The temperature at which saltwater begins to crystallize ("freeze") depends on the chemical composition ("saltiness") of the brine. Chemical composition has a similar influence on the crystallization of magmas.
Figure 4.01. CaMgSi2O6-CaAl2Si2O8 (Di-An) equilibrium phase diagram. Phases at equilibrium are shown as a function of chemical composition (weight percent CaAl2Si2O8) and temperature. Click on the diagram to see a larger, interactive version for more information.
The two curved lines on the diagram show the temperature for each composition at which the magma begins to crystallize when cooled. They are called liquidus curves because the rock is completely liquid at temperatures above the lines (the orange region). The liquidus curves are saturation curves. The curve on the left gives the compositions of magmas saturated with diopside. The curve on the right gives the compositions of magmas saturated with anorthite. The point where the two curves meet at 1274°C gives the composition (42 weight % CaAl2Si2O8) of a magma saturated with both diopside and anorthite. That special point is known as a eutectic point. "Eutectic" comes from Greek words meaning "easily melted," because a rock with the eutectic composition is completely melted at the eutectic temperature. Rocks of other compositions begin to melt at the eutectic temperature but melt over a range of temperatures.
For the "basaltic" magmas of Figure 4.01, the first mineral to appear on cooling will be either diopside (Di) or anorthite (An), depending on the bulk composition of the magma. Once the liquid has started crystallizing a mineral, the composition of the liquid will follow the saturation curve for that mineral as the temperature falls and the proportion of crystals will increase relative to the liquid. When the temperature has cooled to 1274°C, the liquid will reach saturation with the other mineral. Both minerals will then grow until the liquid is completely crystallized. The resulting rock is a mixture of Di crystals and An crystals.
