7.2 Ternary Reaction Curves and Points
Figure 7.01 is a binary liquidus diagram for the system Mg2SiO4-SiO2 at 0.1 MPa pressure. For liquid compositions rich in Mg2SiO4, forsterite (Fo) crystallizes first on cooling. But when the temperature reaches 1557°C, the Fo crystals will react with the liquid to produce enstatite (En). The binary liquidus (saturation) curves for Fo and for En meet at a peritectic point, as discussed in section 4.4 Peritectic Point. If you need to review crystallization around peritectic points, click on Figure 7.01 and use the interactive tools to follow the crystallization of a 20 wt.% SiO2 liquid.
Figure 7.02 is a ternary liquidus diagram for the Mg2SiO4-CaAl2Si2O8-SiO2 system. The peritectic point on Figure 7.01 is on the bottom edge of the Mg2SiO4-CaAl2Si2O8-SiO2 diagram, and is the endpoint of a curved line (a reaction curve) marking the intersection of the forsterite (Fo) and the enstatite (En) ternary liquidus (saturation) surfaces. Click on Figure 7.02 and use the interactive tools to follow the cooling and crystallization of an Mg2SiO4-rich liquid as described in the caption to the large version of Figure 7.02.
If you followed the instructions given in the caption to the large version of Figure 7.02, you will have observed that, in a ternary system, peritectic-like reactions are continous reactions that occur over a range of temperatures. One mineral is consumed (Fo) and another mineral is produced (En) during cooling. Growth of the new mineral (En) around the consumed mineral (Fo) can separate the reactants (Fo + Liq) and stop or slow the reaction. The two minerals reverse rolls during melting (En is consumed and Fo is produced).
Figure 7.01. Mg2SiO4-SiO2 equilibrium phase diagram. Phases at equilibrium for the binary chemical system Mg2SiO4-SiO2 are shown as a function of chemical composition (weight percent SiO2) and temperature at 0.1 MPa pressure. The diagram contains a peritectic point at 1557°C. Click on the diagram to see an enlarged version for more information.
Figure 7.02. Mg2SiO4-CaAl2Si2O8-SiO2 Diagram. Ternary phase diagram for melting the system Mg2SiO4-CaAl2Si2O8-SiO2 at 0.1 MPa pressure based on data from Irvine (1974). The pertitectic point in Figure 7.01 becomes a reaction curve in this diagram. Click on the diagram to see a larger version with the ternary lever rule animated by the "Show Phase %" button.
