Learning Objectives

• Describe the parts of a standard thin section.
• Describe the differences between a standard petrographic thin section, a thin section prepared for electron or ion microbeam analyses, and a thick section.

Prior Knowledge and Skills

• None

Key Terms

• Thin Section
• “Microprobe” thin section
• Thick Section

Guided Inquiry

Use this interactive diagram to answer the following questions:

2.2.1 What are the parts of a standard thin section?

2.2.2 How is a “microprobe” thin section different from a standard thin section?

2.2.3 How is a thick section different from a standard thin section?

2.2.4 How is a thick section different from a “microprobe” thin section?

Use this interactive diagram to answer the following questions:

2.2.5 Are all thin sections square or rectangular?

2.2.6 What are the dimensions of a standard thin section in mm?  In inches? (2.56 cm/inch)

Concept Checks

Concept Check 2.2.1

During field work, you come across a sedimentary rock unit that may contain fossils.  Your priority with creating a thin section is to obtain clear images of the fossil morphology (shape) for identification.  Would you choose to make a thin section or a thick section? Why?

Concept Check 2.2.2

Your research group is conducting a study on fluids in metamorphic rocks.  You wish to analyze hydroxyl stored in garnets in a gneiss by shooting infrared light through them.  Glass and epoxy interfere with the infrared light. Which type of thin or thin section would you choose to make, and why?

If you are unsure of your answers to the Concept Checks, then please read the Summary and revisit the Concept Checks if needed.

Summary

Thin Section:

A thin section is a 30-micrometer-thick slice of rock that is attached to a glass slide with epoxy. The thin slice of rock can be topped with a thin cover glass to enhance the optical qualities of the thin section.  Thin sections are used for mineral identification, petrographic analysis to classify rocks, and textural analysis to describe how a rock formed.  Thin sections can also be stained to identify specific minerals.

Alternatively, the exposed surface of a thin section may be polished to a fine finish for electron microbeam analyses including SEM/EDS, electron microprobe, and ion microprobe analyses.  For SEM/EDS and electron microprobe analyses, it is also necessary to coat the polished surface of the thin section with a thin coating of carbon, which is conductive and disperses the charge of the electron beam.

Thick Section:

Thick sections are just what they sound like: thin sections, but thicker.  Thick sections are used for fluid or melt inclusion studies, Raman analyses, and FTIR analyses, and can also be analyzed in an SEM or electron microprobe.

The standard thickness for a fluid inclusion thick section is 50 micrometers, but thick sections can be made at any thickness.  Thick sections can be attached to a glass slide, or can be prepared so that they can be removed from their mount as a stand-alone slice of rock.  Thick sections can be polished on one or two sides, depending upon the application.  For use in infrared spectroscopy, we wish to prepare some samples with four polished sides in the shape of a prism!

References

Profile of thin section adapted from: “How to make a thin section,” Dave Hirsch, (2012, ret. 11/18/18) https://davehirsch.com/other/thinsections/.

Licenses and Attributions

Figures created by Mark Peale, Elizabeth Johnson, and Juhong Christie Liu, JMU

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