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Cross polarizationantenna

Figure 2.7.3 is the Michel-Levy Interference Color Chart. To better view the text and labels on the full-sized image, click on this link to the original file on Wikipedia. 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.

Crosspolarizedlightmicroscopy

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 2.7.3.

Figure 2.7.9. Quartz exhibiting undulatory extinction under cross polarized light 100x total magnification. Cranberry Gneiss, near Galax, VA.

This video gives an overview of some of the important properties of minerals in cross polarized light. These properties, plus a few others, are explored in more detail in the sections below.

Cross polarizationphotogrammetry

Earth Optics Videos (Nov 27, 2015). Earth Optics Video 2: Cross Polarized Light. CC-BY license. https://www.youtube.com/watch?v=OB7UbgiDGW0

Cross polarizationfilter

In this section, we explore properties that can be observed for minerals under cross polarized light, when both the lower polarizer and the analyzer (top polarizer) are inserted into the polarizing light microscope.

The slice of rock on a standard thick section is about 30 micrometers thick. To determine birefringence, use these steps:

Crosspolarizedlightgeology

If a mineral has cleavage, determining if it has parallel or inclined extinction can be useful to distinguish it from similar minerals. Under cross polarized light, a mineral with parallel extinction will go extinct (dark) when the cleavage direction is parallel to the north-south and east-west directions as the stage is rotated. A mineral with inclined extinction will go extinct with the cleavage at an angle to these directions as the stage is rotated. The cross hairs on the ocular should be used as a guide for this measurement.

The physical property called birefringence(δn, or nγ – nα) 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.

Cross polarizationphotography

This page titled 2.7: Properties Under Cross Polarized Light is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Elizabeth Johnson and Juhong Christie Liu (Virtual Library of Virginia) via source content that was edited to the style and standards of the LibreTexts platform.

Isotropic minerals have crystal structures which have the same structure in every direction. They belong to the isometric crystal system. All other minerals are anisotropic, meaning their crystal structures vary with direction. These are minerals belong to the hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic crystal systems. (See Section 2.8 Interference Figures and Crystal Symmetry for a chart of the crystal systems). The table below summarizes characteristics of opaque, isotropic, and anisotropic minerals.

There are many other textures and features that may be revealed under cross polarized light which are not visible or not obvious under plane polarized light. These can include twinning (Figure 2.7.7), zoning due to compositional changes during crystal growth (Figure 2.7.8), undulatory extinction (Figure 2.7.9) or other deformation textures, and exsolution lamellae (Figure 2.7.10) in which one mineral separates into two minerals, producing a striped effect.

Figure 2.7.1. Explanations of isotropic versus anisotropic minerals, interference colors, birefringence, and retardation. Earth Optics Videos, CC-BY.