Figure 7.02. Mg₂SiO₄-CaAl₂Si₂O₈-SiO₂ Diagram. Ternary phase diagram for melting in the Fo-An-SiO₂ system at 0.1 MPa pressure based on data from Irvine (1974). Melting temperatures of the three minerals forsterite (Fo), anorthite (An), and cristobalite (Crs) are shown just outside the triangle, along with temperatures of three binary eutectic points and one peritectic point. The shapes of the liquidus (saturation) surfaces for each of these minerals, and also for enstatite (En), are revealed by temperature contours. Four cotectic curves and one reaction curve mark the intersections of the four liquidus surfaces.

To see the reaction curve in use click on the "Show Phase %" button and then click on the diagram at composition Mg₂SiO₄ (75%), CaAl₂Si₂O₈ (15%). The selected bulk composition will be marked with a black dot. Then click on the "Change T" button and move the T-slider to lower the temperature. Once the slider is selected, you may use up and down arrow buttons to have more control over the speed of temperature change. When the temperature reaches about 1670°C, the magma will be saturated with forsterite and Fo crystals will grow as the temperature is lowered further. The composition of the liquid will change to become less Mg₂SiO₄-rich along a line that passes through both Fo and the selected bulk composition. The equilibrium weight proportions of Fo and liquid (Liq) are shown the "Phase Proportions" table and will change as the temperature changes.

When the temperature reaches about 1480°C, the Liq composition will reach the reaction curve. Fo crystals already formed will then react with the Liq to produce enstatite (En) crystals. To use familiar saltwater terms, the Fo crystals begin to dissolve and the En crystals begin to precitpitate, changing the Liq composition, which follows the reaction curve. This reaction process will continue until the temperature falls to 1270°C where the Liq composition reaches the reaction point. At the reaction point, all the remaining Fo will react with the all the remaining magma to produce En + anorthite (An).

The spinel field (non-ternary) and high silica two-liquid field have both been removed here to simplify the diagram for petrologists in training. To see the Mg₂SiO₄-CaAl₂Si₂O₈-SiO₂ diagram with the spinel liquidus surface shown click here. Also, the saturation surfaces have been smoothed somewhat and adjusted to fascilitate coding the phase percentages animation.

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