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.
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?