Figure 5.18. Mg₂SiO₄ - SiO₂ diagrams projected from H₂O.

The diagram on the left shows the pressure-temperature 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. Water is included in the experiments so that pressure P(total) = P (H₂O). Mouseover the T-P diagram to see mineral assemblage information for a Mg₂SiO₄-SiO₂ composition of 25 wt.% SiO₂.

The diagram on the right shows the compositions of the minerals as projected from H₂O onto the Mg₂SiO₄-SiO₂ 2-component line. H₂O (supercritical water) is a phase in all equilibrium assemblages. Move the slider to select an Mg₂SiO₄-SiO₂ bulk composition, then mouseover the T-P diagram to see the assemblage changes. To change the T-P diagram on the left to a mineral assemblage diagram (MAD) based on the selected bulk composition, click on the "Show MAD" button. Remember that water should be added to all assemblages.

See how the reactions are easily identified from the composition diagram. As was the case for 2-component systems, one of the phases in the reaction must have a composition that is between the compositions of the other two and will be on the opposite side of the reaction. H₂O is present and can be added to the reactions as needed to balance them chemically.

The data used to calculate the reaction curves were taken from the thermodynamic model dataset SPaC (version 2015) developed originally by Spear and Cheney (1989) using a standard state of formation from the elements at 298.15 K and 1 bar.

Return to Reactions text.   Return to Previous view