From mountains to microscopes

I like pretty things, which is why I’m drawn to geology as a discipline. There is beauty to be found in many aspects of the science – at all scales.

You may have guessed that I’m a fan of large scale formations that result from large-scale processes (mountains! tectonics!). But I’m also a fan of the micro-scale – things that can be observed using a microscope.

Geologists call the special optical microscopes that they use petrographic microscopes and the study of rocks that uses such a microscope is called petrography.

To study rocks under the petrographic microscope, it has to be cut thin enough for light to pass through. Geologists use something called a thin section – or thin slices (usually 30 microns or 30/1,000,000 meters thick) – of a rock or mineral sample. Thin sections are usually mounted on a slide with adhesive and measure 26 x 46 mm but the size and shape can vary depending on the application. Also, there are different ways to treat thin sections, like embedding them with different media, staining them to highlight different minerals, or coating, covering, or polishing them differently so that they are compatible with different types of light or microscopes. If you want to learn more about making thin sections see this excellent website.

Petrographers use polarized light microscopy to observe features in the thin sections. Polarization acts as a filter to isolate waves of light along particular planes. If you think of the way a guitar string vibrates up and down along the length of the string it’s similar to how light propagates through space. The difference is that there are different orientations of the waves and polarized light takes out some of the orientations. The graphic below illustrates how light travels in directional waves and how the polarizer works to filter it.

Cartoon of how polarization affects light.

Polarizers are used by petrographers because the minerals that make up rocks exhibit an optical property called birefringence which means the mineral looks different depending on the nature of polarization. The way the mineral looks under different polarization (plane-polarized versus cross-polarized, for example) can be used diagnostically – that is, it is useful for identifying and characterizing the mineral and it’s history.

Plane polarization is when only a lower polarizer is used and cross polarization is when a lower polarizer and an upper polarizer (also called the analyzer) are used. Properties that can be observed using plane-polarized light microscopy include opacity (degree to which a material transmits light), color, pleochroism (when a material changes color as it rotates relative to the polarizer), refractive index (the speed of light in the material relative to that in a vacuum), and relief (the difference in refractive index between a material and its surroundings).

Under cross-polarized light, minerals reveal very interesting properties, many of which are slightly too technical to describe in detail here. One example; however, is called twinning and it occurs in plagioclase and some other minerals (see below).

Twinning in plagioclase, source.

The textures and patterns that can be observed in thin sections of rocks using a petrographic microscope and not only beautiful but also aid in scientific investigation. For example, petrologists use microscopy to study things like the metamorphic and deformational history of rocks. Further, chemical analysis can be combined with mineral identification from thin sections to determine the environmental conditions under which igneous rocks form, which can be useful in understanding magmatic processes.

So whether you’re looking at a rock through a microscope or observing global patterns there’s always something beautiful and interesting to see.

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