Figure 06. Ab = Jd + Qz diagram.
The diagram on the left shows the pressure-temperature (P-T) stability of Albite (Ab) relative to Jadeite (Jd) and Quartz (Qz). As you mouseover the P-T diagram, specific Gibbs energies (kJ/g) at the P and T of the mouse position are shown for each of the minerals both numerically and graphically with colored dots on the Gibbs energy composition (G-X) diagram on the right. The stable assemblage for any chemical composition will be the one that has the lowest Gibbs energy. The specific Gibbs energy for any mixture of two minerals at equilibrium will be an average based on the relative proportions (weight percentages) of the two minerals. Graphically, the Gibbs energy of a mineral pair must be along a line connecting the Gibbs energy values for the two minerals. As your mouse crosses the Ab=Jd+Qz reaction curve on the P-T diagram, the value of G(Ab) crosses the line connecting the G(Jd)-G(Qz) line.
Notice that the Gibbs energy differences between the phases are small relative to the sizes of the numbers. To see an expanded scale showing Gibbs energies relative to a fixed position for G(Jd), click the "Fixed G(Jd)" button. Gibbs energies shown are derived from the thermodynamic model dataset SPaC (version 2015) developed originally by Spear and Cheney (1989), which uses the standard state of formation from the elements at 298.15 K and 1 bar.
