Figure 5.16. Mg2SiO4 - H2O - SiO2 diagrams.
The diagram on the left shows the temperature-pressure stabilities of Forsterite (Fo), Antigorite (Atg), Enstatite (En), Talc (Tc), Quartz (Qz) and Water (H2O) based on experimental studies of chemical reactions among the minerals. Mouseover the T-P diagram on the left to see the equilbrum assemblages on the Mg2SiO4-H2O-SiO2 diagram on the right for the selected T and P. Now that you have some experience recognizing reactions on composition diagrams, convince yourself that the assemblage changes match the reactions as labeled.
Notice that all the minerals have compositions close to the Mg2SiO4-SiO2 line and that the most of the area of the composition triangle is occupied by H2O-bearing assemblages. Therefore, this 3-component system is a good candidate for a phase projection to reduce the number of components that are plotted. If we limit our attention to only those assemblages that include an H2O phase (supercritical water), the equilibrium assemblage at any T and P is determined by one variable, SiO2/(Mg2SiO4 + SiO2), like a 2-component system.
Click on the "Show H2O Projection" button to see a graphical explanation of a projection from water.
