Figure 4.04. Ab = Jd + Qz diagram.
The diagram on the left shows the temperature-pressure (T-P) stability of Albite (Ab) relative to Jadeite (Jd) and Quartz (Qz). For pressures below the equilibrium line for the reaction Ab = Jd + Qz, albite is stable for all chemical compositions between NaAlSi2O6 and SiO2. For pressures above the equilibrium line, albite is not stable for any chemical composition. This is shown schematically by the small composition lines on the T-P diagram.
What minerals will be in a rock at equilibrium for the two-component NaAlSi2O6-SiO2 system depends not only on the T and P, but also on the bulk chemical composition of the rock. Select a chemical composition with the slider on the right, then mouse over the T-P diagram to see what minerals will be present and in what proportion for that composition at different temperatures and pressures. The mineral proportions are determined by the lever rule.
Notice that two minerals are stable for a randomly selected (throw a dart at the diagram) T and P. Three minerals are stable (at equilibrium) only along the Ab = Jd + Qz reaction curve. These are chacteristics of a two-component system. Which two minerals are stable depends on the chemical composition, as well as the T and P. Click on the "Show MAD" button to show the Mineral Assemblage Diagram for this system. Use the slider to change the bulk composition and the MAD will change when the bulk composition crosses the Ab composition.
This diagram is constructed from the thermodynamic model dataset SPaC (version 2015) developed originally by Spear and Cheney (1989).
