Module 2: Using the Petrographic Microscope

2.6 Properties Under Plane Polarized Light

Elizabeth A. Johnson; Juhong Christie Liu; and Mark Peale

In this section, we explore properties that can be observed for minerals under plane polarized light, when only the lower polarizer is inserted into the polarizing light microscope.

Learning Objectives

In this section, students will learn to:

  • Distinguish opaque minerals from transparent minerals.
  • Identify the color(s) of minerals in plane polarized light.
  • Identify and describe pleochroism.
  • Determine if a mineral has low, medium, or high relief.
  • Describe the relationship between refractive index and relief.
  • Describe mineral form as euhedral, subhedral, or anhedral.
  • Identify cleavage and fracture in minerals.

Overview of Properties in Plane Polarized Light

This video gives an overview of some of the important properties of minerals in plane polarized light.  These properties are explored in more detail in the sections below.

 

Figure 2.6.1. Some techniques that use plane polarized light.  Earth Optics Videos, Creative Commons Attribution License.

Opaque vs. Transparent Minerals

Transparent minerals transmit light, whereas opaque minerals block light from passing through them.  Because most student microscopes only have an option to look at minerals under transmitted light, this means that all opaque minerals will appear dark in plane polarized light.  Some very small opaque minerals, or minerals within a group that contains both opaque and transparent minerals (such as the spinel group) may let some light through and could appear to have some color.

Microscopes with the ability to view thin sections under both reflected and transmitted light do exist, and different opaque minerals will reflect light differently.  This type of microscope is most commonly used for studying ore deposits or other rocks containing many opaque mineral species.

Guided Inquiry

Question 2.6.1. The two figures above show magnetite (Figure 2.6.2) and garnet (Figure 2.6.3) in both plane polarized light (PPL) and cross polarized light (XPL).  Based upon the definition of opaque minerals, which of these are opaque? Why?

Color

Some minerals that are darkly colored in hand sample will also appear to have color in thin section, even though the rock slices in thin sections are typically only 30 micrometers thick.  Sometimes the color of a mineral in plane polarized light can aid in identification (see examples in the figures below) because it is characteristic for a specific mineral.

But, be cautious about relying too heavily on color as an identification tool!  Minerals within a solid solution group can have very different color characteristics in hand sample (as shown in Figure 2.6.6) and under the microscope.

Figure 2.6.4. Garnet (pink) and clinopyroxene (green) under plane polarized light. Eclogite, California, Ward's collection sample, 40x total magnification.
Figure 2.6.4. Garnet (pink) and clinopyroxene (green) under plane polarized light. Eclogite, California, Ward’s collection sample, 40x total magnification.
Figure 2.6.5. Glaucophane in plane polarized light. Total magnification: 2.5x.
Figure 2.6.5. Glaucophane in plane polarized light. Total magnification: 25x.

 

Figure 2.6.6 A. A colorless forsterite crystal (olivine group) from Wannenköpfe, Ochtendung, Eifel region, Germany.
Figure 2.6.6 A. A colorless forsterite crystal (olivine group) from Wannenköpfe, Ochtendung, Eifel region, Germany.
Figure 2.6.6 B. Light green forsterite crystal (Mg-Fe olivine) from Naran-Kagan Valley, Kohistan District, North-West Frontier Province, Pakistan.
Figure 2.6.6 B. Light green forsterite crystal (Mg-Fe olivine) from Naran-Kagan Valley, Kohistan District, North-West Frontier Province, Pakistan.
Figure 2.6.6 C. Fayalite (Fe-rich olivine) crystals from Eifel, Germany.
Figure 2.6.6 C. Fayalite (Fe-rich olivine) crystals from Eifel, Germany.

Pleochroism

Pleochroism is when a mineral changes color as it is rotated relative to the polarizer in plane polarized light.  This effectively shows how the color properties of the mineral vary with direction within the crystal structure.

Testing for pleochroism using a polarizing light microscope is straightforward.

  • First, make sure you are using plane polarized light by removing the analyzer.
  • Locate the mineral you wish to test.  If possible, it is advisable to look at more than one grain of the same mineral, since these will likely be oriented in a variety of directions and will give a better sense of the pleochroic behavior.
  • Rotate the stage and observe the color changes in the mineral grains.

Figure 2.6.7.A. Glaucophane showing strong pleochroism in plane polarized light. Total magnification: 25x.

Figure 2.6.7.B Epidote in plane polarized light. Total magnification: 40x.

Guided Inquiry

Relief and Refractive Index

The refractive index of a mineral characterizes the relationship between the speed of light in a vacuum and the speed of light in that material.  Refractive index is discussed in Section 2.3 Light and Optics.  The refractive index of a mineral can vary according to crystal orientation.  For example, the refractive index of kyanite ranges from 1.712-1.734 (Wikipedia).

Mineral relief is related to the refractive index of the mineral.  If a mineral has a refractive index that is much higher or lower than the surrounding materials, it will stand out relative to the surrounding material and will have thick or distinct .   In a grain mount, the surrounding material is likely to be epoxy or mineral oil.  In a thin section, the surrounding material will be adjacent minerals.  For the purposes of identifying minerals in thin section, it is sufficient to describe relief as high, intermediate, or low.

Mineral Relief Categories:

High relief Refractive index > 0.12 different from surrounding media
Intermediate relief Refractive index 0.04 – 0.12 different from surrounding media
Low relief Refractive index <0.04 different from surrounding media

In practice, it is not necessary to measure the refractive index of the mineral or the surrounding media directly.  Instead, we look at the thickness of the grain boundaries around the mineral and how the mineral appears to either stand out or blend in with surrounding media or minerals.  Examples of high, intermediate, and low relief are shown in Figure 2.6.8.

 

Figure 2.6.8. High, intermediate, and low relief minerals (click on hotspots to identify). Image taken at 25x total magnification.

The relative refractive indices of two materials (minerals, glass, or epoxy) adjacent to each other in a thin section can be determined using Becke lines (pronounced beck-ee).  The procedure for checking Becke lines is summarized in the video (Figure 2.6.1) at the start of this chapter. When the stage is lowered in plane polarized light, the bright Becke line will either move into or out of a mineral.  If the Becke line moves into the mineral grain, that mineral has a higher index of refraction and higher relief.  If the Becke line moves out of the grain, the mineral has a lower index of refraction and lower relief.  These properties are summarized in Figure 2.6.9 below.

Figure 2.6.9. Becke lines move into the material with the higher refractive index when the stage is lowered.  In these figures, “B” indicates the location of the Becke lines as the stage is lowered.  The Becke lines appear above the sample, because after the stage is lowered, the microscope is focused on a plane above the sample.

Guided Inquiry

Mineral Form

Mineral form refers to the ideal crystal form, or the shape a mineral takes when left to grow without barriers or interference with other nearby mineral grains.  Minerals take characteristic shapes according to their crystal symmetries and structures.

For example, here is a cluster of quartz crystals demonstrating 6-sided prismatic form in hand sample: https://skfb.ly/6FnOZ 

Most of the time, however, crystals in igneous and metamorphic rocks do not grow freely.  They run up against other crystals that have already formed, or ones that are growing simultaneously.  This prevents them from being their true shape.

The terms euhedral, subhedral, and anhedral are used to describe how well a mineral displays its ideal form.  Euhedral minerals show perfect or nearly perfect crystal faces.  Subhedral minerals are rounded but still show the general characteristic shape of that mineral.  Anhedral crystals are completely irregular in shape and do not resemble the characteristic form for that mineral.

 

Figure 2.6.11. Subhedral plagioclase (white) and anhedral clinopyroxene (brown-green) and olivine (brown-green, high relief) in a gabbro, 100x total magnification.
Figure 2.6.11. Subhedral plagioclase (white) and anhedral clinopyroxene (brown-green) and olivine (brown-green, high relief) in a gabbro, 100x total magnification.

In igneous rocks, mineral shape helps us determine the crystallization order from the magma and allows us to connect observations in thin section to thermodynamic models.

Guided Inquiry

Cleavage and Fracture

Cleavage and fracture can be used to help distinguish minerals in thin section.

Cleavage is the tendency of minerals to break along atomic planes of weakness within the crystal structure. Minerals may have one or more directions of cleavage dictated by the atomic arrangement and bonding within that mineral.

Muscovite is an example of a mineral with 1 direction of perfectly defined cleavage: https://skfb.ly/6SYYB

Whereas plagioclase has three unique directions of cleavage:  https://skfb.ly/6FYU6

Fractures are irregular cracks within a mineral.  These can form within any mineral, but when only fractures appear in a mineral, it may indicate that it has poor or no cleavage.

Figure 2.6.13. Hornblende in plane polarized light, 40x total magnification.
Figure 2.6.13. Hornblende in plane polarized light, 40x total magnification.

Guided Inquiry

 

References

Figure 2.6.1. Earth Optics Video 1: Plane Polarized Light. Earth Optics Videos, Creative Commons Attribution License. https://youtu.be/ahS5KlXqQXc

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2.6 Properties Under Plane Polarized Light by Elizabeth A. Johnson; Juhong Christie Liu; and Mark Peale is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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