Start with a solid of pure En, which is a mixture of the two components Fo and Si (60:40) which define the system.
At a temperature below 1557°C only solid En exists.
At a temperature above 1557°C the system consists of Fo + L, with a bulk composition equivalent to the starting composition of pure En, a 60:40 mixture of Fo and Si.
This mixture of Fo + L results from the following reaction -
At this temperature, represented by Point 3, 3 phases coexist, En, Fo and L. Applying the phase rule tells us that F=0, no degrees of freedom, making this point invariant.
With the continued addition of heat to the system, in order to maintain equilibrium En decomposes into a mixture of Fo + L. The liquid is enriched in Si and is represented by point P, the peritectic, on the liquidus surface.
As heat is continually added to the system, the system stays at the temperature of the peritectic until all of the En is melted, at which time we are left with a mixture of Fo + L.
At temperatures above P as heat is added, the Fo begins to melt and is in equilibrium with liquid. As the Fo melts the liquid composition moves up the liquidus, becoming enriched in Fo and depleted in Si.
When the temperature reaches the liquidus surface, the last grain of Fo is melted and we are left with a liquid, with a bulk composition equivalent to the initial solid - Pure En (Fo:Si = 60:40).
The isotherm corresponding to the melting temperature of En and the Peritectic tells us that a reaction must occur in the system in order to maintain equilibrium.
