Guiding students through ternary diagrams
An instructor's guide to ternary diagrams

Kelly Deuerling (University of Nebraska Omaha)
Ryan Kerrigan (University of Pittsburgh at Johnstown)

What should students get out of this module?

After completing this module, a student should be able to:

• Identify the three components of a specific ternary diagram
• Normalize data to be plotted with respect only to the three relevant components of the ternary plot
• Interpret composition and trend of data
• Classify with respect to fields on specific ternary diagrams

Why are ternary diagrams challenging for students?

Ternary diagrams are powerful visual tools that allow for data to be directly compared with respect to three main components. Qualitative interpretation is reasonably accessible, however, quantitative plotting or data extraction can be challenging for geoscience students.

Normalizing data (and focusing on the relevant data) is often an issue for students. When there are more than three components presented, students must be able to identify the three of interest and normalize to just those end member components ignoring the excess data. Emphasize to students the need for normalized data to add up to 100% in order to plot as a single point. If normalized data do not sum to 100%, that should cue that there is something wrong in the calculation.

What we don't include in the page?

We have focused this module on plotting data on and extracting information plotted on ternary diagrams. We purposefully have limited this discussion to covering single ternary diagrams, categorization of data associated with the specific ternary diagrams, and general trends in component composition in ternary diagrams.  We do not include:

• Identifying correct initial units for specific ternary diagrams prior to normalization
• Contextual interpretation of specific ternary diagrams. 
• Phase equilibria interpretations in igneous and metamorphic systems.
• "Composite" ternary plots that either project data onto a different plot to understand trends (e.g., Piper plots) or consider more than 3 variables on multiple ternary diagrams (e.g., IUGS classification, ultramafic rock classification).
• Software and web applications that plot ternary data for you. We have linked a few of these below.

Instructor resources

Support for teaching this quantitative skill

• Interactive: How to use a ternary diagram (Geology is the Way)
• Ternary Plotter
• Excel Template for Ternary Plotting
• Ternary Diagrams in Petrologic Phase Equilibria

Examples of activities that use this quantitative skill 

• Feldspar Minerals and Triangle Diagrams is a lab by Eileen Herrstrom that gives a brief introduction to plotting points and fields on ternary diagrams and then develops the feldspar mineral ternary plots using mineral data.