6 2.6 Thickness Standards for Polishing Thin Sections
6 2.6 Thickness Standards for Polishing Thin Sections
Elizabeth Johnson; Juhong Christie Liu; and Mark Peale
In the previous sections of this module, we learned that standard thin sections should be polished to a final thickness of 30 micrometers. But how do we know when we are (finally) to the correct standard thickness? The rock slice is attached to a glass slide and epoxy, so it is not possible to directly measure its thickness with a micrometer. In this unit, we explore how to use the optical properties of the minerals within the rock slice to determine when the desired thickness has been achieved.
Learning Objectives
- Explain how to use thickness standards to determine when a thin section is at the correct thickness.
Prior Knowledge and Skills
- 2.2 Thin section and thick section anatomy
- 2.4 Procedure: how to make a thin section (recommended)
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2.5 Equipment: Thin Section Preparation (recommended)
- Experience using the petrographic microscope to identify minerals (recommended)
- For color-blind students: We are not concerned about naming individual colors in this unit. It is enough to distinguish first-, second-, and third- and higher sets of colors, which are of different brightness. Please ask for help from your instructor if necessary.
Key Terms
- Interference colors
- Birefringence
Guided Inquiry
The color of a mineral under cross-polarized light is called its interference color. Interference colors categorized by first, second, third, and fourth-order colors are shown in Figure 2.6.1:
2.6.1 Which colors are more “pastel” in nature: first-order colors or third-order colors?
2.6.2 Is it easier to distinguish between first-order and second-order colors, or between third-order and fourth-order colors?
Figure 2.6.2 is the Michel-Levy Interference Color Chart. This diagram displays the interference colors in the chart, with verbal descriptions of first-, second-, and third-order colors below the x-axis. The y-axis on the left side of the diagram is thickness of the mineral (or thin section) in micrometers.
The physical property called birefringence is plotted along diagonal lines across the diagram. The value of birefringence for each diagonal line is listed at the top and right sides of the diagram. Minerals are listed across the top and right side of the diagram next to their birefringence value.
Click on the image to link to a larger version to help you answer the questions!
2.6.3 Locate quartz on the diagram (top row, 0.009 birefringence). Find the diagonal line right below 0.009, and follow it down to a thickness (y-axis) value of 30 micrometers. What is the interference color of quartz at this standard thickness?
2.6.4 What color would quartz be in a 40 micrometer thick sample?
Thickness Standards
Spectrum Petrographics created four sets of thin section standards for this project based upon an idea found in Allman and Lawrence (1972). Each rock was finished to a different thickness: 90, 60, 40, and 30 micrometers. We chose four rock