Figure 09. Di-An diagram.
The diagram on the left shows the temperature-composition melting relations of Diopside (Di) and Anorthite (An) at 1 atmosphere pressure. As you mouseover the T-X diagram, specific Gibbs energies (kJ/g) for the temperature of the mouse position are shown for each of the possible phases both numerically and graphically. Specific Gibbs energies for diopside (green) and Anorthite (blue) are shown as colored dots. Specific Gibbs energies for liquids whose chemical compositions are between CaMgSi2O6 and CaAl2Si2O8 are shown as a concave upward curve. 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 phases at equilibrium will be an average based on the relative proportions (weight percentages) of the two phase. Graphically, the Gibbs energy of a mineral pair must be along a line connecting the Gibbs energy values for the two minerals. For a mineral-melt pair, the lowest Gibbs energy will be for a melt whose composition is given by a line from the Gibbs energy of the mineral that is tangent to the G-X curve for the melt.
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 an expanded scale showing Gibbs energies relative to a fixed position for G(Di), click the "Fixed G(Di)" button.
Gibbs energies for the minerals 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. Gibbs energies for pure An liquid is also taken from SPaC (version 2015). Gibbs energies for Di and mixing parameters for Di-An liquids were selected to fit the experimental T-X data shown on the left diagram determined experimentally by Norman Bowen in (1915) at atmospheric pressure (1 bar).
