The diagram on the left shows the pressure-temperature stabilities of Forsterite (Fo), Antigorite (Atg), Enstatite (En), Talc (Tc), 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 blue dots. Gibbs energies 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. Notice that the Gibbs energy differences between the phases are small relative to the sizes of the numbers. The stable assemblage for each composition will be the one that has the lowest Gibbs energy. The specific Gibbs energy for a mixture of two minerals is indicated by a line connecting the Gibbs energy values for the two minerals.