Recognizing Reactions on Composition Diagrams (continued)

For 3-component systems, equlibrium mineral assemblages are likely to have 3 phases, and chemical reactions will involve 4 phases.
Composition Diagram

Figure 12. 3-component Terminal Reaction. En + Ky + Spr = Prp. Click on the image to see a larger version with more information.

Discontinuous reactions may be of two types: "terminal" or "non-terminal."

A terminal reaction is one in which a phase is eliminated from all equlibrium assemblages in the system. Because of the law of mass conservation, in 3-component systems the terminated phase must have a chemical composition inside the composition triangle defined by the other 3 phases in the reaction. Mouseover Figure 12 to see the terminal reaction:

enstatite (En) + kyanite (Ky) + sapphirine (Spr) = pyrope (Prp).

Note that the 3 minerals on one side of the reaction are the corners of the assemblage triangle and the other side of the reaction is the mineral that plots inside the triangle.

Composition Diagram

Figure 13. 3-component Non-Terminal Reaction. En + Crn = Spr + Ky. Click on the image to see a larger version with more information.

A non-terminal reaction is one in which equilbrium assemblages change, but no phase is eliminated. For non-terminal reactions in 3-component systems, none of the 4 phases lies inside a triangle defined by the other 3 phases. Mouseover Figure 13 to see the non-terminal reaction:

enstatite (En) + corundum (Crn) =
sapphirine (Spr) + kyanite (Ky).

Note that the reaction changes the equilibrium assemblage 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.

Both the Figure 12 terminal reaction and the Figure 13
Composition Diagram

Figure 14. 3-component Non-Terminal Reaction. En + Crn = Spr + Ky. Click on the image to see a larger version with more information.

non-terminal reaction are shown on an interactive T-P diagram in Figure 14. Click on the diagram and explore these and the other reactions about an invariant point. If you choose a bulk composition by clicking on the composition diagram and then mousing over the T-P diagram, you will be able to see how particular mineral assemblages and modes are affected by the reactions.

See if you can recognize chemical reactions based on equilibrium assemblages on a ternary composition diagram.



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