Heat them to their melting temperatures, sometimes called their melting points, and they turn into liquid with the same composition as the mineral. Many important minerals are solid solutions of multiple components, but often only limited ranges of compositions are stable.Melts may be equilibrium with partially melted rocks that have compositions different from the melt.Eutectics mean that some magmas may cool to relatively low temperatures before crystallizing.Mineral systems often contain eutectics which means that minerals together melt at lower temperature than they would individually.Other minerals melt incongruently – when heated to their melting temperatures they turn into a different mineral and a melt of a different composition. Some minerals melt congruently – if heated to their melting temperatures they turn into a melt of the same composition as the original mineral.Minerals melt over wide range of temperature.We generally show ternary equilibria on triangular diagrams. Ternary systems involve three components.We typically show binary equilibria on temperature-composition diagrams. Binary systems involve two chemical components.We typically depict them on pressure-temperature phase diagrams. Unary systems, contain only one component.There are many different kinds of phase diagrams. We depict phase equilibria on phase diagrams.To simplify analysis and broaden applications, petrologists often study model systems involving only one or a few components.Thermodynamics deals with chemical systems, which are defined by the chemical components that make up everything in the system. The Laws of Thermodynamics govern phase equilibria.Unstable equilibrium means that chemical reactions are occurring. Metastable equilibrium means chemical energy is not minimized but no reactions are occurring. Stable equilibrium means chemical energy is at a minimum. Minerals and mineral assemblages may be at stable, metastable, and unstable equilibrium.Phase equilibria involve chemical reactions when one or more phases react to form different phases.The term phase refers to any compositionally and physically distinctive substance.

8.0 Examples of crystallization paths for Fo-An-Qz melts

Ternary diagram of feldspars