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Even when viewed with the naked eye, this sample of gabbro has visible crystals. When sliced thin and placed under a microscope with supplemental light, it is stunning! Photomicrograph by Kali Sipp.

 Kali Sipp is from Wild Rose, Wisconsin, and recently graduated with a degree in geoscience from Northland College in Ashland, WI. Kali’s senior project included taking photomicrographs of slices of rocks in the geo lab. Those photos grace the hallways of the Museum, and they write about them here. Kali was a Summer Naturalist/Geology Intern at the Cable Natural History Museum.

It was 7 o’clock at night at the Northland College Geology Lab, and for the past four hours, I had been peering down a microscope, absorbing the stunning images of a rock cut so thinly that light leaked through it.

The microscope I had been using magnified the tiny sample hundreds of times, revealing hidden intricacies between grains mere tenths of a millimeter in size. Each view was a tiny stained-glass window. However, magnification alone was not what made this mineral thin section so colorful and vibrant.  

Light, normally chaotic, is forced to be orderly as it passes through a slotted filter called a polarizer. The light then goes through the crystals of the sample, where the light changes speed and direction within their matrices before exiting back out and through a second polarizer, perpendicular to the first.

The result seen is an image of crystals on the slide, now dyed in new colors that changed when you spun the thin section around. It was a feast for the eyes.

But this polarized light microscopy was not just a light show, it was a method for study. Individual quartz crystals that would otherwise appear as a clear mass in plain light suddenly sprung out in a rainbow of colors. Other minerals produced only a small array of colors. Some, like garnet, turned completely black under the polarized light. Using polarized light would prove useful for this particular sample.  

It was a rock I found in the Upper Peninsula of Michigan: a mere curiosity when I collected it as a greenhorn geologist. I wanted to know more about it. The garnets in it were evidence of drastic changes in its lifetime, as garnets only form under high heat and pressure, ultimately altering the original structure and composition of the rock. So, it begged the question: what was this rock before its transformation?  

I set out on a study with my professor, Tom Fitz, to find out. Our hikes across the Northwoods to find other outcrops of its kind yielded nothing. But we did not give up hope. Instead, we sent our sample away to get a thin section cut out of this rock to study it much more closely. The result was marvelous.  

When I spun the microscope’s turntable, biotite crystals shimmered with a brown hue. They were accompanied by quartz grains that raved in different colors. Hornblende and feldspar also flashed with each turn of the slide. Chlorite stubbornly remained green, even under polarization; while in contrast the garnets turned opaque under the polarized light, only to become transparent in comparison to the hematite that sat next to it unchanged when the polarizer was removed.  

All of these minerals were vexing, as they were not what we were expecting to find in the sample. So, for several weeks, I pored over studies published by other scientists about the geology of the U.P. of Michigan to see if anyone else had documented a rock similar to mine. The online search yielded nothing for a while, but then, a breakthrough!  

One study described the geochemical makeup of a rock they had collected, very similar to mine in both location and chemistry. But it was the thin section photographs the study provided that would cause me to declare I had found a probable match. The photomicrographs displayed rocks that could have been a sibling to mine!

Together with the geochemistry, it was enough to convince me I had figured out what my rock was: a metadiabase sill—fancy talk for a rock that cooled from magma in between two other layers of rock, then was metamorphosed by heat and pressure.   I could not have figured out what my rock was without the microscope. It was a tool that transformed my way of thinking and looking.

Now, everywhere I go, I see rocks and wonder what they would look like under the scrutinizing gaze of the microscope. What minerals lay hidden? What structures or other hidden features are sequestered away from prying eyes, waiting to be revealed?   The more I learn, the more I want to explore the unseen world that surrounds us.    

Emily Stone’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.   For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Our exhibit: “The Northwoods ROCKS!” is open through mid-March. Our Fall Calendar of Events is ready for registration! Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.