Phases are physically separate regions of homogeneous chemistry.
Solid phases may exhibit solid solution, i.e., have a range of compositional variation. To introduce phase diagrams, we will consider the characteristics of a series of phase diagrams of increasing complexity, beginning with a binary mixture of diopside and anorthite. The system remains at this temperature until all the diopside present in the original rock melts. In these and the following equations f is the fraction of liquid and is the composition with the subscripts E, s and l denoting the eutectic liquid, the solid and the liquid, respectively. Melting will continue until the composition of the residual solid reaches the value 1, i.e., the diopside component has melted completely and the residual solid is pure anorthite.