The diagram on the left shows the temperature-composition melting relations of olivine at 1 atmosphere pressure. As you mouseover the forsterite(Fo)-fayalite(Fa) T-X diagram on the left, Gibbs energies for olivine (green) and a Fo-Fa liquid (orange) at the mouse position T are shown as concave upward curves on the Fo-Fa G-X diagram on the right. For any bulk composition (weight percent Fe2SiO4), the stable phase or phase pair at equilibrium will be the one with the lowest Gibbs energy. At low temperatures, olivine is the stable phase. At high temperatures a Fo-Fa liquid is the stable phase. At intermediate temperatures, the stable phase or phase pair will be olivine alone, olivine plus Fo-Fa liquid, or a Fo-Fa liquid alone, depending on the bulk composition as shown on the T-X diagram.
To hold a particular P and T, click on the P-T diagram. To resume mouseover changes to P and T, click on the "Change PT" button on the upper right. To see the phase assemblages on an expanded scale showing Gibbs energies relative to a fixed position for G(Fo), click the "Show Assemblages" button.
Olivine and Fo-Fa liquids, both nearly ideal solutions, have very similar Gibbs energies. In order to show clearly the topology of the G-X relations, Gibbs energy values for the Fo-Fa liquids and intermediate olivine compositions have been adjusted to magnify differences and to match the T-X relations.