Problems of two-pyroxene geothermometry

It is possible to formulate a two-pyroxene (MgFeCa) geothermometer using non-ideal but disordered site models for the pyroxenes. Experimental data on any two of the following three sets are needed: (a) the site occupancy data in orthopyroxene, (b) the site occupancy data in clinopyroxene (both sets of data with varying composition and temperature), and (c) synthetic phase equilibrium data on coexisting pyroxenes. A geothermometer based on site occupancy data is very sensitive to small variations of composition and may be useless for common petrological purposes but immensely useful for studying disequilibrium and kinetic history of the rocks.