- Ternary eutectic diagrams summarize experimental data on the melting and crystallization of idealized, three-component igneous rocks made of stoichiometric minerals with no solid solution.
- Liquidus surfaces on these diagrams, which are saturation surfaces for individual minerals, identify the temperature above which a rock is completely melted for each bulk composition.
- Liquidus surfaces for two minerals may meet along a cotectic line, which gives the temperatures and chemical compositions followed for magmas in equilibrium with two minerals when crystallizing or melting.
- Liquidus surfaces for three minerals may meet at a ternary eutectic point, which gives the temperature and chemical composition of the last drop of liquid during crystallization of the three minerals and the first drop of liquid formed during melting of a rock made of the three minerals.
- When one mineral is crystallizing in these simple ternary systems, the liquid composition must follow a straight line away from the composition of the crystallizing mineral.
- When two minerals are crystallizing in these simple ternary systems, the liquid composition must follow a cotectic line.
- When only two phases are present in ternary systems, the binary lever rule may be used to calculate their proportions from their compositions and the bulk composition.
- When three phases are present in ternary systems, the ternary lever rule may be used to calculate their proportions from their compositions and the bulk composition.
6.8 Summary
