5.6 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. 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 5.12 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.

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 5.13 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 5.12 terminal reaction and the Figure 5.13 non-terminal reaction are shown on an interactive T-P diagram in Figure 5.14. 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. Test Yourself

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