For 3-component systems, equlibrium mineral assemblages are likely to have 3 phases, and chemical reactions will involve 4 phases.
Figure 12. 3-component Terminal Reaction. En + Ky + Spr = Prp. Click on the image to see a larger version with more information.
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.
Figure 13. 3-component Non-Terminal Reaction. En + Crn = Spr + Ky. Click on the image to see a larger version with more information.
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
Figure 14. 3-component Non-Terminal Reaction. En + Crn = Spr + Ky. Click on the image to see a larger version with more information.
See if you can recognize chemical reactions based on equilibrium assemblages on a ternary composition diagram.