4.10 Summary

  • Igneous and metamorphic rocks form at high temperatures, commonly over long periods of time, so they may have approached chemical equilibrium during their history.
  • Minerals that occur together at chemical equilibrium are said to comprise an equilibrium mineral assemblage.
  • If the minerals observed together in a rock represent chemical equilibrium, then the rock is likely to have experienced the conditions at which those minerals are found to be at equlibrium in laboratory experiments.
  • Results of laboratory experments may be summarized using phase diagrams showing the stabilities of equilibrium mineral assemblages and chemical reactions among those assemblages.
  • The chemical compositions of phases such as minerals can be represented as a combination of chemical components.
  • The maximum number of minerals that that are likely occur together at equlibrum at a random T and P is equal to the minimum number of chemical components needed to describe the chemical compositions of all the minerals in a rock system (Mineralogical Phase Rule).
  • The minimum number of chemical components needed to describe the chemical compositions of all the minerals in a rock system is equal to the minimum number of dimensions needed to plot the compositions of all the minerals.
  • If a rock has more minerals than components, then the minerals in the rock cannot all have been in chemical equlibrium. Petrologists use this clue to help interpret the history of igneous and metamorphic rocks.
  • Gibbs Phase Rule (f = c + 2 - p) is derived from the laws of thermodynamics (i.e. chemical equilibrium), counting temperature and pressure among the variables that could be changed (f) for a system with c components and p phases.