Module 2: Using the Petrographic Microscope
This section contains two ideas for synthesis assignments that require students to pull skills and knowledge together to create resources and apply what they have learned. Students should first complete Sections 2.1–2.8, and should understand how to search for additional resources using Section 2.9.
Exercise 2.10.1: Create a Help Guide: Distinguishing Among Similar Minerals in Thin Section
- Compile characteristic optical features of common minerals using online resources and textbooks.
- In a group, construct a help guide which lists distinguishing features which differentiate among minerals that are similar in appearance in either plane or cross polarized light.
Part 1: Look up mineral properties
Students should familiarize themselves with the optical properties of many common minerals using resources in Section 2.9 and in mineralogy and optics books available in class. They should record the following information for these minerals:
- Name and Mineral Formula.
- A Labeled Sketch showing important features and a scale.
- Characteristic Properties (these may include optical properties such as pleochroism, optical sign, relief, birefringence colors, inclined or parallel extinction; also textural features such as twinning, exsolution features, morphology, cleavage, etc.).
A list of common or important igneous and metamorphic minerals could include:
- Orthoclase and microcline
- “Hornblende” or other amphibole group minerals
- Serpentine-group minerals
- Oxides (magnetite, ilmenite)
- Carbonate minerals
Part 2: Which Minerals are Similar?
How do we quickly distinguish between different minerals? Some minerals can be difficult to distinguish from each other, especially the ones below. Ask students: which 1-3 observation(s) allow you to tell the difference between each pair of minerals?
- Olivine vs. Clinopyroxene
- Clinopyroxene vs. Orthopyroxene
- Quartz vs. Plagioclase
- Orthoclase/Microcline vs. Plagioclase
- Hornblende vs. Pyroxenes
- Biotite vs. Hornblende
- Biotite vs. Muscovite
- Chlorite vs. Epidote vs. Serpentine
- Garnet vs. Oxides
- Calcite vs. Dolomite (trick question)
Part 3: Gallery Walk: Create a Table of Distinguishing Features
In this exercise, students are divided into groups of 2-3. Each group is assigned one of the comparisons from Part 2 (for example, Biotite vs. Muscovite). Each group writes their best 1-3 characteristic differences between their minerals on the white board or on paper hung throughout the room. One each group has compiled their best observations, groups rotate to the next comparison. For example, the Biotite vs. Muscovite group moves to the Chlorite vs. Epidote vs. Serpentine list and adds or edits as necessary. Student groups can rotate a few times until everyone is satisfied with the lists.
The product of this exercise is a giant “cheat sheet” on the board or walls of the classroom, meant to help students differentiate among minerals more easily. This is a great way to prepare students for a mineral identification quiz.
Exercise 2.10.2: What is this Mineral?
Students will be able to:
- Correctly identify an unknown mineral in thin section using optical techniques under a polarizing light microscope.
This activity can be completed individually or as small teams (2-4 students). Each group is provided with a thin section containing one or more unknown minerals. Mineral grains should be large and pristine enough for students to be able to observe optical properties and obtain interference figures on the minerals.
Part 1: Document the observations.
Students should compile their observations through sketches, photos, and written notes. Observations can include features in plane and polarized light, and optical interference figures.
Part 2: Narrow the possibilities.
For each observation, students should look up possible mineral species using the Michel-Levy interference chart and other resources. Optical mineralogy textbooks often contain flowcharts or tables that list minerals according to specific properties (such as relief or birefringence).
Part 3: What is the Mineral?
Students should narrow the list of possibilities to a few minerals. Other groups of students can peer-review the evidence and interpretations, to see if a unique mineral identification can be made.