A plot of temperature versus composition of a solid solution mineral. Unlike a phase diagram, there is a region where the material will exist in a partially solid and a partially liquid phase. The line dividing this region and the solid is known as the solidus line; that separating the liquid region is the liquidus line.

When a material with the composition at A is cooled at equilibrium, the solid which precipitates will have the composition given by the intersection of the horizontal temperature line AB and the solidus line, point C. As the temperature slowly decreases, the solid remains in equilibrium with the melt. This means that the composition of the solid changes to that lying on the solidus line at a given temperature (C shifts to the right). Therefore, when the solid reaches the composition of the original (directly below A) the solid and liquids share the same composition, so all subsequent crystals will have the original composition of the melt.

The analogous situation holds for melting, with the composition of the solid shifting to the left until it lies above point B, at which point melt and solid share the same composition. All subsequent melting will therefore have the original composition of the solid.