4.6 Mineralogical Phase Rule

On the previous pages, it was shown that the maximum number of phases that can be at equilibrium for a chemical system (e.g. a rock) at a randomly selected T and P is equal to the number of chemical components needed to describe the system.
1-component system (e.g. SiO2 or Al2SiO5)
• all the phases have the same chemical composition
• only 1 phase is likely to be stable at a random T and P
2-component system (e.g NaAlSi2O6-SiO2)
• all the phases have chemical compositions that can be plotted on a line
• only 2 phases are likely to be stable at a random T and P
3-component system (e.g Mg2SiO4-SiO2-Al2O3)
• all the phases have chemical compositions that can be plotted on a plane
• only 3 phases are likely to be stable at a random T and P

Because in most cases a rock is formed at an aribitrary T and P, rocks are not likely to have more minerals than components.
These observations can and have been generalized, notably by Victor Goldschmidt (1911) as the Mineralogical Phase Rule: "In any rock the number of minerals will not exceed the number of chemical components." This rule applies to rocks that have achieved chemical equilibrium and is one of the most useful tools in a petrologist's chemical toolkit. If a rock has more phases than components, then the minerals in the rock cannot all be in chemical equlibrium. Perhaps a reaction was stopped in progress, such as the hydration reactions that may occur during cooling of igneous or metamorphic rocks. Alternatively, it may mean that not all of the components have been recognized and counted. If another component is added to the total, another mineral can be added without violating the rules of chemical equilibrium. "Add a component; add a phase!"

With the Mineralogical Phase Rule, petrologists have a guideline they can use to help them understand the mineral assemblages they observe in rocks. To use the Mineralogical Phase Rule, petrologists must know the chemical compositions of minerals present and be able to identify the chemical components needed to describe those mineral compositions.

How many chemical components are needed to describe a rock?