# The use of visualization and sketches of thin sections to encourage a better understanding of phase diagrams: Binary and ternary phase diagram exercises

Jennifer M. Wenner and Drew S. Coleman
,
University of Wisconsin Oshkosh and University of North Carolina

#### Summary

In these homework exercises, students manipulate two- and three- component phase diagrams. At various points during their interpretation of melting or crystallization of a composition, they are asked to visualize/sketch the resulting rock (in thin section) if it were quenched at that point. They are also required to know how to determine how many phases are in equilibrium and the proportions (or percentages) of those phases at any given point during the evolution of a given magma.

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## Context

#### Audience

This exercise is designed for a sophomore or junior level required course in petrology.

#### Skills and concepts that students must have mastered

Students should understand Gibb's phase rule (and the condensed phase rule), the Lever rule, and how to determine liquid and solid (both instantaneous and bulk) compositions.

#### How the activity is situated in the course

This is a stand-alone exersise.

## Goals

#### Content/concepts goals for this activity

This activity is meant to help students read and understand phase diagrams.

## Description of the activity/assignment

In these homework exercises, students manipulate two- and three- component phase diagrams. At various points during their interpretation of melting or crystallization of a composition, they are asked to visualize/sketch the resulting rock (in thin section) if it were quenched at that point. They are also required to know how to determine how many phases are in equilibrium and the proportions (or percentages) of those phases at any given point during the evolution of a given magma. These exercises require that students understand Gibb's phase rule (and the condensed phase rule), the lever rule and how to determine liquid and solid (both instantaneous and bulk) compositions. In all, there are 4 binary and 6 ternary phase diagram exercises that are each 1-3 pages in length. I have also included an exercise that introduces phase diagrams and the phase rule (developed with Daniel Brabander). We developed these exercises while at Boston University because we felt that conventional exercises taught the students how to manipulate the diagrams but students could not make the connection to what they are seeing in hand sample and in thin section. We have since found that students were better able to apply the concepts of phase diagrams to their hands-on laboratory exercises.

## Determining whether students have met the goals

Students have met the goals of this activity if they answer the problems completely and accurately. 