Figure 5.13. Non-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 lowered, Spr and Ky will react to form En and Crn. To see how the composition diagram changes, click and reclick the "Show Reaction" button.

This is a discontinuous (no solid solution), non-terminal reaction because no mineral is eliminated from the equlibrium assemblage diagram. The reaction:

Spr + Ky = En + Crn

can be deduced from changes to the composition diagram because the reaction changes the tie line connecting the 2 minerals on one side of the reaction to a tie line connecting the 2 minerals on the other side of the reaction (a tie line "flip"). The reaction can be balanced using formulas for the reaction minerals enstatite (MgSiO₃), kyanite (Al₂SiO₅), sapphirine (Mg₄Al₈Si₂O₂₀), and corundum (Al₂O₃):

Spr + 2 Ky = 2 En + 6 Crn
Mg₄Al₈Si₂O₂₀ + 2 Al₂SiO₅ = 4 MgSiO₃ + 6 Al₂O₃

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

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