Figure 6.03. Al2SiO5 stability diagram. Equilibrium Temperature (T)-Pressure (P) stability of three crystalline phases (polymorphs) with the chemical composition Al2SiO5: andalusite (And), kyanite (Ky), and sillimanite (Sil). Click on the image to see a larger version with more information.
The three aluminosilicate reaction curves meet at a "triple point" at temperatures and pressures within the medium P/T (Barrovian) facies series of regional metamorphic rocks. The intersection of the reaction curves at metamorphic conditions is especially valuable. For example, if sillimanite is observed in a metamorphic rock, that rock must have been heated above the triple point temperature (545°C). Similarly, if in a prograde sequence the mineral kyanite is replaced by sillimanite, the metamorphic pressure must have been above the triple point pressure (0.46 GPa). Other one-component mineral polymorphs such as calcite and aragonite or quartz and coesite offer T-P constraints, though with only two phases and one reaction curve there is no invariant point at normal metamorphic conditions.
Figure 6.04. T or P Limiting Assemblages. Intersecting reaction curves define fixed points that can be used to place limits on temperature (T) and pressure (P) for assemblages of minerals involved in the reactions. Click on the image to see a larger version with more information.
Bounds to the T-P conditions of metamorphism are helpful, but perhaps not as informative as actual temperatures or pressures. To this end, petrologists have calibrated geothermometers and geobarometers based on the chemical compositions of certain solid solution minerals.