Three Component Systems

Dexter Perkins
University of North Dakota
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Initial Publication Date: July 21, 2008 | Reviewed: October 19, 2012


This exercise involves plotting mineral compositions on triangular (3-component diagrams) and using those diagrams to determine possible mineral reactions.

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Introductory petrology course

Skills and concepts that students must have mastered

Prior to being able to complete this exercise, students should be familiar with the phase rule. They also should have been introduced to the way mineral compositions are plotted on a ternary diagram—but only introduced—they will work out the details as they do the exercise. They should understand what a metamorphic reaction is, and know how to balance reactions.

How the activity is situated in the course

This is one of many exercises that students do first as homework, and then in groups during class. Finally, all groups report results and we compare answers and discuss the exercise.


Content/concepts goals for this activity

After doing this exercise, students should:
  • be more familiar with the phase rule and its implications
  • know what constitutes a degenerate reaction
  • be able to plot mineral compositions on a ternary diagram
  • be able to look at a ternary diagram and infer possible mineral reactions
  • know how to balance reactions
  • add and subtract reactions to derive others

Higher order thinking skills goals for this activity

Triangular diagrams are just abstract tools that petrologists use to depict real-life phenomena. This exercise helps students gain the ability to make that abstract/real-life connection.

Other skills goals for this activity

Description of the activity/assignment

This is a problem set that can be assigned as homework or done in class. I do both.

The problem set involves mineral equilibria in a 3-component system. Students examine the equilibria and mineral assemblages, and apply the phase rule. They also learn how to derive and balance mineral reactions, and how to use a systematic approach when doing a complex problem.

Determining whether students have met the goals

The project is successful if people complete the assignment, regardless of whether they get the correct answers or not.

More information about assessment tools and techniques.

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