To many people, a rock is just a rock. If anyone has been house shopping, you know how realtors will mention that the countertops are “granite” and you might wonder why, from house to house, they all look so different. Turns out (much to the chagrin of geologists in the housing market), those countertops are likely neither granite nor all the same type of rock. That’s because rocks are made of different minerals and rocks get their names (mostly) from what minerals they contain.

Now, much of the time, once you’ve been trained to identify minerals, you can use those skills to give a rock its proper name. The countertop has quartz, plagioclase feldspar, potassium feldspar and biotite? That is likely granite. At another house it is all calcite…now that’s marble.

Slicing Up the Rocks

However, the people who study how these rocks form take it at least one step further and that is looking at rocks under the microscope. However, this isn’t your garden variety “make everything bigger” microscope. We don’t just take a sample in the same form it comes out of the ground. Instead, geologists make what we call “thin sections” … which are exactly what it sounds like: very thin slices of rock, usually glued to a glass slide.

How thin? Well, most slices of rock on thin sections are cut and polished down to 30 micrometers. That is less than the thickness of most people’s hair strands. Why do we bother with making thin sections so, well, thin? Because once you cut the rock that thin, light can pass through much of the material of the rock. Yes, see-through rocks.

If that isn’t cool enough, the microscopes we use to look at these thin sections utilize a very special kind of light called “polarized light”. That might sound familiar because good sunglasses are polarized, so they only let light that is vibrating in one plane through to your eye, thus reducing the exposure of sunlight to your relatively fragile eyes. In a petrographic microscope, the light is first polarized, then the thin section is placed in that beam of light and then that light passes through a second polarizer.

A weird thing happens to polarized light if you place a second polarizer oriented at a right angle to the first. That light vanishes. Okay, it doesn’t vanish as much as it gets blocked. The light is vibrating all in one direction hits a filter that only lets light vibrating at a right angle through, so like a cow trying to fit through a fence, nothing gets through. So, why can you see stuff in the microscope when the light is “doubly polarized”?

Bending the Light

That’s because minerals are crystals and crystals can refract light. This means the light will be split into two “rays” that are vibrating at a different angle that the starting light. So, you slap a thin section full of minerals (aka, a rock) between the first polarizer and second polarizer, the mineral will refract the light. That refracted light hits the second polarizer and is recombined as some of the light is allowed through. The refraction and recombination of light is dependent on the crystal structure of the mineral and the orientation of the minerals.

Each mineral type has a specific crystal structure, so they will show up differently in doubled polarized light, but their optical properties will also be specific for that mineral type. That means we can identify minerals using how they appear in doubly polarized light under a microscope!

Even in singly polarized light, some minerals betray their identity. In the image above, the brown mineral is hornblende. Most of the other minerals are white (clear), but hornblende is one of the few minerals that shows color in singly polarized light.

However, throw the second polarizer in the light of sight and you get this:

Suddenly, there is a lot more color and the rock looks a lot darker. That’s because some of the light is being blocked by some minerals and other minerals are refracting the light. Hornblende suddenly has the bright yellows, blues and pinks. The white, grey and black minerals that look like blades are plagioclase feldspars.

The Fine Details

On top of that, different things that can happen to minerals like reactions or growth defects or “twinning” will also show up when you examine them in doubly polarized light. The image below shows a cluster of big plagioclase feldspar crystals showing both twinning (when crystals grow with changing orientation) and zoning (when conditions change in the magma as it crystallizes).

I’ve zoomed in (from 4x to 10x) on the most impressive crystal to show just how thin the zoning can be in a crystal. The scale at the top left is only 0.2 millimeters, so each band in the crystal is some small fraction of that sub-millimeter scale.

If I zoom in more, from 10x to 40x, the first thing you notice is that the image is a little blurry. That’s just because of the high magnification. However, you can see just how fine the zoning layers are and you can see little inclusions in the crystal. Usually those are imperfections that trap a little bit of the magma and then quickly cools into glass. We call them “melt inclusions” and they tell us a lot about the conditions that the crystals formed.

That is a very quick look at how geologists examine rocks, at least using light. It might be a technique that has been around for hundreds of years, but it is a necessary and vital part of the research process! It can illuminate much about the rock without needing to do anything more than make a thin section and use your eyes. Once you have that context, the geochemical or isotopic analyses that follow can give us even more insight, all built off of your observations.