CoachA ternary phase diagram represents the equilibria of a system of three components at a given pressure: each point of a triangle stands for a composition, the corners for the pure components, and the liquidus surface is drawn as contours of temperature or as the fields of the minerals that crystallise first. A cotectic line is the line along which the liquid is saturated in two minerals at once and crystallises both as it cools. The cotectic lines descend from the binary eutectics on the sides of the triangle and meet at the ternary eutectic, the lowest temperature at which liquid exists, where the three minerals crystallise together.
