5: Optical Mineralogy

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5.1.jpg — Figure 5.1: Thin section on the stage of a petrographic microscope

KEY CONCEPTS

Light entering a crystal may be absorbed, refracted, or reflected.
Optical mineralogy involves studying rocks and minerals by studying their optical properties.
Today, most optical mineralogy involves examining thin sections with a petrographic microscope.
Petrographic microscopes have polarized light sources that illuminate a thin section.
We examine thin sections in two modes: using plane-polarized light or using cross-polarized light.
In plane-polarized light we can distinguish opaque and nonopaque minerals; we can see crystal shape, habit, cleavage, color and pleochroism, and relief.
In cross-polarized light, we distinguish anisotropic from isotropic minerals, we see interference colors related to birefringence, and we can see twinning and related features.
We use cross-polarized light to learn a crystal’s optic class and optic sign, to measure extinction angles and sign of elongation, and to measure 2V.
A combination of optical properties allows us to identify minerals in thin section and to interpret geologic histories.

For an Alternative Approach

This chapter contains the standard and fundamental information about optical mineralogy. But there are many ways to approach this topic. For an alternative approach, and to see many excellent videos, go to “Introduction to Petrology” by Johnson, E.A., Liu, J. C., and Peale, M. at: https://viva.pressbooks.pub/petrology

Prologue: An Introduction to Optical Mineralogy

The principles of optical mineralogy and mineral microscopy can be confusing. A standard approach into complicated science topics is to start with a discussion of underlying principles and to build to more complicated concepts. And we will do that.

But because mineral optics can seem arcane, before we jump into the underlying fundamentals, we will begin with a video that gives some background and a broad overview without all the details of optical theory and microscopy. The video finishes up by discussing the most important aspect of optical mineralogy, which is viewing rocks and minerals in thin section. And everything that follows this box in Chapter 5 is building to that end.

Video 1: An Introduction to Optical Mineralogy (9 minutes)