Figure 5.12. Terminal Reaction on Mg2SiO4-SiO2-Al2O3 Composition Diagram.

The minerals enstatite (En), kyanite (Ky), and sapphirine (Spr) form an equilibrium mineral assemblage at conditions of 950°C and 1.4 GPa. If the temperature is raised, those minerals will react to form pyrope (Prp). To see how the composition diagram changes, click and reclick the "Show Reaction" button.

This is a discontinuous (no solid solution), terminal reaction for Pyp (Pyp is not stable in any asseblage on one side of the reaction). The Pyp producing reaction:

En + Ky + Spr = Prp

can be deduced from changes to the composition diagram as Prp appears in the middle of the En-Ky-Spr assemblage triangle. The reaction can be balanced using formulas for the reaction minerals enstatite (MgSiO₃), kyanite (Al₂SiO₅), sapphirine (Mg₄Al₈Si₂O₂₀), and pyrope (Mg₃Al₂Si₃O₁₂):

14 En + 2 Ky + 1 Spr = 6 Prp
14 MgSiO₃ + 2 Al₂SiO₅ + Mg₄Al₈Si₂O₂₀ = 6 Mg₃Al₂Si₃O₁₂

The minerals forsterite (Fo), quartz (Qz), corundum (Crn), and pyrope (Prp), also plot on the composition diagram, but they are not affected by this reaction.

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