exhibits an incongruently melting compound (En) along the binary join Fo-Si. This system also exhibits a ternary eutectic and a ternary peritectic. Things to note about this system are:

1. Melting and crystallization relationships in this system are ternary except along the Fo-An join where spinel appears on the liquidus.
2. The En-An join or tie line (dashed green line) divides the triangular area of the Fo-An-Si diagram into two smaller triangular areas:
   • Fo-En-An triangle
   • En-Si-An triangle.
   These two triangles (shown on the inset of the diagram) tell us something about the final solid compositions that will be present as a result of crystallization of a liquid and whether that liquid will move to the peritectic or eutectic as a result of crystallization. At the peritectic and the eutectic the three phases in equilibrium with liquid are Fo + En + An and En + Si +An, respectively, - the same phases that make up the two small triangles.
   
   • For equilibrium crystallization any staring composition that begins in the Fo-En-An triangle will move to the peritectic, where crystallization will cease and the last liquid will have a composition of the peritectic.
   • Compositions that start in the En-Si-An triangle will move to the eutectic, where crystallization will cease and the last liquid will have a composition of the eutectic.
3. Also in the system the boundary curve separating the fields of Fo + L from En + L is a reaction curve, labelled with a double arrow, and involves a reaction relationship during crystallization:
   En ===> Fo + L
   This is the same reaction we studied when examining the binary Fo - Qtz system.