Phase diagrams provide a graphical means of presenting the results of experimental studies of complex natural processes, such that at a given temperature and pressure for a specific system at equilibrium the phase or phases present can be determined.

SYSTEM - Any portion of the universe which is of interest and can be studied experimentally.

PHASE- any particular portion of a system, which is physically homogeneous, has a specific composition, and can be mechanically removed or separated from any other phase in the system.

In petrology we generally deal with primary phases - any crystalline phase which can coexist with liquid, i.e. it formed/crystallized directly from the liquid.

EQUILIBRIUM - The condition of minimum energy for the system such that the state of a reaction will not change with time provided that pressure and temperature are kept constant.

In experimental petrology there are three practical criteria used to test for equilibrium.
  1. Time - with time the system does not change its physical or chemical makeup.
  2. Approach equilibrium from two directions,
    e.g. the melting point of Albite. Melting point of albite = 1110°C + 10°C.
  3. Attainment of equilibrium by using different reactants and procedures.
    To determine the melting temperature of Albite
    Use both to determine Ab melting point.
One final term to be defined prior to examining phase diagrams.

COMPONENT - the smallest number of independant variable chemical constituents necessary to define any phase in the system.

For example, experiments carried out in the H2O system, show that the phases which appear over a wide temperature and pressure range are ice, liquid water and water vapour. The composition of each phase is H2O and only one chemical parameter or component is required to describe the composition of each phase.