- Binary solid solution diagrams summarize experimental data on the melting and crystallization of two-component igneous rocks that contain solid solution minerals.
- Liquidus curves on these diagrams identify the temperature above which a rock is completely melted for each bulk composition.
- Solidus curves on these diagrams identify the temperature below which a rock is completely crystalized for each bulk composition.
- The temperature at which melting begins for a solid solution mineral depends on the chemical composition of that mineral.
- The composition of the first liquid formed when melting begins for a solid solution mineral is almost always a different chemical composition than the mineral melting.
- At equilibrium, the composition of the solid solution mineral will change as it is melted or as it is crystallized.
- Solid solution minerals crystallized from a magma are commonly zoned chemically.
- The liquidus and solidus curves for some solid solution minerals are coincident at a minimum, an azeotropic point, where the solid solution mineral melts to a liquid of the same composition.
- Solid solution minerals with a minimum on the liquidus are also likely to have a solvus that bounds a region at lower temperatures where two mineral are stable. When solid solution minerals of intermediate composition are cooled slowly into a two-mineral region, the single mineral will unmix (exsolve), commonly forming a two-mineral intergrowth with textures like those observed in perthite for alkali feldspars.
5.10 Summary
