Through the Looking Glass

The past month has been focused on looking at my returned thin sections under the petrographic microscope. This microscope was developed for use in optical mineralogy and is integral in identifying microstructures and the mineralogy of rock samples. There are two primary views with the petrographic microscope: plane polarized light and cross-polarized light. Plane polarized light (PPL) is when the light shining through the microscope is polarized, meaning it vibrates in a single plane. A PPL image would look something like this:


PPL image of a quartzite sample. Field of view is 1mm.

We then have cross-polarized light (XPL). This is when the light is passed through two polarizers and vibrates in two directions. When we place a thin section in between the polarizers, the minerals (depending on what kind) can split the light from the lower polarizer and join again at the upper polarizer, also called the analyzer. An XPL image looks like this:


XPL image of the same quartzite sample as above. Field of view is 1mm.

These images are helpful in both describing structures and identifying minerals within rocks. I’ve been looking at many of my thin section slides and the ones that stand out most in terms of evidence of deformation have been the quartzite samples. The peridotites have not shown to be very helpful as most of them are massive in texture and therefore do not record evidence of deformation. In contrast, the quartzites tend to show plane to flattening strain at the microscale, which in hand sample exhibits as foliation. In the next post I will delve into this further with specific examples from my first batch of thin sections.